7704A (et. al.) Inverter Regulator Question


The SMPS LVPS in my 7704A is in tick mode. I've found a couple of bad transistors and have ordered the last of them as well as resistors to make a load array somewhat like the one Dennis built. With those, I hope to finally get this PS working; however, until all of those arrive, I got to wondering about something in the inverter regulator.

The custom Tektronix chip, U3105 (in the 7704A but may have a different designation in other models of the series) regulates the inverter by changing the point in the cycle where a set of windings in T3030 are shorted to ground via Q3102/Q3104.

I notice there is also a diode, a 15V zener diode, and an RC network all connected in series between the collectors of both Q3102/Q3104 and ground. Is this a fail-safe overvoltage protection circuit for the inverter should Q3102/Q3104 or U3105 fail or is this a snubbing circuit to clip noise spikes (which I don't quite understand would be necessary), or something else? The manual doesn't seem to mention it - at least not that I've found.

Barry - N4BUQ

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