[frsimen]

The 22V from the mains power supply will have several volts of ripple on it. The peaks will be somewhat higher than 22V but with 160V supply from the variac you shouldn't be getting anywhere near a voltage that will cause trouble. Try measuring the AC voltage that is applied to the bridge rectifier with the power switch off, that will explain what is going on, I think. I would expect the voltage to be at least 17V but not more than 21V when fed from 240V.

The rest of the circuit works quite well down to just 6 volts on the 22V line. Check carefully the wiring around the voltage selection switch, it may have been wired incorrectly by someone, so that it is wired permanently in the 120V position.

The Zener diodes are used for in two ways in this circuit. The voltage reference is provided by A2CR4 and that is fed by a resistor to control the current through the Zener.

Many of the other Zener Diodes are being used to provide overvoltage protection. A2CR5, 6, 7 and 8 all are used for that purpose. You will notice that their voltage rating is rather higher than the voltage which normally appears across them. They don't have a current limiting resistor either. If the voltage across them becomes excessive, they will conduct and pass a lot of current as there is no limiting resistor. They will fail short circuit and that should shut the power supply down. They play no part in regulating the output of the supply.

The unfortunate A1CR10 protects Q17 from overvoltage in a similar manner.

Paula