Hey Robert,
Your problem drove me back to thinking about this. I would love to be able to respond to your individual points by quoting them like some posters do, but can’t see how to do that. So I’ll just talk about what I think I know, hoping it may help you a bit….
Firstly, I can’t find a schematic of this part of the 332A. The manuals I have been able to find seem to have all the schematics except the series pass assembly and pre-regulator. I have found an early 332B schematic which is quite different from my 332B/AF, but the component ID scheme is quite different.
Assuming that the design philosophy is the same, the way the pre-regulator works is as follows:
The pre-reg works at a nominal 100V AC (85V in the B/AF) derived from a tap on the line side of the main power transformer (hence the reason why probing with an unisolated set-up is hazardous). The control is via a single control transistor in a conventional diode bridge.
At every mains zero crossing, the control transistor is turned on with each mains transition via a rectified but unsmoothed voltage sourced, in the A version, from a bridge rectifier CR1201-1204 (this schematic from the A I do have). In my book this is labeled “-V”.
The control transistor switches ON when -V goes nominally to zero (ie, mains transition). There is a bunch of stuff between the pulse transformer and the control transistor which then locks it ON until a pulse hits it from the pulse transformer driven by the UJT (or excessive current in Q107 as sensed by a small resistor in its emitter…although I don’t know if this is present in the A). The UJT is a conventional oscillator which responds to the voltage across the last series pass transistor (I’ll call it Q8 as per the B, but the A may be different) in a chain of 8, the first 7 of which do nothing more than absorb the bulk of the up-to-1200V or so coming from the HV transformer fed by our control transistor, which is the one I think you call Q107. (If the control loop is working correctly, then in the steady state these first 7 transistors are simply saturated with almost zero voltage drop. They only see action during dramatic range or load changes, and then only briefly.)
Now, the UJT circuit is such that it starts running when the voltage across Q8 is around 40V, and when that voltage gets to about 55V or so, it’s running flat out well beyond 20kHz.
(In the B/AF, there is extra circuitry here to prevent the UJT running for a very brief period at the mains transition, but that circuitry is absent in other B schematics I have seen, and probably in the A as well.)
So, with each mains transition, Q107 gets turned on and stays on until Q8 calls “enough!”. But if Q8 already has enough (such as high input voltage or light load, which I think is where you were seeing issues) Q107 could well be being turned off immediately, which may look like loss of drive that you are seeing. I think you mentioned that you were running some parts of this independently of the overall control loop, so perhaps it will help to watch the voltage across your Q8 as you monitor your drive to Q107.
Apologies if you already know all this…..
Roman