Back to Part 1 of TS-940 page: Click here









4.4 Experience from Persons who modified the TS-940. 2

4.5 : Pin Diode Modification for TS-440. 14

4.6: So summary of Pin Diode Modification. 14



6.1 Observation of the Phase Noise problem by Thomas Hohlfeld DF5KF in 2005. 24

6.2 Extensive History of the Phase Noise Issues upto about June 1987. 24

6.3 Data Sheet for Problem BA718 OpAmp. 25

6.4 What is Phase Noise and how to measure it by John Grebenkemper, KI6WX.. 25

6.5  Phase Noise measurements on Elecraft K2 by John Grebenkemper, KI6WX.. 47

6.6 Significant Phase noise improvements developed by Thomas Hohlfeld DF5KF.. 48

6.7 Information on Ultra Low Noise OpAmps available in 2008. 54

6.8 Further assessment and Phase noise improvement evaluations planned by Jeff King ZL4AI 58

6.9 First plots of phase noise comparisons below show some very exciting results: 58

6.10  Real Experience of the Ultra Low Noise LT1028 significant receiver performance improvement 59

7.0 SM-220 mods to remove Ghost Signals generated by the 940. 65



PLL BOARD 0: Reconnect Connectors. 68

PLL BOARD 1: Remove the Black Foam from Behind the Board. 68

PLL BOARD 2: Remove the Wax from the VCOs. 69

PLL BOARD 3: Replace all Electrolytic capacitors... 70

PLL BOARD 4: Tune through all Frequencies in USB mode 30 Hz to 30 MHz to verify PLL Board is working correctly... 70

PLL BOARD 5: Identify which PLL is not locked. 71

PLL Board  6:   PLL Board and RF Board and PLL out of lock. 76

PLL Board  7:   PLL Board and setting voltages:  Comprehensively updated in 2012.. 77






Power Supply HEAT SINK RUNS TOO HOT.. 87





VK5KYO includes Larger Computer Fan and relocated Rectifiers. 93

28 Volt Crowbar safety circuit for the TS-940. 93

TS-940 Power Supply Improvement in 2012: Read this page first. 93




4.4 Experience from Persons who modified the TS-940


First from


TopBand: : [WSVHF] TS-940 Specs

km1h @
Thu, 05 Mar 1998 10:30:41 EST

To all those on Topband who is the results of the TS-940 tests. 
73  Carl   KM1H
--------- Begin forwarded message ----------
From: (km1h @
Subject: [WSVHF] TS-940 Specs
Date: Tue, 09 Sep 1997 22:35:15 EDT
Message-ID: <>
As an addition to the Sherwood receiver info,  here are some specs on a TS-940.
All tests were run on 28MHz during the past few days on a customers unit.
TS -940 late serial #  with factory phase noise updates:
MDS SSB  -135 dBm
MDS CW cascaded 500Hz filters   -137 dBm
Sensitivity 10dB S+N/N   .12uv
Phase Noise    -131dBc   at 10 KHz
Filter rejection CW  >90 dB
2 tone dynamic range Wide  95 dB @ 20 KHz
2 tone dynamic range Narrow ( CW Filters)  77 dB @ 2 KHz
3rd Order IP  +1dBm at  20 KHz
Wide Band transmitted noise  -75 dB below full carrier
The same TS-940 but with PIN diode mods to RF and IF boards:
MDS SSB  -137dBm
MDS CW   -142 dBm
Sensitivity  ~ .1uv  
Phase noise  -131 dBc  @  10 KHz
Filter rejection ( CW )  >90 dB
2 tone Dynamic Range Wide  102 dB @ 20 KHz
2 tone Dynamic range Narrow  ( CW Filters)   83 dB @ 2 KHz
3rd Order IP   +5 dBM @ 20 KHz  
Wide band transmitted noise  -90 dB below full carrier
A few notes and comments:
Although the PIN diode improvement is evident in the numbers the audible difference is much greater. 
First of all the receiver is noticeably quieter. The IMD performance shows an "apparent" improvement of 
about 10-12 dB under crowded band conditions. This follows along with conversations I had with Dr. Ulrich
Rhode several years ago when I first started using PIN diodes. The cumulative effect of multiple strong signals degraded IMD performance in a stock receiver a lot more than a simple 2 tone test would indicate.
The receiver is a pleasure to use in lowband pileups now. 
The improvement in wideband TX noise is due to, I believe, the use of PIN's in those paths that were 
common to TX and RX on the IF board. The stock diodes either generated noise or allowed RX path noise 
into the TX path. The same appears to hold true in the opposite scenario. This TX noise is something I 
was recently made aware of by a local on 6M and bears closer examination and possible additional
improvements.  The noise does not change dB levels when going from full power to the 20-30mw from the
transverter port so it can not be blamed on thermal noise in the subsequent linear amp stages. 
Magazine reviews of the TS-940 were of early production. Kenwood at first refused to admit to phase noise
problems. They then went thru two different mods before they were satisfied. The improvement between early 
and late models is about 15 dB. 
Serial numbers in the mid 8 Million group  and up had factory mods. 
Kenwood Service Bulletin 917 may be retrofitted to the earlier radios. It
is a fairly simple mod. 
Other KW radios such as the TS-850 and TS-930 could also benefit from PIN mods. They both have transverter
ports and are quite reasonably priced on the used market. 
I cant speak for other brands but a quick review of a few Service Manuals shows an awful lot of commonality
across all brands with respect to diode switching schemes.  
73....Carl   KM1H 
----- Submissions: Subscription/removal:  
--------- End forwarded message ----------





-----Original Message-----
From: thomas hohlfeld []
Sent: Wednesday, 27 April 2005 10:42 a.m.
Subject: RE: ts-940


Hi Jeff,

thank you for your reply and congrats for the informative web site. I read about the PIN diode mod in a web message.

 I replaced diodes D9 through D20 of the RF board by the PIN types BA479. I did not replace D3-D8 because these

switch a frequency too low for the BA478 to be an effective PIN. I checked the BA478 and found them to be good at frequencies above 2 to 3 MHz. The BA479 have been offered in different versions and it is important to use those

which are designed for  HF (not VHF). Certainly, other PIN diodes may also be useful. The IMD of my 940 improved by

about 5 dBm after this mod.


For re-alignment of the receiver, I mainly followed the instructions of the  service manual for the RF and IF

amplifier stages (I did not align oscillator and PLL circuits). I have a sweep generator (Rohde & Schwarz SWOB 5

equipped with log amplifier), which was very helpful to optimise the bandpass filters on the RF board.


I established RX sensitivity with a HP8640B RF generator together with a home made audio voltage detector to

determine an audio increase by 10 dB. I have two of the HP8640B, so that I am able to determine receiver IMD.

The HP8640B are quite famous, since they produce a very clean RF signal and are sold at a reasonable price.


By the way, I own two TS940 and use one of them for experimental modifications which are more ´critical´, so

that I would not really recommend others to reproduce them. If you are interested anyway, I will report on that

later. It’s past midnight now.


73 for today,




>From: "Jeff King" <>

>Reply-To: <>

>To: <>

>Subject: RE: [ts-940] Re: Why don't more people use this group?

>Date: Mon, 25 Apr 2005 09:09:42 +1200


>I found your review of the FETS most interesting.

>My reason for writing is to ask you to tell me more about the pin diode modifications you have undertaken.

>Where are these diodes and what do you do to replace them?

>What else did you do to realign receive?

>How did you establish the 0.15 uV sensitivity.?

>Yours sincerely

>Jeff King ZL4AI






>From: "Jeff King" <>

>Reply-To: <>

>To: <>

>Subject: RE: ts-940

>Date: Fri, 29 Apr 2005 17:05:30 +1200

>Hi Tom,

>Thanks this information is very interesting.

>I really appreciate your advice on aligning the 940 very helpful information.

>I also read [below] about pin diodes being changed in the IF board RX / TX circuits. Have you tried changing any of those?

>Is the Temic's- Vishay (former Telefunken) BA479 (G or S suffix) a suitable diode? Spec sheet attached.

>Yours sincerely

>Jeff King ZL4AI



-----Original Message-----
From: thomas hohlfeld []
Sent: Monday, 2 May 2005 9:23 a.m.
Subject: RE: ts-940


Hi Jeff,

Thank you for the comments on the pin diodes. Yes, I used the BA479G. These were quite inexpensive and it was

easy to obtain them when I did this mod two years ago. Today, I checked for a source in the internet

and found a German distributor who may still have the BA479G. Check at  (a button at the lower left will switch language into English).

They do not look too expensive (0.31 Euro when 50

are ordered).I checked the BA479G with an RF generator and scope and found it to be good at 7 MHz and

higher. I am not sure if it is very effective at 3.5. Nevertheless, I decided to replace D7 through D20

on the RF board. I think I also exchanged D21 (don’t know any more, but makes sense). I did not change

D23 and D26 in the preamplifier because these probably are already pin diodes and their exchange might

impair the AGC characteristics. There are some switching diodes also in the IF section of the 940, but I

did not replace these, because XF-1 already cuts the bandwidth at the front of the IF amplifier and

there may not be too much intermodulation behind. May be I will do this later when I have to take out the

IF board for some other reason.


I told in my last mail that I did other mods in my 940s. It may take quite long to describe them all, so

for the first time here is a short list:


(1) I exchanged the PLL amplifier IC (IC18, PLL board) into a pair of extremely low noise OPamps,

which lowered  phase noise. However, this required to tackle some problems with PLL instability.


(2) I have the optional 250 Hz CW filter for the 455 kHz IF. When working PSK in SSB mode, I missed the

possibility to activate this filter. I found out how to modify the 940  to allow for activation of the

CW filter in SSB, including the control LED at the narrow CW filter switch.


(3) When I bought my first 940, it came without the AT unit. So I built one with an automatic antenna tuning

board (kit) and built an interface which nicely communicated with the antenna tuning control circuit of

the 940. Later, I got the original AT-940.


(4) Follow hyperlink




(5) I equipped my 940s with the piexx boards which allows to control the 940 via the serial interface of

a PC.


Best 73s for today, Jeff


Thomas, DF5KF





The following 2 emails have had some irrelevant content edited out.



>From: "Jeff King" <>

>Reply-To: <>

>To: "'thomas hohlfeld'" <>

>Subject: RE: ts-940

>Date: Fri, 27 May 2005 19:35:16 +1200

>Hi Thomas,

>Thanks for your email which I am still thinking about, a lot.

>Anyway I have been trying to get BA479G diodes from

>a supplier.

>The supplier advises they have the BA479 but cannot tell if it is a G or S.

>I have asked them to put a resistance meter on it and await those

>results. [They replied they could not help.]

>Attached you will find the datasheet. As you can see on the second

>and third lines on the first page

>Reverse impedances are:

>G: 5 k Ohms

>S: 9 K Ohms

>It appears to me that the S might be better, or no worse than the G.

>Do you think S would be suitable to use without further testing.

>My worry is installing an S and finding it does not work well.

>I have an interesting article from Radcoms Pat Hawker on Pin Diode

>replacements in July 1995. If you would like this I will send that when

>I have it scanned.  This article explains that the BA482 replaced in the

>Omni VI with the HP 5032-3081 resulted in impaired performance because

>the HP put through 0.5 dB less signal.

>Your sincerely

>Jeff King





-----Original Message-----

From: thomas hohlfeld []

Sent: Tuesday, 31 May 2005 9:15 a.m.


Subject: BA479 etc.


Hi Jeff,

Thank you for your mail and your thoughts on the PIN diode mod. Yes, you are

perfectly right in that this mod will probably be a tradeoff between

linearity and a slight loss of sensitivity. I would be very interested to

read Hawker´s article you mention.


Your mail also made me re-think about this mod. The G version of the BA479

has been suggested to be better for short wave purposes, but it is true that

there is not much evidence in the datasheet. My guess is that the BA479

should simply be measured in a test setup, and this is what I started



I set up a simple circuit with two RF generators (300 mV out) fed into a

hybrid combiner, the output of the combiner going into one end of the diode

under test. The other end of the diode was coupled via a step attenuator

into a spectrum analyser (my shack is a museum of old instruments, hi). I

fed a forward current of about 10mA through the diode, uncoupled from RF of

course.  The generators were adjusted between 2 and 15 MHz, always 500 kHz

apart (two tone signal).


I hadn’t much time and did only some very short measurements. Anyway, the

results were interesting. Here they are:


1. A conventional all-purpose diode (1N4148) produced a horrible spectrum of

intermodulation products. In addition, the insertion loss was high (up to 10



2. Next, I inserted one of the original diodes which I replaced in my TS-940

(of course I did not through them away). The difference was impressing. At

10 MHz and above, these diodes produced very little intermodulation

distortion with very low insertion loss (1 dB or so). Below 10 MHz, the

intermodulation became worse and was poor at 3.5 and below. Interestingly,

insertion loss moderately increased below 7 MHz (reaching 3 dB at 3.5).


3. I also tried a BA479G which I left from another project. These produced

very low intermodulation signals, even below 2 MHz (which surprised me). At

10 MHz and above, the BA479´s have a slightly higher insertion loss than the

original Kenwood diode (1-2 dB worse). Below 10MHz the BA479 showed less (!)

insertion loss and were clearly superior with respect to intermodulation.


To summarize, the BA479G is better than the original TS-940 diode only at

frequencies lower than 10 MHz. As you will imagine, I am thinking about

returning to the original diodes at 10 MHz and above. It may take a couple

of weeks until I will have time.


I can also take some digital photographs of the intermodulation spectra and

mail them to you, if you are interested (may take 2-3 weeks). Let me know if

your e-mail server has limitations in file size.


Best regards for today and vy 73,






-----Original Message-----
From: thomas hohlfeld []
Sent: Friday, 10 June 2005 8:56 a.m.
Subject: RE: BA479 etc.


Hi Jeff,

thank you very much for including these excellent articles on the web page.

It’s so useful that I’ll print it out.


Now, here is part #1 of my measurements on the RF properties of the 940 Front end diodes. To start, I concentrated

 on the diodes in front of the filters (diodes # 3, 5, 7 ... 19), which see the whole RF spectrum from by the antenna.

 The forward current of these diodes is 25 mA in my 940, which appears quite a lot, but probably was chosen because

the diodes need to be very linear here.

The attached file (25mA-all.jpg) shows the schematic test setup on top. I already described it in my previous mail,

but this time I measured at a ´realistic´ diode current of 25 mA. The two RF generators were set close to 7, 3.5 and

 1.75 MHz, always 250 kHz apart. The spectra (figs 1-12) show the two carriers at the center. All additional peaks are

 3rd and higher order products, indicating the non-linearity of the diodes. To get an ´ideal´ reference, I also

measured with the diode replaced by a wire bridge (figs 1, 4 and 8). Here, the low remaining 3rd order signals

(less than -60dB) reflect some non-linearity of my test setup (my homemade hybrid combiner may not be perfect).

As I already noticed in my earlier mail, a general purpose diode (1N4148) produced a horrible spectrum of side

products at all three bands (figs 2, 6, 10). The original Kenwood diode (1S2588) was very good at 7 MHz (and at

higher QRG´s, not shown), but worse at 3.5 and quite poor at 1.75 MHz (figs 3, 7, 11). In contrast, the BA479G

was very good and provided the least side products (figs 4,8,12). If you look closer at the distance of the carriers

from the top graticule (sorry, background is quite dark), you will notice that the carrier attenuation of the BA479 is

low at all bands and not much different from the original Kenwood diode.









Part #2 of my measurements will follow, where I studied the diodes behind the input filters (D4,6,8...20 and D21)

which are run at a lower forward current. The measurements are already finished but the figures need to be arranged.


Good luck for today!







-----Original Message-----
From: thomas hohlfeld []
Sent: Monday, 13 June 2005 4:31 a.m.
Subject: TS-940S - pin diode part#2 and some other considerations


Hi Jeff,

here comes part #2 of the pin diode measurements, which concentrates on the

diodes between the bandpass and the preamplifier of the TS-940.


First a short comment on the 940´s front-end: when the RF has passed the RX

bandpass, three diodes follow before the signal reaches the preamplifier

Q10. Of these, two are switching diodes (one of D6,8 ... D20, D21), while

one (D26) is a pin diode. Only the two switching diodes are candidates for

replacement. For test purposes, I assumed that D26 is fully open and D23

closed (AGC at highest sensitivity). In this case, we deal with two

switching diodes in series. They are also in series with respect to their

forward current, which is determined by R31. I measured 17 mA in my TS-940.

The impedance, which these diodes see, is also important. I estimate it

about 50 Ohm, because the bandpass filters of the TS-940 are constructed



Therefore, my test setup was similar as before with the following changes:

(1) I tested two diodes in series, (2) I used a forward current of 17mA and

(3) I also measured at 10.1 MHz. Because the 1N4148 was so poor in part#1, I

did not consider it further. The attached jpg file again contains a plot of

the test setup.


Here are the results (see attached jpg file): At 10.1 MHz (figs 1-3), the

original Kenwood switching diode (1S2588) is as good as the pin diode BA479.

The same also applies to higher frequencies, which I do not show here. The

insertion loss of the two BA479 in series is slightly higher than that of

two Kenwood diodes, but is still less than –1 dB (more on this below). At 7

MHz (figs 4-6), the 3rd order products are clearly increased by the Kenwood

diodes (fig 5) in comparison with the reference (diodes shorted, fig 4). The

BA479 (fig 6) is clearly better than the Kenwood diode, although a minimal

increase is also seen compared with the reference. At 3.5 MHz the Kenwood

diodes generate a lot of intermodulation products (fig 8) and the BA479 (fig

9) is obviously superior. The same is true for the 1.75 MHz band (figs

10-12). In summary, the BA479 is better than the original Kenwood switching

diode at 7MHz and the lower bands at a forward current of 17mA. There is no

relevant difference at 10 MHz and higher.


It would also be interesting to know how the diodes behave at frequencies

below 1.75 MHz. Unfortunately, my combiner is not appropriate for a lower

QRG. But I compared the generation of harmonics by the Kenwood diode and the

BA479 at lower frequencies and found the BA479 still to be better than the

Kenwood diode even at frequencies down to 100KHz.


The above measurements also show that the BA479 has a slightly higher

insertion loss than the Kenwood diode. This is a well known disadvantage of

pin diodes (also adressed in the excellent articles on your web page). In

the case of the BA479, however, the effect is less than –1dB and therefore

probably negligable in the 50 Ohm system of the TS-940. To confirm this, I

also used a dB meter to determine the insertion loss of two Kenwood diodes

in series compared with two BA479 in series. At 17 mA diode current, two

Kenwood diodes produce a loss of -0.2dB at 1.75 and -0.1 dB at 3.5 though 28

MHz. Two BA479 in series cause a loss of –0.8 dB at 1.75 and 3.5MHz and

-0.7dB at 7 through 28MHz. So there is a clear difference, but probably

without much importance.


What are the consequences? As long as a broadband antenna feeds the TS-940,

the front-end before the bandpass (diodes D3, D5... D19) will probably be

improved by changing into suitable pin diodes, such as the BA479. Those who

use the TS-940 only with a beam antenna (e.g. 20-10m), which is unlikely to

deliver large signals at 80 and 160m, will probably not have much benefit.

The two switching diodes between the bandpass filters and the preamplifier

may also be replaced by pin diodes at the lower bands (D6, D8 ... D14 and

D21). I would not recommend to exchange D16, D18 and D20, because the

original diodes are already excellent at the higher bands and the pin diodes

would add nothing else than a (minimal) increase of insertion loss.


In addition to the front-end diodes, there are also numerous switching diodes

in the IF unit. Particularly those before the 8MHz crystal filters may be

considered for replacement by pin diodes. I can imagine that this will

improve narrow-band intermodulation. I’d be curious if anybody else has

experience with this. If not, I may check out this point in future.



Your last mail says you ordered 40 BA479 diodes, so it seems you found a

source. I for myself also ordered 50 BA479 from Schuricht, here in Germany.

It’s good to have some on stock for future projects and people say these

parts are likely to be replaced by SMD types in future. If you still need

BA479, let me know and I will try to help. By the way, there are probably

excellent alternatives. The TS-940 uses in its front-end an attenuation pad

which is part of the AGC (D23 and 26, MI204). I tested these and found they

are even a little better than the BA479. The problem with these is that they

are hard to obtain.



Thank you for mailing the discussion on the reversed Q10/Q4 problem.

Although I was unable to find a difference in my test setup (as I reported

earlier), it is certainly possible that there is an asymmetry of the

internal capacitances that did not become apparent in my measurements. I

think I should try out this mod and do some measurements, but it may take a

little until I’ll have time. It would also be interesting to see how the

increase in sensitivity, if it really occurs, will change the receiver’s

dynamic range.



Finally, my congratulations for your exciting web page! You did a very good

job in digging out all the fascinating information about and around the

TS-940. It’s a pleasure to contribute.


Best regards,

















4.5 : Pin Diode Modification for TS-440


A similar sort of mod for the TS-440 maybe found at:





4.6: So summary of Pin Diode Modification


Until further research verifies otherwise, only replace (odd numbered) diodes that carry less than 10 MHz. [Above 10MHz BA479 has more attenuation than the Kenwood 1s2588, so leave the original 1s2588 in place.] It maybe better to hold off any replacement until research is completed.



-----Original Message-----
From: Jeff King []
Sent: Monday, 13 June 2005 9:53 p.m.
To: 'thomas hohlfeld'
Subject: RE: TS-940S - pin diode part#2 and some other considerations



Hi Thomas,


Thanks for this.

Just to verify I understand you correctly.



At 17 ma you advise


The above measurements also show that the BA479 has a slightly higher

insertion loss than the Kenwood diode. This is a well known disadvantage of

pin diodes (also addressed in the excellent articles on your web page). In

the case of the BA479, however, the effect is less than –1dB and therefore

probably negligible in the 50 Ohm system of the TS-940. To confirm this, I

also used a dB meter to determine the insertion loss of two Kenwood diodes

in series compared with two BA479 in series. At 17 mA diode current, two

Kenwood diodes produce a loss of -0.2dB at 1.75 and -0.1 dB at 3.5 though 28

MHz. Two BA479 in series cause a loss of –0.8 dB at 1.75 and 3.5MHz and

-0.7dB at 7 through 28MHz.



so Jeff King concludes that

Two BA479s at 17 ma show

-0.6 dB more insertion loss at 1.75 MHz

-0.7 dB more insertion loss at 3.5 MHz

-0.6 dB more insertion loss at 4 MHz through to 28 MHz


This means the BA479 will reduce the signal and therefore will not improve the receiver performance. In Jeff’s opinion this is considerable increase in loss of receiver performance, and not really viable to use a BA479 as a replacement.


Transmitter performance between 1.5 and 7 MHz will improve because of the much better BA479 IMD performance you describe.



On 31-5-2005 Thomas advised at 25 ma


3. I also tried a BA479G which I left from another project. ………  At

10 MHz and above, the BA479´s have a slightly higher insertion loss than the

original Kenwood diode (1-2 dB worse). Below 10MHz the BA479 showed less (!)

insertion loss and were clearly superior with respect to intermodulation.



This means the only diodes that should be replaced are

D13, D11, D9, D7. in operating at 1.5 MHz to 8.5 MHz and 25 ma


D5 and D3 operate at less then 1.5 MHz, and the performance of the BA479 is unknown in that region.


I wonder if you could document the actual insertion loss of a BA479 performance at 25 ma between 1.75 MHz AND 10.0 MHz?


I look forward to your reply.

Yours sincerely

Jeff King



Diodes on RF board

Original Kenwood Diode

Original Spec

Operating Frequency MHz


Replacement Spec


1s2588 [(L30) DIODE TW-4000A, $3.15]



Don’t replace until testing verifies suitability






Don’t replace BA479 -0.3 dB loss is too great






Don’t replace until testing verifies suitability






Don’t replace BA479 -0.3 dB loss is too great






1.5 -> 3.0

Don’t replace BA479 -0.3 dB loss is too great





3 - 4

Don’t replace BA479 -0.35 dB loss is too great






4 - 7

Don’t replace BA479 -0.3 dB loss is too great





7 - 8.5

Don’t replace BA479 -0.3 dB loss is too great





1.5 -> 3.0

PIN type BA479         

SI-D 30V 50mA 100MHz          




3 - 4

PIN type BA479         

SI-D 30V 50mA 100MHz         




4 - 7

PIN type BA479         

SI-D 30V 50mA 100MHz         




7 - 8.5

PIN type BA479         

SI-D 30V 50mA 100MHz         







D16, 15



8.5 - 14

don’t replace BA479 has 1 - 2 dB higher insertion loss than 1s2588 > 10MHz


D18, 17



14 - 20

don’t replace BA479 has 1 - 2 dB higher insertion loss than 1s2588> 10MHz


D20, D19



2 -- 30

don’t replace BA479 has 1 - 2 dB higher insertion loss than 1s2588> 10MHz





2 -- 30

don’t replace BA479 has 1 - 2 dB higher insertion loss than 1s2588> 10MHz




Pin Diode

2 -- 30

don’t replace BA479 has 1 - 2 dB higher insertion loss than 1s2588> 10MHz






Whether you install many pin diodes depends on how crowded the bands are at your location.

More information below explains this:



-----Original Message-----

From: thomas hohlfeld []

Sent: Tuesday, 21 June 2005 9:16 a.m.


Subject: RE:


Hi Jeff,

yes, I think that 0.3-0.4 dB is almost nothing. Remember that one S unit makes 6 dB (20-fold more!). The additional loss caused by the pin diodes therefore is a very small fraction of an S unit. A few meters of RG58 antenna cable with conventional plugs cause more attenuation than these pin diodes.


By the way, I also measured the attenuation between the TS-940 antenna plug and the preamplifier input. All frontend switching diodes plus the relay contacts plus the bandpass filters (14 MHz) have an attenuation of 2 dB. The same measurement with my Icom IC751 was 5 to 6 dB. This shows that the TS940 is indeed an excellent construction.


Based on the results of my pin diode measurements I have now replaced most of the switching diodes with BA479G. The only exception are D20, D18, D16 and D14 which I did not change. With this change (plus turning around Q10), the RX sensitivity (MDS) is -135 dBm, an excellent result fully competitive with the top Rigs marketed today.


Best regards,






From: Traian Belinas []

Sent: Monday, 20 June 2005 10:29 p.m.
Subject: Re: pin diode, reversed FET

Hello Jeff, 

0,35 dB is an INSIGNIFICANT amount of loss, it is even dificult to detect a such low difference....
The positive aspect of the reduced IMD (especially the second order IMD reduction) is by far more important than the little loss.
My only doubt in such case is the parts and labour cost, and if the mod is really needed for you there.
If you consider it as being needed (if you have unwanted strong signals at your QTH, and please consider the broadcast bands also), then go for this mod with confidence.

Thomas is right, the lower bands are really crowded here, especially during the evening. You are a lucky OM being there in ZL...
After 22.00 local time, the 80m band is full here, I can hear even the italian and DL stations making local QSO's with other I and DL stations there respectively and having some big signals, as not to mention the russians which are everywhere and really strong, like locals, and S9 + 30 to 40 dB signals are usual. The thousands of GU43, GU74 and GS35 power tubes are really put to work out there, hi.
In my case, the added city QRN is also high, the normal noise is to +20 dB, so I use the attenuator for to get the noise lower, at a reasonable level as it have no sense seeing the S meter to S9 + 20 only because of the noise and so to loose a big part of available  Rx dynamic range. The PIN mods are usefull here in Eu. 

Traian, YO9FZS

All the Best,



From: Traian Belinas []
Sent: Wednesday, 22 June 2005 10:26 p.m.
Subject: Re: pin diode, reversed FET

Jeff King wrote:


Hi thanks for this. It did really help my understanding.

In New Zealand we have max of 4,500 hams.

The bands are not crowded

This is the strongest ZL stations.

So on 80 meters at night I can tune whole band and only hear about 2 or more other stations.

My 80 antenna, you have seen diagram is only about 13.5 off ground so it does not work well.

Usually I hear Australian stations and occasionally some USA.

Yes when propagation is there I hear those strong Russians here too.

Now back to Pin diodes.

My simple understanding of how the pin diode works is that the clean pin diode prevents other unwanted multiple frequencies up and down the band. This would seem to be an advantage during transmitting because it prevents unwanted additional off frequency splatter signals on the band. 

Until I read your email yesterday, I did not comprehend to the fact that of course this works in reverse for receive!!! It the existing diodes generate those off frequency signals then of course the a nearby signal will be picked up off frequency in reverse the same way and that off frequency signal will be heard as interference on top the signal you want to hear. Hence Pin Diodes significantly improve the selectivity of the receiver. Traian please confirm this understanding is correct.

Not quite so.
The PINs will have big advantage for Rx only.
The problem with the normal junction diodes is that when they conduct (when the filter is switched in line), the far out of band signals may cause 2nd order IMD and the close or in band signals may cause 3rd order IMD. Note that the in band mean inside the BPF which may have many MHz bandwidth...
Actually the diode act like a mixer (such simple one diode mixers are used, especially at SHF)! You may see how it hapen if consider the unwanted signals which will be mixed by the diode...
For example, when receiving the 14 MHz band here in EU, if big signals exist in the 41m broadcast band, they may cause second order IMD (a 7110 and 7150 kHz BC stations may produce a ghost signal at 14250 kHz as second order IMD; the same will hapen for different bands/frequencies...). Also, when receiving the 40 m ham band, two signals on the 41m BC band may cause 3rd order IMD apearing as ghost unreadable or carrier signals on the 40m amateur band. These are only particular examples, as when the propagation is good, many hundreds of signals arrive simultaneously at the RX diode bandpass filters input which may cause a lot of trouble, especially the stronger ones, the band may seem noisy or ghost or unreadable signals may appear.
The advantage of the PINs is that they act (theoretically) as controlled resistor, they can rectify and mix only for the signals at frequency lower than the one corresponding to its carrier lifetime and their switching characteristics regarding the produced IMD are much better than for the normal diodes.
You may understand now why PINs having large carrier lifetime specification as the BA479 are better for HF than the ones having very small carrier lifetime: they may maintain the same good IMD behaviour at lower frequencies; and the bigest problem is at the lower bands, as Thomas measurements confirm, it is just a practical confirmation of the theory...

Regarding the TX, the signals switched during Tx are few, they are the mixing products from the Tx mixer and IMD is not a problem, as all are originating from the same signal, so a PIN will not make large improvement for Tx. Actually, the Tx IMD are generated by the final amplifier...

Jeff, please note that the proffesional Rx, if not using relays for the BPF switching then they are using good PIN diodes.

So, using PINs instead of the existing diodes is a good thing, especially if the above mentioned problems seemed to occur (so include the presence of the strong signals of the broadcast bands!).
But if big signals are not present, if have a good quiet location like yours, and when considering the cost and the effort involved, it may not worth doing it. It is only a decision of each of us, depending also on the local Rx conditions...
This is what I intended to let you know before.

The mod shall be more usefull for me here, but I am not decided because I will have to sell all the radios some day, so no reason for spending $ and effort for such mod, and I consider also that some buyers don't like buying the modified radios...


Now I must admit I am not sure if I Need these pin diodes. Now propagation is not good I am only really listening on 20 at about 3 UTC. According to Thomas [if one want to avoid losses] I probably should only be interested in changing the diodes below 10 MHz, so this is really only going to improve 40m and 80m

Yes, this is done on some Rx, as the Icom R9000, where the Rx BPF are splitted in two banks (LF+MH and HF)  switched by good PINs.
the other diodes are normal.
[The TS950 SDX Rx BPFs are also separated in two banks by a HPF filter which attenuate the LF/MF signals when using the higher bands, for avoiding the 2nd order IMD caused by the LF/MF broadcast signals. This is the only difference between the TS950SD and TS950SDX regarding the RF board/front end , and some amateurs that are not aware of this are still speaking of the "big difference" between the two radios !!!]

Now I’m convinced not to do the PIN diode mod. I think you are right I do not need it here, where there is little interference.


I look forward to your reply. 

Yours sincerely

Jeff King









From: [] On Behalf Of Martin Sole
Sent: Tuesday, 11 July 2006 2:31 p.m.
Subject: [ts-940] S meter slam mod

I saw a note here the other day/week about the S meter pinning over on power down and an unpublished fix.
I have a radio here that is doing just that, what was the fix? Cannot see it anywhere in the various references checked so far.
Martin HS0ZED

From: [] On Behalf Of Jeff King
Sent: Wednesday, 12 July 2006 9:53 a.m.
Subject: RE: [ts-940] Re: S meter slam mod: A Fix does exist but is not documented out in the public forum.

Garey,  Not all 940s make an S meter slam on shutdown.  On

KI4NR [a Kenwood repair expert] explains,


One other thing .....on all the older 940   4, 5 and early 6 mil serial number ...the IF board is different. The gain distribution in not the same. All the 940 ... late 6 and newer  had better IF boards. They have more gain TX & RX  the radio are hotter sensitivity wise, better AGC compression. I use a 5 mil TS 940 with a later model 8 Mil IF board in it  .......much , much better !!

Also Kenwood put an  S meter slam mod in those boards. all the older 940 when you shut the radio off, pin the S meter over. The newer boards are fix for that.

If someone can get hold of a later than late 6 mil board, that person will find that extra capacitors / diodes / transistors have been patched onto the rear of the board. There may also have been changes on the on-board components.

If someone could fully document all the changes on the late 6 mill or later board then we would all know what to do to improve the receive. Kenwood must have made a factory modification which has not been publicly released.  Once the late 6 mill plus board has been documented, one would have to identify the changes that were due to already published service bulletins. The remaining changes are the secret to improving the receiver the way Kenwood did. If someone could access the unpublished Kenwood Service Bulletins for these changes then that would be the ultimate information for any 940 owner.

 Yours sincerely

Jeff King ZL4AI,


From: [] On Behalf Of Garey Barrell
Sent: Wednesday, 12 July 2006 3:31 p.m.
Subject: Re: [ts-940] Re: S meter slam mod: A Fix does exist but is not documented out in the public forum.


Hi Jeff -

Well, mine is not only a "mid 7 mil" unit, it's even been to KI4NR for a complete workover, and it still pegs the S-Meter at turn-off.! Always has.
Typical Kenwood, nothin's ever easy. (Or documented!) :-)

Good to hear from you!
73, Garey - K4OAH

Drake 2-B, 4-B & C-Line Service Data

From: [] On Behalf Of DavidGriffin
Sent: Thursday, 13 July 2006 12:08 a.m.
Subject: [ts-940] Re: S meter slam mod: A Fix does exist but is not documented out in


Mine does not.



<hr size=2 width="100%" align=center>

From: [] On Behalf Of Martin Sole
Sent: Thursday, 13 July 2006 1:22 a.m.
Subject: Re: [ts-940] Re: S meter slam mod: A Fix does exist but is not documented out in


Sooo... the question has to be, what's your serial number, the radio's I
mean. Starting to look like a cutoff in the 7 mil region.

Martin, HS0ZED

From: [] On Behalf Of Martin Sole
Sent: Thursday, 13 July 2006 2:52 a.m.
Subject: Re: [ts-940] Re: S meter slam mod: A Fix does exist...


A quick check now I have the 9 mil radio fixed, at least as far as getting the power supply going anyway, shows no tendency to pin the
meter needle on power down in the way my 7 mil radio does. All this one does is drop to zero immediately followed by a slight blip to about S6.

Now to investigate the difference.
Martin HS0ZED


Editor Comment: It looks like Martin is going to investigate and solve the mystery of the S Meter Slam / Receive performance improvement. Here is a question asked 2 days earlier:


Jeff King wrote:

Hi Martin,

Thanks for your communication.



Re The S meter slam. If you find the solution to that I would be eager  to know. As KI4NR advised Kenwood fixed that from 7 mil or 8 mil on, but Kenwood never published the fix. I emailed Kenwood Australia about this  but they refused to give the information. Whatever Kenwood did to the  8 mil board improved reception and finding that out would be so useful.

Yours sincerely

Jeff King ZL4AI




From: [] On Behalf Of Martin Sole

Sent: Friday, 14 July 2006 1:22 a.m.


Subject: Re: [ts-940] Re: S meter slam mod: A Fix does exist...


For those who may have an interest I believe I have identified the cause of and solution for the S meter end stop slam on power down.

Resources were a 7 mil radio with the 'problem' and a 9 mil radio without.


Observation shows a couple of differences in this area, an additional  100u capacitor under the board and a diode in place of a jumper.

Reference TS940 service manual revised edition 1987-3 pages 92 and 93 on  page 92 IC1 is located in area H,2. and on the board layout, page 93 at



On the 7 mil radio IC1 is fed with 15 volts through R206 and R178. On the 9 mil radio a diode is located between R206 and R178 in the position of J5 on the board layout. Additionally a 100u capacitor is fitted between the junction of this new diode (cathode) and R178 (+ve side) and ground (-ve side).


Overall this gives an increase in the time taken for the supply to IC1 to fall. Increased from approx 300mS to approx 800mS.

I used an ordinary 1N914 type diode and a 100uF 25 volt electrolytic.



Martin, HS0ZED



Jeff King wrote:


 Thank you so much for investigating and evaluating the effect of the new components which eliminate S meter power off slam. That is very useful

information which many 940 owners will value. I really appreciate your  efforts.


Two questions:


On the 9 mill IF board did you identify any other component changes that  will effect (make improvement to) the receiver?



Any particular reason you used a 1n914 as opposed to the more commonly  available 1n4148?

[With your permission All you findings will be placed on the TS940 page.]

Yours sincerely

Jeff ZL4AI?



-----Original Message-----

From: [] On Behalf Of Martin Sole

Sent: Saturday, 15 July 2006 3:53 p.m.


Subject: Re: [ts-940] Re: S meter slam mod: A Fix NOW exists... are there any 9 MILL IF changes that IMPROVE THE RECEIVER?


Hi Jeff,


So far I have not dug any deeper into the various board differences. I did note a couple of resistors and capacitors under the board of the 9

mil unit that do not appear to be on the 7 mil board.

A couple of things that I have noted is that the 9 mil radio sounds quieter, equally as sensitive but less background noise. It could be alignment, not sure yet. I do know that the noise blanker in the 7 mil radio is vastly better than the 9 mil radio but I put this down to my own alignment. On many of the 940's and 930's that I have seen over the years most have their noise blanker incorrectly set up such that they are usually only partly effective.

Actually I do not know exactly what the diode was that I used for the meter mod, only that it was a small silicon signal diode, could well have been a 1N4148.

Like Garey, K4OAH, I'm not all that sure that the slam is a big problem. Most older radios, Collins, Drake to name but two generally pin their S meters at switch on and also when muted by a connected transmitter, Have never heard of a lot of meter failures. Either way I think it is better engineering to avoid such where possible.


By all means add whatever you want from this to your most useful resource.


Martin HS0ZED





dean hunsinger wrote:

I have a 9 mil s/n radio and at shut down the meter does not hit the > other end just a slight rebound from off position.Is there a freq that  this group uses?




From: [] On Behalf Of Martin Sole

Sent: Sunday, 16 July 2006 2:20 p.m.


Subject: Re: [ts-940] shutdown

Hi Dean,

That sounds about right from what others have said and what I found.  Seems to be somewhere in the 7 or 8 mil range that the change got implemented.


Martin HS0ZED












The Phase Noise improvements are probably the most significant improvements possible to the 940.

These will upgrade a 1984 design to a 2008 design.


What follows in this section is:

1.      Observation of the Phase Noise problem by Thomas Hohlfeld DF5KF in 2005

2.      Extensive History of the Phase Noise Issues upto about June 1987

3.      Data Sheet for Problem BA718 OpAmp

4.      What is Phase Noise and how to measure it by John Grebenkemper, KI6WX

5.      Phase Noise measurements on Elecraft K2 by John Grebenkemper, KI6WX

6.      Significant Phase noise improvements developed by Thomas Hohlfeld DF5KF

7.      Information on Ultra Low Noise OpAmps available in 2008

8.      Further assessment and Phase noise improvement evaluations planned by Jeff King ZL4AI







6.1 Observation of the Phase Noise problem by Thomas Hohlfeld DF5KF in 2005


From: hohlfeld_thomas []

Sent: Sunday, 3 April 2005 10:08 p.m.


Subject: [ts-940] VCO phase noise


Hello to everybody


A couple of years ago I came across some test reports on the 940, which claimed that the phase noise of VCO3 is rather poor. I measured

it and, indeed, is was worse than reported for other rigs of the time. A comparison with an IC-751 (my former rig) indeed favoured the ICOM.

Thereafter, I added the PLL modification recommended by Kenwood, which improved phase noise by 5-10 dB. Still, the ICOM is better. Any

suggestions about further mods to improve the PLL circuit (PLL3)?

For example, I can imagine that replacing IC18 on PLL unit by a modern low noise op amp will improve phase noise.


When measuring the phase noise of my 940, I noticed weak unwanted signals about +/- 20 kHz apart from the main signal. Indeed, the rig

receives weak spurious signals about 20 kHz apart from an S9+40 carrier from a high quality RF generator (HP8694B). The spurious

signals (not the main signal) are slightly shifted when the digital display is dimmed. I checked another TS-940 with a similar result. My

conclusion is that the DC-DC unit (which oscillates at about 20 kHz) somehow interacts with the VCO. Attempts to improve uncoupling the

DC-DC unit from the power supply did not help. Did anybody else observe this and, perhaps, could find a solution?


Certainly, the phase noise and weak spurious signals are minor problems. The 940 remains an excellent rig I. But why not thinking about making a good thing better?


Any suggestions are appreciated.

73 de Thomas, DF5KF






6.2 Extensive History of the Phase Noise Issues upto about June 1987


John Delevore in Edition 1 of Low band DXing speaks specifically about his frustration with phase noise in both the 930 and 940.


Link below is 13 pages of very important observations and dissatisfaction with phase noise in the 940 , when first released.



Suggest reading these 13 pages carefully.


Key Points are:


  • Kenwood mod 911 (which ZL4AI has never seen) quoted as AR-81-031, 18 Aug 1986 reduces phase noise by 2 to 3 db


  • Kenwood mod 917, 2 March 1987, reduced phase noise further to 8 dBc / HZ


  • Lowe Modifications by Technical Director John Wilson reduced phase noise further to 13 dBc / HZ. Lowe Modifications are introduction of simple loop filters shown at the bottom of page 11. These can be added to the coax connectors as the VC1 and VC2 lines enter the RF Unit.


Points of interest is:


Lowe did the modifications to transceiver 7060088:  manufactured in June 1987.

Lowe was UK Kenwood (Trio) agent so would have had in June 1987 Kenwoods latest modification versions including 917.

Why did John Wilson do his tests against a transceiver fitted with only mod 911?



Kenwoods mod 917 is similar to Lowe’s Mod. It is adjusting the filtering of signals in and out of IC18 which is dual OpAmp.

It appears this OpAmp is very noisy.





6.3 Data Sheet for Problem BA718 OpAmp




Please note there is no data at all on this sheet about this OpAmp being low noise.

One could conclude this means the OpAmp is noisy.




6.4 What is Phase Noise and how to measure it by John Grebenkemper, KI6WX












































6.5  Phase Noise measurements on Elecraft K2 by John Grebenkemper, KI6WX


Well worth reading:


John shows that phase noise varies per band. So you really need to measure phase noise across all major bands

160, 80, 40, 20, 15, 10 to characterize a receiver performance.








6.6 Significant Phase noise improvements developed by Thomas Hohlfeld DF5KF


Thomas has probably done more than any amateur to improve the 940. His work is truly impressive.

This modification is probably the single largest improvement to the 940 receiver in its history



Phase noise of the TS-940


Thomas Hohlfeld, DF5KF



The relatively mediocre phase noise of Kenwoods TS-9 0 is largely produced  by the PLL loop of the  main  VCO.  Kenwood developed   a modification and published a service bulletin for improvement. Read here how much this modification helps and  what else can be done.




I own a TS-940 for several years and I am still happy with it. Its general performance is competitive with many contemporary high performance radios. Nevertheless, the TS-940 isn’t free of problems. One is the excess noise generated in the RX when there are strong out-of-passband  carriers close to the tune  frequency. This is caused by noise surrounding the receiver’s local oscillators and, hence, is produce  within the receiver. This ´phase noise´ is mixed with receive signals and appears as a noise sideband of the receive carrier.  It may not  be evident  when  performing  a  QSO  under standard  conditions, but can be  disturbing when weak signals are to be  detected  with strong signals nearby.  Local  oscillator  phase  noise  has  been  designated  as  a  main  limiting  factor  of  modern

receiver performance  [1,  2].  A good discussion of oscillator phase noise and   its consequences on system performance is given in [3]. I studied the problem more closely in my TS-9 0 and evaluated some possible cures.


1.  Measurement setup for phase noise determination

The measurement setup was simple: I connected my HP8640B RF generator to the TS-940 antenna input.  The  HP8640B  is  known  for  its  clean  signal,  with  phase  noise  below  that  of  most  commercial receivers (<140  dBc/Hz [4]).


The TS-940 was switched to CW mode, RF attenuator off, AGC off an RF gain set to maximum. Band width  was  set  to  maximum  (2,   kHz)  and  the  RX  tune   to  14.100  MHz.  The TS-940 audio output was fed   into a true RMS voltmeter.  A -125  dBm  signal  from  the  RF  generator  produced   a  defined   audio  output,  which  was  10  B  above  background   noise  (= audio  out  with  RF  generator replace  by 50 Ohm termination).


Then,  the  RF  generator  was  set  to  14.200  MHz  (+100  kHz  carrier  offset)  an   the  generator  output increased until the audio noise reached the same audio output as the -125  dBm in-passband  signal. The measurements were repeated   with stepwise reducing the carrier offset.  Lower carrier offsets required lower generator outputs to produce the same audio output.


Phase noise was calculated as follows:

Phase noise [dBc/Hz] = -125 dBm (reference) – dBm RF generator output – 33.8 (*)

(*) correction for 1 Hz bandwidth




2. Improvement of phase noise by Kenwood´s recommended modification

The results of all measurements are depicted  in figure 1. The grey line represents the phase noise I obtained before any modifications.  The  phase  noise  dropped   from  about  -100  dBm/Hz  at 4  kHz offset in a log/linear fashion  down to -142  dm/Hz with carrier offsets increasing to 50 kHz. There was no relevant change at higher offsets up to 100 kHz.




Next, I installed the Kenwood modification as described in Service Bulletin #917. As shown by the blue line in figure 1, this reduced phase noise by 5 to more than 10 dB, an encouraging result.

Kenwood´s  re-design  of  the  PLL  apparently  moves  the  loop  filter  behind  IC18  (BA718),  which allows for a better suppression of the noise produced  by this OpAmp. As a consequence, one would  expect  less  voltage  noise  FM  modulating  the  VCO  varactor  diodes.  I could   not find a detailed datasheet of the BA718, but I assume it is not a particularly low noise OpAmp.





Figure  1:  Phase  noise  versus  carrier   offset  from  the  tuned  frequency  (14.1  MHz).  The actually measured offset points are marked by black dots. These were later connected by the coloured lines.

All  measurements  have  been  performed  on  the  same  PLL  board  and  the  same rig,  using  the same  measurement  setup,  but  were  performed  on  different  days.  Offsets <3 Hz could not be measured with the available setup.


3. Replacement of IC18 by an ultra-low noise OpAmp

Noise  in  PLL  systems  can  be  generate  by  many  causes.  One is the  loop  amplifier  stage  itself.  A good  discussion  of  the  origin  of  PLL  noise  is  given  in  the  ARRL  Han book [2].  During the past years, ultra low noise OpAmps have become available, which might be more appropriate for a good

PLL design than the BA718. Therefore, I constructed a small replacement board which contains two LT1028 OpAmps, each in an 8-lead SMD (SOIC) package. The LT1028 is a little more expensive than conventional OpAmps, but still affordable.  Figure 2 shows the layout of the PCB, the complete board and part of the TS-9 0 PLL unit after replacement of IC18.







Figure 2:

A: PCB layout of the loop amplified board, matching the pins of the dual OpAmp BA718.

A  capacitor  (C1,  100  nF)  was  included  across  the  voltage  supply. 

B:  the finished unit before soldering into the PLL board.

C: PLL board after installation of the new board replacing IC18.



The  evaluation  of  the  LT1028  modification  started  with  the  original  PLL  board,  without  the Kenwood modification. The results are given in figure 1 (green line). At lower frequency offsets (5 to  20  kHz),  the  LT1028  modification  appears  to  produce  less  phase  noise  while  at  higher  offsets (>50  kHz)  the  Kenwood  modification  appeared  to  be  slightly  better.  Both modifications in place (figure 1, red line) produce a limited further improvement above the single mods.

After  these  modifications,  some  weak  spurs  became  detectable  during  the  measurements  which  I not  recognize  before,  because  they  were  covered   by  phase  noise.  As to  my  knowledge,  this  is quite  normal  for  conventional  PLL  designs.  (By the  way:  the  origin  of  one  spur,  which  appears when a strong carrier is about 20 kHz off-passband , is interesting: this one results from coupling of the  20  kHz  magnetic  field   from  T1  on  the  DC-DC-unit  over  a  long  distance  into  the  chokes  L65, L69 and  L72 on the RF board , causing a slight 20 kHz modulation of the VCOs.)




4. Comparison with other rigs

I  was  curious  how  the  modified   TS-940  compared  with  other  rigs.  Since I don’t own one of the contemporary top performance (and top expensive) rigs, I use  an FT-857, which I recently bought.

It is technically similar to the FT-897. As figure 1 (yellow line) shows, the FT-857 is superior to the original TS-940, but the TS-940 modifications reduce its phase noise clearly below that of the 857 (exception: 3 kHz).


The  phase  noise  levels  of  the  OMNI VI+  an   K2  transceivers  for  a  10 kHz  spacing  are  -123  and -126  dBc,  respectively  [1, ].  Although comparisons with performance data given in the literature are  limited   (different  measurement  techniques  etc.),  a  comparison  with  figure 1  suggests  that  the

TS-940, with the above modifications, is still very competitive.




In  summary,  the  phase  noise  of  the  TS-9 0  can  be  substantially  improve .  The modification described   in Service Bulletin #917 is highly effective.  It can be even improved   by replacing IC18  by an ultra low noise OpAmp, such as the LT1028.


73  e Thomas,





5. References


[1]   Raczek T: The DX Prowess of HF receivers. QEX Sept/Oct, 1-5 (2002)


[2]   The ARRL handbook for radio communications. ARRL headquarters, Newington,

84th ed., 10.1 ff. (2007)


[3]   website: _frequency/radio_impact_ultralow_phase/


[4]   web document: f





6.7 Information on Ultra Low Noise OpAmps available in 2008


The LT1028

Described as

LT1028 - Ultra Low Noise Precision High Speed Op Amps

looks very low noise when compared to other OpAmps:,C1,C1154,C1009,C1026,P1234


Compare these very low voltage noise figures to other conventionl OpAmps


Voltage Noise

-          1.1 nV/rt Hz Max. at 1kHz

-          0.85 nV/rt Hz Typ. at 1kHz

-          1.0 nV/ rtHz Typ. at 10Hz

-          35nVP-P Typ., 0.1Hz to 10Hz


Gain-Bandwidth Product

  • LT1028: 50MHz Min.
  • LT1128: 13MHz Min.



Another possible replacement is desribed as

LT1037 - Low Noise, High Speed Precision Operational Amplifiers


  • Guaranteed 4.5 nV/Root-Hz 10Hz noise
  • Guaranteed 3.8 nV/Root-Hz 1kHz noise
  • 0.1Hz to 10Hz noise, 60nVP-P, Typical
  • Guaranteed 7 Million Min Voltage Gain, RL = 2k
  • Guaranteed 3 Million Min Voltage Gain, RL = 600 Ohm,C1,C1154,C1009,C1026,P1213#applicationsSection




ZL4AI had earlier communications with DF5KF and suggested to DF5KF the LT1028, because of its ultra low noise figures, maybe a better choice than the LT1037 that was available in Germany. With the passage of time it appears DF5KF agrees.



The following design notes seem to provide really god information on OpAmp Selection.,C1,C1154,C1009,C1026,P1234,D4193,C1,C1154,C1009,C1026,P1234,D4187,C1,C1154,C1009,C1026,P1234,D4187


ZL4AI is still to review these notes to decide which is the best OpAmp and

the values of surrounding isolating resistors and capacitors that should be supplied to minimize noise from the OpAmp.

At this stage ZL4AI is still to measure the frequency the OpAmp operates at. This will be major factor in amp selection.

Probably the LT1028 is the must suitable OpAmp.




6.8 Further assessment and Phase noise improvement evaluations planned by Jeff King ZL4AI


Well DF5KF’s work has got ZL4AI excited at the improvements that can be achieved.


ZL4Ai intends to:

-          measure phase noise across all major bands (before changing the OpAmp) to draw comparison graphs

-          against the K2,

-          against the earlier work by John Wilson, and

-          against the data collected by DF5KF and,

-          against the ICOM 745 data by KI6WX


Install dual LT1028 or alternative low noise amps if they appears a better match to required operating circumstances


Investigate the simple loop filters used by John Wilson. It appears that by careful matching of the OpAmp input / output filtering that even lower phase noise performance maybe possible. DF5KF does not seemed to yet have investigated the this further filtering potential improvement.



6.9 First plots of phase noise comparisons below show some very exciting results:


  1. The TS-940 5 Mil, DF5KF mod at 14 MHz has better phase noise than the K2, beyond 5.5 kHz offset from the carrier,


  1. Icom 745 and 765 appear to have 4 to 5 dB less phase noise than the K2 or DF5KF 5 Mil 940 at 5.5 kHz offset: We do not know which bands 745 or 765 phase noise measurements were taken on. We know K2 Phase noise plots (above) vary significantly band to band.  So the difference shown, may not be correct as these 745 or 765 phase noise plots maybe for band other than 14.1 to 14.2 Mhz.

o   If Icom 765 data is on 14 MHz, then the DF5KF 5 Mil 940 has less phase noise than the 765 from 8 kHz offset from the carrier.

  1. ZL4AI 20 Mil 940, (Manufactured May 1992):

-          Measured with HP 8640B generator and HP 400FL AC Voltmeter using KI6WX method. [Note: VERY important to use an AC voltmeter that reads AC to at least 1 KHz, and preferably 2 Khz. Most cheap Multi meters do not read AC voltage and are not suitable because the CW tone on a 940 is approximately 700 Hz to 1 KHz]

-          has less phase noise than DF5KF 5 Mil 940 Kenwood Modified, so later modifications undertaken at factory decreased phase noise more than just the Service Bulletin Modification: This is why very late serial numbers perform better,

-          Between carrier offsets 3 KHz and 8 KHz 20 Mil 940 only has 3 dBc to 0 more phase noise than a K2,

-          Between carrier offsets 3 KHz and 12 KHz 20 Mil 940 only has 8 dBc to 0 more phase noise than a IC-765,

-          When ZL4AI adds the LT1028 to 20 Mil 940 it looks possible the 20 Mil 940 may have less close in phase noise than the K2 as follows: 

§  3  kHz off set 2 dBc less,

§  4 KHz off set and 5 KHz off set same as K2,







6.10  Real Experience of the Ultra Low Noise LT1028 significant receiver performance improvement


Jan 2102 ZL4AI got around to installing a pair of LT1028s. Mounted on a perf board with small wires from the ICs to the solder in wire. Does not look neat like the board by Thomas. Took abaout 2.5 hours to install and commission.

How does it perform. Outstanding. I would say this improves the SSB listening pleasure by 20%. 20% of what you may ask!  Well I do not have any technical justification but the 940 is really much much much better to listen to. [It is worth pointing out what I anticipated this mod would do is reduce splatter from nearby QSOs becoming a drone noise interfering with reception. This may have improved, but noticeably. Probably will have to undertake the phase noise tests again to detect the difference. What did change that was not expected was general listening to the receiver became better. I did not expect this to change. So it was an unexpected pleasant outcome. I have owned and used the 940 for 10 years so did not really expect that such a change could be achieved.] Wish I had done this 3 years ago. For the effort put in this single modification improves your 940 more than any other modification that could be undertaken. Try it out and tell me what you think!!!

Below are others experiences.


From: [] On Behalf Of sv9dru
Sent: Wednesday, 8 June 2011 10:12 a.m.
Subject: [ts-940] Phase noise PLL improvement

  Just wanted to convey my experience with the DF5KF PLL mod.

It does provide a very significant improvement in the Rx performance and it is a worth while task.
In my case, after having all the other Rx mods (including changing the 2 FET orientation in the RF and IF boards and the Kenwood PLL improvement mod), I ordered 3 pairs of LT1028 ICs from ebay (Taiwan) and installed 2 of them as suggested by Thomas in his excellent article.
That turned a nice and quiet TS-940 into a FANTASTIC 940 Rx.....!!! Significantly better and more pleasant than my TR7.

Good job my friend DF5KF !
Marinos, sv9dru / ki4gin


From: [] On Behalf Of sv9dru
Sent: Wednesday, 8 June 2011 8:31 p.m.
Subject: [ts-940] Re: Phase noise PLL improvement (construction details)

  For anyone interested I have some more details of the project done without having to use a specialized PC board or having access to surface mount soldering equipment:

I actually ended up using a general purpose perforated board for the mod. Got the lt1028 chips as seen at the following link:

As you can see they are already mounted on small DIP adapter boards. If you leave them on these boards you have at least 2 advantages over the option of desoldering them and resoldering on a small custom made board:

1) You do not need any specialized (hot air) equipment for the task and you do not risk damaging the chips in the process.
2) these small boards can be mounted exactly on a general purpose perforated board and you can work their interconnections from both sides of the composite board.

Detailed instructions: First I desoldered and removed the 4 pins from each side of the lt1028 little boards in order to be able to mount them flat on the respective insulated surface of the general purpose board. I then used short component leads to interconnect the IC boards to the underlying one.
The combination (3) boards will fit in the space left after IC18 is removed. I did have to sandpaper the sides of the Lt1028 boards to make them fit within the 9 hole required size of the general purpose board, and also carefully adjusted the height of the composite board to make it slightly shorter than the metal shield present around the area of IC18. You only need to install 8 small leads for mounting the new IC on the board, as the lead #9 is neither soldered nor used.

I ended up using a small conventional 100n cap mounted on the front (same as the chips themselves) surface from:

The cap and its interconnections were on the front while the other interconnections were done with small caliper insulated wire on the back surface of the structure.

I do regret not having taking any photos of my project before installing it in its place !! I was so anxious to see if the whole thing would work, and since everything worked A-ok from the first powering op of the radio, could not find the courage of disassembling the rig one more time just to take pictures of it..

I am sure that one can construct the little PC as described in the original article and if using the proper tools (hot air soldering paste etc) the whole project would be a breeze, but if one is willing to invest some more time doing it as described above, would be rewarded with a very nice looking and performing structure...IC18

No matter what construction technique you use, the result will ABSOLUTELY BE WORTH YOUR EFFORT!!

Marinos, sv9dru / ki4gin


Subject: Phase noise PLL improvement some ideas for even more improvement.
Date: Thu, 9 Jun 2011 22:45:41 +1200

Hi  Marinos,


I am really so fascinated with your posting. I suggested the LT1028 to Thomas who originally was proposing using an LT1028.

I have brought all the parts about 2 years ago but have not found time yet to do the mod yet so its great to hear how well it works.

You will see I analysed its potential on the TS-940 page.


When I read Thomas's application idea I have wondered if he matched the gain of the LT1028 with the original OPamp BA718.

I know on quick read the LT1028 had greater gain that the original op amp.


What I have been thinking about doing is measuring the frequency the op amp worked through, and comparing the gain of the LT 1028 with original Opamp BA718.

I do have the data sheet for the original BA718 opamp showing the gain if your interested. Anyway if the LT1028 was much greater gain and this was proven by taking measurements I was thinking of putting in an attenuator of resistors to reduce the LT1028 gain the same as the original op amp. I suspect this will reduce the phase noise even further, and will make the receiver even better.


As you will see on TS-940 page it is close to a K2 or an ICOM 765 low phase noise, and reducing that gain may make it equal the K2.


I am really interested in any developments you make. I am also sure Thomas will be really interested too!


Yours sincerely


Jeff King ZL4AI

in Dunedin New Zealand.



From: Marinos Markomanolakis M.D. []
Sent: Thursday, 9 June 2011 11:14 p.m.
Subject: RE: Phase noise PLL improvement some ideas for even more improvement.

Hi Jeff,
I am glad to see the results of the modification and grateful that technically inclined guys like you and Thomas are able to suggest these rather delicate mods to improve the already excellent TS-940.
I have made 3 K2s (sold them all eventually) so I would have to say that the TS-940 has nothing to worry about when compared to the K2. If anything, at least subjectively, it has a better SSB performance after the kenwood PLL mod alone, and suspect it is much better with the lt1028.
Since currently I do not have a K2 for side by side tests, can not comment any further though.

As for any additional improvement with attenuating the lt1028 to match the BA718, I would be very interested to know myself about it, but since I am lacking either the theoretical knowledge or the specialized equipment necessary for measuring the results, would have to wait for input from you guys ......

At any rate, if you have the small PC board for mounting the 2 lt1028 ICs, it should be a very straight forward job to do the mod yourself. I am sure you will not regret it!

Marinos, sv9dru / ki4gin

From: [] On Behalf Of hohlfeld_thomas
Sent: Friday, 10 June 2011 8:44 a.m.
Subject: [ts-940] Re: Phase noise PLL improvement

Hello Marinos,
I am really happy to see you were successful with this PLL mod. You did a great job by realizing the mod with the general purpose perf board.
This modification started with a very inspiring communication with Jeff, ZL4AI, who proposed different Opamp types and kindly provided the datasheets. We discussed several Opamps and I ended up with the LT1028, because I discovered some on a used board in my junkbox. First I examined and compared the audio noise of the LT1028 and the original BA718 on my lab bench because I did not have a good datasheet of the BA718. As expected, the LT1028 was far better. So I decided to try the replacement and, as you experienced, there was a remarkable improvement of phase noise. If I remember correctly, I also experimented with the addition of very weak negative feedback (many MegOhms from Opamp output to inv input) to compensate for the somewhat higher internal forward gain of the LT1028. I did not notice a difference and removed this feedback. My interpretation is that the capacitive/resistive feedback of the circuit is more important for the amplifier characteristics than the Opamp intrinsic gain.

Thank you very much again for your kind feedback,

Thomas, DF5KF



From: Hugh Coleman []
Sent: Tuesday, 19 July 2011 10:02 p.m.
To: jaking
Subject: pll boards

Hi yes I am using the same one that Thomas did he even sent me his template and permission to use. Just let me know how many you want and where to ship. You can use my paypal account at the boards are $10 and what ever the shipping is. I can use USPS priority shipping for $5 if you want a good quality shipping or regular post if not.


Here is the link to the mod.


The parts from digikey are about $20 just two op amps and a chip.


I have the parts here:









[For the record ZL4AI spent hours trying the Lowe [by John Wilson] Modification on top of Kenwood Service Bulleting 917 modifications.


I did not install the 2.2K resistor because the Kenwood 3.3K replacement was already sufficient. The result was very disappointing resulting in more distorted sound for me, and I ended up removing the Lowe Modifications, except for replacement on the PLL board C184 C185 C186 C187 with 4.7UF 35V tantalums, which did improve the stepping of the VFO tuning on 10 KHz steps. Maybe the Lowe Mod does not work as well as the Kenwood 917 mod. Others have reported the Lowe mod is better than the Kenwood 917 mod, so maybe I did not put iit in correctly. Next required step is to speak to John Wilson because I’m sure he knows what he is doing and did have really good success with this mod.]



7.0 SM-220 mods to remove Ghost Signals generated by the 940








Inrad are providing filters for experimenters who want to create their own modifications. For more information on this topic, please see our document entitled VHF Filters for Experimenters. The table below lists the first IF for many of the better radios.


Frequency MHz


Main Rx

Sub Rx















FT-1000MP, MkV




FT-1000MP, MkV
















IC-746, IC-756PRO I, II, III




IC-756, 765, 775












Orion, Orion II




TS-450, 690, 850, 870, 950SDX














Adding this narrow roofing filter is a really outstanding improvement to the receiver. Inrad’s statement about the filter is not correct and bizarre because the filter offers a huge improvement to the receiver by narrowing the first IF from 15 KHz down to 5 KHz. For crowded bands, the roofing filter provides the 940 receiver enhanced performance equivalent to the best current receivers. [The best way to install the filter is by cutting tracing traces on the IF board and mounting the filter on a board with relays so it can be switched in when needed  instead the existing 15 KHz roofing filter. A section describing this is still to be written up for this page]



Thomas Hohlfeld provides an outstanding explanation of the installataion of this filter and its benefits.








PLL BOARD 0: Reconnect Connectors

 [Kenwood] 940S Question -Solved

Fri, 7 Jun 2002

At the suggestion of one of the list members, I lugged the xcvr off its self  and removed the cases and re-seated all of the connectors on the boards which

I had moved. PLL and one underneath. Also, I re-seated the connectors on  the Digital A board above the PLL.

The "missing bands" are now working fine.

I think the connectors on the Digital A board are vulnerable to improper  seating, and being propeRLY PULLED LOOse, ESPECIALLY when that board is

lifted and tilted aside to get to the PLL board. There is one very long  connector on the front left of that board which seem to work itself loose at

one side and needs to be looked at.

Thanks to all

Randy K8BUX

ZL4AI: Notes: Re seat is the wrong description. Each and every time you pull those connectors off & on the spring clamp inside opens up a little. Eventually the connector fails to connect to the pin, the circuit is lost and you get a PLL failing to lock. You have to remove each connector from the shell using a sharp tool, and then using small pliers close the spring clamp back together. This is the only way to make the connectors connect again. It takes quite some time to get to the connectors in the inside edges of the PLL board. This re-clamping connectors has fixed the PLL failing to lock for me anumber of times.


PLL BOARD 1: Remove the Black Foam from Behind the Board

From: "kt4xw" <kt4xw@...>

Date: Sun Jul 25, 2004  9:41 am

Subject: Re: TS-940 Very low output. update  kt4xw



This morning I had a chance to look at the rig again, and found out some things that were interesting. The power adjust control on the

front panel, along with the carrier control in cw all seem to work. The output power goes up and down with adjustment. With the power out

adjustment VR2 all the way up, the SSB power jumped to 100w, but I still only had 3w or so CW. The IC meter showed 4 amps with no output

on SSB, so I check the current with a ohm meter and verified it was around 1.2a. A adjustment of the IC0 control fixed that. With 100w

out on SSB, the IC meter read over 16a. The ohm meter read 8.5a to 9a Adjument of the IC meter adjust pot fixed that. Then, on a fishing

expedition, I look at the micro processor board, and fixed several fish eyed solder joints, no help, but made me feel better. Then,

under the Het. Osc. on the PLL unit there was a piece of black conductive foam that had deteriorated. Also, it had a green/white

residue covering it. I cleaned all of it, and removed the rest of the foam. It helped alot. I had to readjust VR2 back down to 110w or so,

and the CW output jumped up to 15-18w. But still, that is it on CW.  

Thanks for all of your help!

Keith Spainhour, KT4XW

PLL BOARD 2: Remove the Wax from the VCOs

From: Garey Barrell <k4oah@...>

Date: Thu Oct 9, 2003  1:49 pm

Subject: Re: [ts-940] Welcome  k4oah

Fred -  

This is an indication that one or more of the PLL's are unlocked. It will only get worse!  

The two VCO's under the speaker (two layers down, of course!) are "potted" with a sort of beeswax. Over time this wax becomes contaminated and the VCO's become unreliable.

I fixed mine by using a heat gun (judiciously) to melt the wax out of these two compartments. Standing the transceiver on end, tipping it toward me and putting a piece of cardboard under the shield can to catch the wax as it drips out. Some will run out 'under' the shield, but it can be picked off with a Q-tip stick or other.  

The alignment was not changed in either circuit, but it wouldn't hurt to check the adjustments in those two areas after the "meltdown". They are simple peaking adjustments.  

Solder troubles are more common in the TS-440 and TS-930, but could be a problem in the 940 as well. My AVR board had a LOT of solder problems, but I have not reworked VCO areas. 

73, Garey - K4OAH







PLL BOARD 3: Replace all Electrolytic capacitors.

ZL4AI found that most of the electrolytic capacitors were much lower values than their specification. Especially test C71, C172, C143, C97, C93, which I found to be close to only 50% of rated capacitance. With a 28 year old radio these capacitors will have deteriorated.  It is best and easiest to just replace all the electrolytic capacitors. Takes about 1 hour to replace all and costs about $10






PLL BOARD 4: Tune through all Frequencies in USB mode 30 Hz to 30 MHz to verify PLL Board is working correctly.


In summary: a fault on the PLL board can show up and did show up in ZL4AI’s 940 as internally generated signals which showed up as S Meter readings when no antenna was connected. This existed for 9 years but ZL4AI never realised the PLL board had a fault. ZL4AI just presumed the receiver had the occasional birdie which produced an S meter reading. When the PLL board was exchanged with an error free board there were the occasional whilst birdies as you tuned through in side band mode, but no birdie ever produced a an S meter reading. If your 940 dos have internally generated S meter readings then something is wrong on your PLL board.


ZL4AI found that when a PLL board had a fault which showed up first as an background humming noise through the tuning range of VCO #3, the following symptoms were present:

-          Adjusting PLL voltages and VCO voltages enabled the PLLs to lock and the normal display returned,

-          Voltage on VCO #3 could not be set below 12.4 volts regardless of where L71 was set,

-           The humming noise was

§  Intense and loud on the entire 28.01 to 28.99 MHz range

§  vaguely present in background noise on VCO #2 from 19.499.99 MHz down to 9.50 MHz,

-          At many odd frequencies for a spot when tuning 30 Hz to 30.0 MHz a birdie would should which resulted in an S meter reading of S3 to S7.  This birdie would only occur over say 1.5 KHz but was all generated internally from the defective PLL board.

-          When the board was replaced with a fully functional PLL board, throughout the entire tuning range there were birdies, but none of these caused the S Meter to lift off S0.




PLL BOARD 5: Identify which PLL is not locked

From: "k8aicurt" <k8ai@...>

Date: Tue Nov 30, 2004  1:57 pm

Subject: Re: PLL unlock  k8aicurt

Well, I finally got it working. There is a line on the PLL board that goes to the control unit that's labeled "UL". This line goes low if one of the individual PLL IC's is in unlock from both the PLL board and the carrier board. If you have dots on the display, first disconnect connector #2 on the PLL board and check the voltage at connector #5 pin 5 4. If the voltage is "high" (~4.6V) then the unlock is on the carrier board. If the voltage is still low (~0V) then replace the #2 connector and then check the voltage on the individual IC's.

Check the voltage on IC8 pin 2, IC9 pin 2, IC19 pin 2 and IC17 pin 7. The one(s) that has(have) a low voltage on them are the PLL's

out-of-lock. Troubleshoot that PLL circuit.  

Curt, K8AI



From Jim/G3KAF:




ZL4AI: Notes: It is very easy to carefully follow the procedure below and identify exactly which PLL is unlocked and which component is not operating.

Then you can easily solder joints around the components which fixes most unlocks. Components do not fail that often, so start with soldering all the joints. Once you identify which PLL use the Figure 3 below listing of components in that PLL,  print out a copy of the circuit board from the Workshop manual, and use a highlighter to highlight all the components in the defective PLL. Then solder all the leads on these components.



The TS940S along with other Kenwood transceivers are prone to a common fault - that of displaying a row of dots instead of a

frequency readout. This is caused by one or more of the Phase Lock Loops (PLL) going out of lock.


A number of suggestions have been put forward to cure the fault. The most common two being to remove all the wax from parts of the PLL

board by heat or other methods and the other is to resolder all the joints on the underside of the board. Either method may cure the

problem but (unless great care is taken) they could induce additional faults thereby adding to the problem and obscuring the original

fault. An alternative method would be to narrow down the fault to a particular part of the board or boards as suggested below.


In the case of the TS940 there are a total of 6 phase lock loops, any one of which could be out of lock and cause the 'DOT' problem. Only 4

are on the PLL board - the other 2 are on the CAR board. At this point I suggest you click here and download the relevant pages of the

TS940 service manual (4.7MB) which is in PDF format.


ZL4AI: On Figure 3 page 6 identify the 6 PLL circuits; This diagram shows all the components in the PLL.

































































































Note for PLL3 is on both the PLL board and the RF Board.




Before making any tests on either board it is best to check that the supply voltages are correct. On the PLL board connector 5 pin 5

should be +5V and pin 3, +15V.


Each individual phase lock loop produces a permanent voltage of around +4.75V if it is in lock and just above 0V if it is out of

lock. The signals from all 6 are gated together using diodes such that any one PLL producing a 0 voltage will 'override' the other

signals and produce 0 volts at the final output designated UL.


First we need to decide whether the fault is in the PLL board or the CAR board. With the rig powered up, connect a voltmeter (I use an

oscilloscope) to pin 4 on connector 5 of the PLL board (be careful, pin 5 has +5V on it and pin 3 has 15V as previously mentioned). This

is the final lock/unlock signal from all 6 phase lock loops. As we are out of lock, this voltage should read around 0 volts or a little

above. Now unplug connector 2. If the voltage on pin 4 of connector 5 is still around 0 volts then the fault is on the PLL board. If the

voltage is now around 5 volts then the fault is on the CAR board.


For arguments sake let's assume the fault is on the PLL board (the most likely scenario) then we may now proceed to narrow down the

fault to 1 of the 4 phase lock loops.


We can leave connector 2 unplugged at this stage. Check the output of each phase lock loop as follows. With the rig powered up, check the

voltage on IC8 pin 2, IC9 pin 2, IC19 pin 2 and IC17 pin 7 (yes pin 7). Whichever output is giving a low voltage of around 0V is the loop

which is out of lock.


circuit diagram page 96 shows:


Output of each PD Location shown as connected to in PLL diagrams

1.8V on : IC8 pin 2 as 180 mV p to p 

[ZL4AI measures this as 4.6V when locked.  probably the 4.6V label on pin 1to ground  should be on pin 2 and probably theses labels should be written on pin 3]


4.9V on : IC9 pin 2

PD LPF Q13/Q14

4.7V on : IC19 pin 2

PD LPF Q28/Q29

5.1V on : IC17 pin 7 shown as connected to IC8 pin 2, IC9 pin2, IC19 pin 2 & CAR-1-UL1




4.9V on : IC1 pin 2


4.9V on : IC2 pin 2




Say for example we find that all levels are showing +4.5V except for the output of IC17 pin 7 which is showing around 0V. By looking at

the circuit diagram it becomes obvious that we need to fault find around the components associated with IC15, IC16 IC17, Q22, and Q23.

We have the option here of taking the board out and resoldering all the joints and components associated with those semiconductors or we

can narrow it down further by checking the voltage levels and comparing them to those shown on the circuit diagram. The choice is



If you have an oscilloscope available you can in addition check the RF signal on the base and emitter of Q22 and it's subsequent input to

pin 9 in IC15. Check the output signal on pin 8 of IC15 and it's subsequent input to pin 1 on IC16.Check the output on pin 7 of IC16

and it's subsequent arrival at pin 8 on IC17. Check also the RF input signal on the base of Q23 and it's output which goes to pin 1 on



With the voltage readings and/or oscilloscope measurements it should be possible to narrow the fault down to a single component. Before

changing it though, I would suggest that you resolder the component as most faults appear to be due to bad soldering on the board rather

than to a component failure.


Good Luck............Jim G3KAF







PLL Board  6:   PLL Board and RF Board and PLL out of lock


kc0bi <> wrote:
Hello everyone.  I am troubleshooting a TS-940S and it has at least two problems - I tackled the easy one first

- it had a bad optical encoder and I replaced it with a known good pull from a TS-430S.

This unit definitely has PLL problems - the exact frequency that it fails changes with heat but the general problem is this:  Above  10 MHz it works correctly in USB and LSB - below 10  MHz it works  only in USB.  There is an area between approximately 9 MHz and 10 MHz where it is probably unlocked but the frequency display still works - and that point changes with heat. Below 9 MHz it gives the "all-dots" display indicating PLL unlock (but seems to still work in USB). I am using the built-in 100 kHz calibrator as a signal.  After a period of time the nice sounding sine wave becomes a very "ratty" sounding tone.  This is true regardless of which sideband and at any frequency I've tried.  Does anyone have an idea of where to start looking? One of the PLL's must be losing lock. Thanks in advance and 73,

 Harold W0HJW (formerly KC0BI)


From: [] On Behalf Of Bill K0ZL
Sent: Wednesday, 19 October 2005 11:47 p.m.
Subject: Re: [TS930S] TS-940S Phase-Lock-Loop Problems?


Drop your RF unit down (rig up on it's left side) and resolder all around the VCO area, which is about the front 2-3" of the board. Also resolder around the RF RX preamp and first mixer area (that is on the same unit, the narrower shielded area, about mid-way back.


Be patient, you have about a two hour job there. Use bright light and drugstore reading glasses to make sure you get them all and watch for bridges as you solder. Much easier to find and clean them "as you go" rather than discover them later.

Next, get the PLL unit out, scrape the adhesive pad from under the VCO nearest the front right cornner of the board (as the rig is facing you), and Resolder that area about 1 sq inch.


Next, do the PLL/VCO service note [Editors Comment this is Service Bulletin 900], which should be in the files section of the group. Requires an RF probe for your DVM.


73, Bill K0ZL





PLL Board  7:   PLL Board and setting voltages:  Comprehensively updated in 2012


ZL4AI: in year 2013. Much easier to use an oscilloscope.. Much easier that trying to make an RF probe. With an oscilloscope you can get a picture of how clan or distorted the signal your era measuring is.


From: Alan & Susie Carlton []

Sent: Saturday, 22 April 2006 4:09 a.m.


Subject: TS-940 PLL Problem


Jeff, I found your web page interesting and have tried some of the mods.  I am having problem with the PLL unlocking and got in contact with Cliff Holland, Kenwood’s Service guy in Irving Texas, and he had me melt the bee’s wax from the cans and re-solder the whole PLL Board, and re-align it.


The problem that frustrates me is he claims KENWOOD SERVICE MANUAL IS APPARENTLY NOT TRANSLATED INTO ENGLISH CORRECTLY for a proper alignment procedure, e.g., Page 67 call out the test equipment needed, and page 68 & 69) of the manual clearly calls for RF Voltage measurements with an RF Probe, due to the PLL voltage being @100+MHz, but Cliff says to use a DMM in DC measurement.  What does not make sense is (following the schematic) we are adjusting slugs in coils or transformers and these are tuned circuits, which should be an AC/RF voltage………  However, with my fluke RF Probe I can not get more than a volt on TP calling for 8 volts. 


Apparently, when the PLL is aligned properly (He says) it will stay locked when the Bee’s Wax is poured back in the metal cans.


I have already tried the mod about PLL Unlocking adjusting L-22, L-23, & L24,  It all went fine as they said to peak each of these.  L-24 even gave the greatest change just as they said, but it did not help the unlock situation.  One thing that bothers me is the mod said the voltage will be about 250mV; well, I started with 300mV on a Fluke RF probe, and they peaked to 700mV……  Nothing seems to do what they say it will do.  


Any advise would be most appreciated,


Alan, N5GKY  





From: Jeff King []

Sent: Saturday, 22 April 2006 8:04 p.m.

To: 'Alan & Susie Carlton'

Subject: RE: TS-940 PLL Problem



Thanks for your interesting email. What you state sounds very similar to my experiences.


I am just an amateur who wanted to fix his own radio. That’s why I started collecting the information on the page. I am sure Cliff knows a lot. His advice often appears remarkable, because of his long experience. When I communicated with Cliff a couple of times by email I found Cliff’s answers were quite short. …. But I did really appreciate that he replied…… I suspect Cliff is aware of some service information others have not seen.


I am sure Cliff is correct about the poor translation of the Japanese. I find the workshop manual very difficult to understand in places. See my comments about important information Kenwood missed out regarding S meter settings.  Cliff holds knowledge there about Japanese DB scales that others were not aware of.



- On my radio, I have not been able to set PLL frequencies, so I just left them until I gathered some more knowledge of what to do:

So I really appreciate your email, because what you find may also help me.




Referring to Items numbers listed in the service manual


Item 2 PLL1: bottom of page 67, 

Could not measure any RF voltage reading, and at TP1 frequency jumped all over the place. I was confused by this and just left it.

Test data said it should be 8 volts and 3.5 – 4.5 volts, but then

the circuit diagrams shows 80 mV p-p

I concluded there was something wrong with the information in the manual.



-Item 3: PLL2 TP3: mine measured 47.699 MHz to 47.700 MHz, with RF voltage = 0.128 and DC 10.3 volts.

So I left that too, because the readings I got did not make sense, compared with the manuals values.


As well for both the above, how do you set the frequency at the frequencies the manual calls for????????????????????/

This seemed to be critical instruction that was missed out of the manual????????????????????????/



Item 4: PLL-IF TP5: With the radio set at 14.200 MHz last time I first measured 24.5 MHz and RF V at 0.411v. Way too high.

Then I followed Service Bulletin 900, which supersedes the manual. It told the critical information to adjust the frequency to just below 1.8 MHZ. I then adjusted L22, L 23, and L24 to give me a peak of 264mV. SB 900 said it should be 250 mV.

I looked through my notes and when previously setting this I ended up peaking it at 698 mV.  I suspect but I am not sure, that I did not read the instruction to set the dial frequency to 1.700 MHz.


My RF probe is home made following ARRL Handbook.



I agree with you, in theory voltages should be measured with an RF probe.

Maybe Cliff is right about using a DVM and reading DC?????? Cliff may well have seen service advice on how to correctly set these voltages.

It would be very useful if you could get Cliff to tell us how one sets up these PLL voltages.


Maybe there is frequency setting like 1.799 MHz (SB-900) we are supposed to put the radio at before setting these voltages.


I would be most grateful if you would advise whatever you discover.


Yours sincerely

Jeff King ZL4AI






Yahoo! Groups Links


jeff_king169 wrote:

> I cannot understand the instructions in the service manual. Can anyone help, who has successfully setup the PLL-1?

> Page 68 of the manual Control adjustment 2 states:


> Item PLL-1 100-110 MHz

> Measurement Test equipment: RF V.M, FREQ.C


> Measurement Unit: PLL

> Measurement Terminal TP1


> Adjustment Unit: PLL

> Adjustment Part L10


> Adjustment Method: ADJ to 8V at 110MHz


> Specification / Remarks: 100 Mhz: 3.5-4.5 V


> All thats sounds ok, but here is what I measured Frequency jumped all over the place 0.067 Mhz to 0.432 Mhz.

> Couldnt get any RF voltage reading, with my home made Rf probe and  Digital volt meter.

> The circuit diagram shows 80 mV P-P at this point!


> Can anyone tell me what L10 adjusts to? Are you adjusting the counter  to read 110MHz or the voltage to read 8 volts?

> Does this specification setting mean ideally it should show exactly 8  volts when you adjust the frequency to 110 MHz?

> What does the specification mean?

> The voltage could be as low as 100 mHz or the voltage as low as 3.5 to  4.5 V?

> Or do you change something else on the radio to check the voltage at 100 MHz as 3.5 to 4.5 volts?

> If so what do you change?


> Does any one know how to make this adjustment correctly


> It seems to me there is something missing from the instruction here.


> For example when you set the PLL-IF at 100 to 170 mV you have to carry out an extra step, which is to set the band to 1.797 MHz. If you dont

> set the band to 1.797 mHz the voltage reads 411 mV. I found the  instruction to set the band to 1.797 Mhz in Service Bulletin 900. That

> information IS NOT IN THE SERVICE MANUAL. Without knowing this band  setting instruction you cannot set achieve the lower voltage.


> Is there similar CRITICAL instruction information missing from the  settings for PLL-1?


> Any advice from anyone who knows how to make this setting would be

> really appreciated


> Yours sincerely Jeff King ZL4AI




From: [] On Behalf Of Martin Sole
Sent: Thursday, 14 September 2006 1:23 p.m.
Subject: Re: [ts-940] How do you Set PLL-1 ? Advice from someone who has successfully set please


Hi Jeff,

Just a few thoughts. PLL1 is responsible for the 100Hz steps. It does  this by tuning in 10kHz steps which get divided by 100 at the output,
IC10. So I guess then that every 10kHz of real tuning should cause PLL1 to go from 100MHz to 110MHz if we ignore the 109.99 rollover.

The front panel encoder steps in 10Hz or 1/10 of the IC10 output (100Hz) so for every 10Hz step the vco of PLL1 should move by 10kHz. This should
start at some frequency xx.xx0.00 and finish at some frequency xx.xx9.99.

If the loop is in lock then the frequency should be stable at whatever the dividers state it should be and adjustment of the inductor, L10,
should cause the tune voltage to vary such that it adjusts the capacitance in the tuned circuit in order to maintain the same frequency.

What they want you to do is have a tune voltage of 8 volts when the vco is at 110MHz and a tune voltage between 3.5 to 4.5 volts at 100MHz. If
the tune volts at 100MHz is more than say 4.5 volts then by the time the vco tries to get to 110MHz there is not enough voltage available from
the tune line supply. Same if you have the volts set too low at 110MHz then when you tune the radio such as to cause the vco to want to go to
100MHz the tune line cannot go low enough to pull it there. There also needs to be a margin such that any drift, which causes the tune line
volts to vary so as to keep everything on frequency, can be accommodated from the available supply range.

As I look at Fig 3 I see that PLL1 moves in 10kHz steps, gets divided by 100 so gives 100Hz steps at its output. This is mixed and fed as another
frequency, but variable in the same 100Hz steps, to mix with the output of PLL2. This causes PLL2 to move in similar 100Hz steps which then get
divided by 10 to give us our 10Hz steps.

Of course you do need to have the correct divider data for IC9 coming from digital A else all bets are off !

Martin, HS0ZED

p.s. It's a bit early and the coffee has hardly had time yet so might be
a bit out here and there.



Editors note: The above procedure for setting frequencies to get the correct voltages  is explained much more fully in the ts-930 service manual. Obtain a copy of the TS-930 service manual and read it.

As well,  discovered home made RF probe at 200 to 300 mV probably only reads about 85% to 90% of actual voltage,

So the actual figures will be as much as 15% larger than those recorded above.










29 Jan 2013. More Comprehensive PLL set up instructions are now available at:







-----Original Message-----
From: thomas hohlfeld []
Sent: Monday, 2 May 2005 9:23 a.m.
Subject: RE: ts-940


(4) One of the voltage regulators on the control board warms up the bottom of the 940, which causes a small but detectable shift of the BFO on the IF board. I removed this regulator from the control board and mounted it on the big aluminium heat sink at the rear side of the TRX.


Best 73s for today, Jeff

Thomas, DF5KF



>From: "Jeff King" <>

>Reply-To: <>

>To: "'thomas hohlfeld'" <>

>Subject: RE: ts-940

>Date: Fri, 27 May 2005 19:35:16 +1200


Your mod (4) is also very helpful for me. I have heard about a TS-940 that is supposed to have this problem of frequency drift after operating for 30 minutes. It apparently shifts frequency slightly then jumps back. Just sometimes: comes and goes.  Sounds like you have solved the problem. Wow thanks. Can you please tell me which regulator was it that you shifted to the heat sink?


>Your sincerely

>Jeff King




-----Original Message-----

From: thomas hohlfeld []

Sent: Tuesday, 31 May 2005 9:15 a.m.


Subject: BA479 etc.


You also asked for the IC that warms up (my mod 4). I believe it was either  IC7 or IC6 on the control board, but I am not entirely sure. Next time I open my 940 I will see and let you know. Warming up of the IC caused a slow shift of the 100 kHz BFO (L19). Indeed, L19 was quite sensitive to changes in temperature in my 940. Perhaps you should verify with a counter that the jumps in frequency you mention are really caused by this BFO.

Best regards for today and vy 73,






-----Original Message-----
From: thomas hohlfeld []
Sent: Friday, 10 June 2005 8:56 a.m.
Subject: RE: BA479 etc.


As my TS940 is open now, I had a look which of the voltage regulator IC I  had moved back to the heat sink. It is IC7 on the control board, next to relay RL2. I include an additional jpg file which shows (upper figure) the place on the control board where I replaced IC7 by a connector with the three cables leaving (arrow). The lower figure shows the voltage regulator at its new place (arrow). The heat sink needed to be unscrewed in order to drill a hole for mounting IC7. I mounted IC7 isolated from ground. If you decide to do this mod, be very (!) careful not to make any mistakes. The primary 22V line may cause severe damage when connected to the 15V regulator output.





Good luck for today!













 FAN AND TEMPERATURES Forum : Elmers : Kenwood TS 940s  Forum


Kenwood TS 940s  Reply 

Anonymous post on February 19, 2001 

Can someone help? My fan in back of my TS940s has developed annoying rattling sound, nothing appears to be loose. Is there any quick fix to this problem? Thank you 


by AL7BB on February 19, 2001 

This fan has sealed "permanently lubricated" bearings, if it is like the one on my TS-940.

With this type of motor, I have had some results in lubricating motors like this by taking a straight pin and pricking a small hole in the cover over the bearings, and injecting a light oil through the hole.

This will possibly extend the life for a short period, but in my experience, it is time for a new set of bearings, or in this case, a new fan motor.

Bill, AL7BB 



by WG7X on February 21, 2001 

My TS-940S and two of those owned by fellow hams have developed the noisy fan. Like another poster, I oiled my motor, and that helped for a little while. Eventually, the motor failed and had to be replaced.


You should also be aware that there are two of these fans. Number one, and usually the one that fails, is the one on the back of the transformer. Number two is the fan on the finals. This fan can be seen inside the rig. Look down through the slots on the top cabinet in the right hand side in front of the finals. This fan probably would be a bear to change. Thankfully, the other fan seems to fail more often. These fans go for about $32(US). I bought two just in case.


Sometimes I also use an auxiliary fan over the transformer. Of course, this introduces a little extra fan noise into the shack. I might eventually replace the back fan with a full time muffin fan on the back transformer. I tried that while waiting for the replacement fans to arrive. A six-inch fan does a great job of cooling the power supply. In fact, it does a BETTER job of cooling the rig. I know this because I used a Fluke DVM with a temp. probe to measure the temp of the cooling fins. With the stock fan, the thermistor kicks in at about 45 deg C. With the muffin fan I was able to keep the temp at about 20 deg C.

Gary WG7X



by AL7B on March 22, 2001 

I replaced my fan about 8 years ago with one used for cooling a 486 computer chip. I was too cheap to pay Kenwood $35 for a new motor. After tweaking the brushes a few times I gave up.

I did have to pack foam around the opening to force the air through the fan, but it has worked fine over the years.

Something to think about anyway and hope this helps.


Anchorage, AK 








From: Victor Zelenin <>


Date: Tue, 27 Sep 2005 17:09:12 +0400

Subject: TS940 Mods


Dear Jeff,

Thank You for the nice page about Kenwood TS940S.

I have a small question to Thomas DK5KF and to Jeff as experts.

I am waiting replay from both of you.


DK5KF wrote the mod N4  Voltage regulator heats up and causes a shift  in  BFO on IF Board".


In my TS940 there is the shift in BFO due the warm up. The shift is about 150 Hz per first 30 min of operation.

Thomas advice to be careful with the IC7 voltage regulator on the Control Unit.



From my assumption it is god to reduce the voltage drop on IC-s on the Control Unit via AVR Unit thru reduction the signal 21T from 23.2V (see service manual page 103) to 21 V.

May be the name of the signal has a sense? To drop the voltage we shall use D14 with 22 V or a bit less.


U (i/o) IC7 = 23.2-15=8.2 V

21-15= 6V

6:8.2 *100 =73%

So we would have a 27% reduction the dissipated power from all the 3 voltage regulators in Control Unit.




Additional advantage of my suggestion is:  the Q6 will not blow  when AC 220 Volt is low. It is happened in winter time in Russian country side due electrical heating of houses.


When AC =190V, the voltage drop on Q103 too low, it current amplification is low (beta is function from emitter-collector voltage), due that Q6 is trying to give more drive to Q103 till Q6 had burn with D14. I have changed two times Q6 with D14 by the conditions.


Best regards Jeff and Thomas

Victor UA2FP

Kalinigrad, 27 September 2005




-----Original Message-----
From: thomas hohlfeld []
Sent: Sunday, 16 October 2005 9:44 a.m.
Subject: RE: Fw: TS940 Mods


Hi Victor, hi Jeff

sorry for the late reply. I had to visit a couple of conferences during the last days.


Victor's suggestion to reduce the 21T Voltage from 23 to 21 Volts, bringing down the heat production of IC7, makes perfect sense to me. Let us know whether this will help to reduce the BFO shift of your rig during warm-up.

It is interesting that the service manual says that D14 is either an MTZ 22 or an MTZ 24 type (legend below the AVR schematic, page 103). It seems therefore that Kenwood has also tried different diodes here. One of my 940 rigs is presently open, so I did a few measurements. The voltage across D14 is  22.7 V.  At Pin 21T of the AVR unit I have 21.2 V. It appears thus that my TS-940 has the MTZ 22 diode.

It may be interesting to you that, despite the lower 21T voltage,  I had to replace Q6 a couple of years ago.  I chose a standard NPN type in a TO-220 case. This one gets only a bit more than handwarm and there were no problems any more.



In addition to Q6, other parts on the AVR board also become quite hot, for example C12 after a couple of hours. I found that diodes D10-D13 are the reason. They dissipate a lot of heat to the board (which turned dark below the diodes) and to other parts, such as C12. I removed these diodes from the AVR board, connected an appropriate bridge rectifier with sufficiently long wiring and mounted this rectifier on the black heat sink. D1 and D2 ran also hot and were moved to the heatsink as well. This has helped, C12 remains cool now.


Best regards and 73s for today,











quickfaststang <> wrote:

I just bought a ts-940 off of ebay. Had it on for about 3 hours just  receiving and i noticed the unit was getting pretty hot,

i also noticed the fan or fan's are not working, Now i believe they are tempature controlled,

So my question's are: Do these radio's normally get real hot, before

the fan kicks on ? Also, I am sure this is a common problem, So What

usually is the cause of this, Is it the motor probabally burned out

or the thermistor etc. ?

Also, I believe there are 2 fans one in the tranformer area and one

in the final section, I dont believe any of them are working, but my

main concern would be the tranformer fan first ! 



120205. Power Supply 04.htm






ZL4AI 940 had a very hot TS-940 power supply.

On removing the power supply heat sink the thermistor 101 actually fell out of its housing metal housing. It had become unglued. It had been sitting in the correct position but probably was not making adequate contact thermal heat transfer. Hence not turning on the cooling fan.

To verify the thermistor changed resistance I connected a ohm meter and heated the thermistor with soldering iron. It went from approx 16,700 ohms at 12 degrees C to 220 ohms at say 400 degrees C. I have no idea if these are correct resistance values but they give an indication of what happened.

If anyone has the temperature table for  Thermistor 101, please send to


On page 8 of the promotional brochure above, its shows that the heat sink should run at or below 40 degrees Celsius.







After these fail, they release 36.1 volts down the 28 volt supply lines, and 23.9 volts down the 21 volt supply line.


Q101 and Q102 can fail at any time.

36.2 volts can destroy the final amplifier transistors: MRF422 [Vceo 40V and MRF485 [has Vceo 35 volts]





IF 28A READS 36.2 THEN 36.2 IS BEING APPLIED TO YOUR FINAL AMP TRANSISTORS. [ZL4AI measured 36.2 volts when one of his Q101/102 failed]






ZL4AI found after replacing the original power supply transistors 2n5885 (60 volt version) 

with the 2n5886 (80 volt version), the heat sink temperature seemed to  decrease.



Original 2N5885 is by Motorola [60V, 25A]


Replacement could be any brand of 2n5886, [80V, 25A]

Motorola and ON semiconductor are recommended brands


A possible ever stronger replacement is the NPN 100V 30A, NTE181

ZL4AI  has not used the NTE181 yet but intends to review the NTE181. NTE181

Veb is down to 4V compared with 5V on the 2N588X:

Hfe min is 25 compared with 20 on the 2N588X

If these changes are critical needs to be verified before the NT181 can be used.

The up rated stronger transistor maybe a safety advantage.


From: G3JVC

Power supply regulator failure in a TS930s, or TS940s (usually one will go short circuit) will cause the 28 volt supply to the PA board to rise to 40 volts, if over voltage protection has not been fitted to the power supply output, serious damage to the PA board transistors will be certain, before the PSU internal fuse blows!

A better, cheaper, direct TO3 replacement for the 2N5885 is the MJ15003, (RS Components 296-267) this is a 140 volt working transistor, rated at 250 watts, and more than able to cope with the power supply demands of the TS930s and TS940s circuitry.
73, John. G3JVC.










If 28A reads > 29 volts:


From: Traian Belinas []
Sent: Wednesday, 7 February 2007 5:14 a.m.
Subject: TS940 problem!!!
Importance: High


Yes, the Q101/102 may have been gone, indeed...
It is a known problem!
Also, they may be ok and other to be the problem: check the Q101, Q102, Q1, Q2, R1, D3, Q3, Q104 and the soldering under the board.
For my previous TS940, the Q101/102 were ok, the problem was their Q1 driver transistor, and the finals and drivers were ok!

For further investigation:
- disconnect the 28V line from the AVR to the Final Unit (the FB connection, I think) for avoiding further damage to the Final Unit;
- disconnect the connector 3 and so avoiding powering the rest of the radio from the bad 28V line (don't forget to check the Q103 first).
- make checking on AVR without power (the above transistors, diodes, R)
- replace the defective parts;
- replace the F1 fuse with a low value one (1 amp is ok) or with a low value fusible R (10-20 ohm 1/2W is ok); place a light load to the 28V line (500 ohm is ok)
- apply power and check the 28A line voltage
- from now on you may work on AVR unit with power on for checking, but take care anyway;

Editor note: At this point the AVR is in test mode loaded against a 500 ohm dummy load.


Take a deep breath, relax for a while, and then go on for it!
Don't forget to reconnect the connectors and replace the F1. back to original.








 -----Original Message-----

From: [] On Behalf Of Garey Barrell

Sent: Friday, 28 October 2005 4:48 a.m.

To: Bill Stewart


Subject: Re: [Kenwood] TS-940S Cooling Fan Problem


Bill Stewart wrote:


>I have a TS-940S, s/n 6040XXX, which has a sluggish cooling fan (on left, facing front, behind the power trans.). Sometimes it will not start. I have shot WD-40 in it and tried to get oil to go down the shaft but sometimes it still won't start and if it does start, it turns slowly & erratically. I measured the voltage to be around 18VDC with the fan disconnected and connected, a fluctuating .5 to 1.5VDC with the erratic rotation.

>Questions: 1. Is this no-load to load voltage drop normal?

>                 2. Does the voltage and fan speed increase with temp. rise?

>                 3. If the above is a problem, are there any mods. to replace this fan or maybe make the fan run all the time?

>The fan runs ok if it gets beyond the low start-up point (on a seperate DC supply).

>Any comments will be appreciated, thanks...Bill K4JYS



Bill -


The problem is that the motor is drawing too much current at start-up.  The supply voltage is applied through a series resistor and the excess current drawn because of the gummed up bearings is high enough to drop the supply voltage too low to start.  The correct fix is to replace the motor.  East Coast Parts has them but they are expensive, ~$30.   You  might be able to find a hobby motor that will fit, if you look long enough.


A "temporary" fix is to put an 18V Zener diode across the motor dropping resistor so that if the voltage drop across the resistor is more than 18V, the Zener conducts, supplying enough voltage to start the motor.  Once the motor starts, the voltage across the resistor drops below 18V and the Zener is out of the circuit.  BUT this is only a temporary fix!  If the motor freezes up, the 940 WILL overheat significantly.


73, Garey K4OAH



-----Original Message-----

From: [] On Behalf Of Garey Barrell

Sent: Monday, 31 October 2005 4:10 a.m.

To: Bill Stewart


Subject: Re: [Kenwood] TS-940S Cooling Fan Problem


Walt -


I have been through this several times.  The problem is gummy or worn  bearings in the fan motor.   The "locked rotor" current is too high, dropping the supply voltage (supplied through an 18k, 1W resistor) too  low to start the motor.   The fan will start sometimes if you spin the  blade to get it started.   The fan will start and run fine if the supply  is stiff enough.   All Bill has to do is measure the voltage across the 18k resistor and he will see about 22V.   The transistor will be saturated, and about a volt across the motor winding.   An 18V Zener across the resistor will allow the motor to start, and once it starts the voltage across the resistor will drop to about 12V, and about 9-10V across the motor.   But once the motor stops altogether, which it will, the transceiver WILL overheat, damaging AVR board parts first, and if you're unlucky, the PS pass transistor will short putting 40V across the final and driver transistors.   The fan motor will sound cheap then...


73, Garey - K4OAH







VK5KYO includes Larger Computer Fan and relocated Rectifiers.


-----Original Message-----

From: Joan de vk5kyo []

Sent: Friday, 22 September 2006 10:12 a.m.


Subject: TS940 Modifications


Hi Jeff


You may be interested in the mods I completed to my 940.

Since you run such a good site for this radio, I'll be quite happy if

you want to share it with others.


Joan made these significant changes:

This looks a very good improvement but has not been verified yet.

If you ask Joan for clarification,

Please send on all feedback so the revised versions can be updated here.







28 Volt Crowbar safety circuit for the TS-940



TS-940 Power Supply Improvement in 2012: Read this page first

120205. Power Supply 04.htm








Back to Part 1 of TS-940 page: Click here





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