CROWBAR CIRCUIT FOR THE 940:

Circuit is same as applied in the TS-930 below:

ZL4AI found the above article difficult to follow because

the crowbar circuit modification is not well separated from the  

change to 2SC708s modification.

The crowbar is better, explained below.

From ARRL Handbook 2004 page 11.39

The crow bar circuit functions as follows:

The Zener hold off diode D3 blocks the positive regulated voltage from appearing at the base of Q6 until it avalanche voltage is exceeded. 

In the case of the device selected, this voltage level is a 33 V, Which provides for small overshoots  that might occur with sudden removal of the output load (switching off a load, for instance).

In the event the output voltage exceeds 33 volts, D3 will conduct, and forward bias Q6 through R22 and C20, which eliminates short duration transients and noise.

When Q26 is biased on, trigger current flows through R23 and Q6 into the gate of SCR Q7, turning it on and shorting the raw DC source, forcing F3 to blow.

Since some SCRs have a tendency at high temperature to turn themselves on, resistor R24 shunts any internal leakage to ground.