The ALC (Automatic Loading Control) 100 volt 5 watt zener in Henry RF amplifiers is a known problem for going bad. It will first short causing a HV surge, then the HV surge will normally blow open the zener and the main circuit breaker will trip. After resetting the circuit breaker, the amplifier will appear to function OK, however the ALC circuit will no longer be operational. Sometimes after resetting the circuit breaker it will trip again because the zener did not open after the HV surge.
To test the ALC zener, remove one end from the circuit and test with a basic volt/ohm meter. You should have infinity ohms in one polarity direction, reversing the meter leads, you should have approximately 100-200 ohms in the opposite polarity direction.
In the Henry 3K-A, the ALC zener is diode D206. The zener connects parallel across the HV bleeder resistor which is closes to the B- side of the HV power supply. The anode of the zener goes to B- side of the HV power supply and the cathode side of the zener goes to plus side of the last bleeder resistor in the bleeder resistor series. For the Henry 3K-A this is the junction of D206 and R116. The junction of D206 and R117 then goes to the RF deck ALC voltage threshold adjust pot. See the Henry 3K-A schematic for complete details.
The way the circuit works, the negative ALC voltage from your transceiver will mix with this -100V ALC reference voltage, which then feeds back to the tube BIAS circuit. If the voltage goes higher then the ALC adjustment pot threshold is set to, the gain of the tube will be clamped off. This prevents you from over driving the input gird and damaging the tube. Its also prevents the tube from conducing non-linear when being over driven, which will cause distortion and splattering.
This circuit is not really required. Since this zener is a common failure point, I personally removed D206/R117/R118 totally from my 3K-A amplifier. I found D206 to be a real headache, shorts all the time, poor design. I just manually watch the grid drive meter and don't over drive the amplifier grid input.
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