Low voltage on plates of Traynor YCV40

[mojotronix]

Hello,
Wow, haven't been here in years. I am so glad Ampage is still kicking.
I have a YCV40 that is driving me nuts.
This has a regulated power supply including the B+
Only getting 90 volts on output plates.
Replaced U3, Q8, and an open fuse resistor (R100).
All other components seem to measure fine.
Anyone with experience on these?
Thanks for your time.
StanManual-Service-YCV40-1v2.pdf

[g1]

There are lots of versions of the regulator/standby circuit, so you need the exact schematic version.
Are you positive this is it?

[Enzo]

Were those other parts bad? Don't test parts, test circuits. You have 90v on the tube plates? OK, what voltage is on either end of the 100 ohm resistor, and on either side of the FET? Is the 300v supply good or is it real low as well?

[mojotronix]

Yes, 100 Ohm resistor was open. FET was shorted (may have happened while testing). Optocoupler probably was good, but replaced anyway.
HV supply is good prior to regulation.
300v supply is low. But isn't dependent on the 410v supply anyway?
Disconnecting that preamp supply from circuit gives me another 90v on the plate supply, but still not right.
I hear what you're saying about testing circuits, but if I don't fully understand how a circuit works, I test individual components to find the culprit.
Thanks for your knowledge, Enzo.
Any other advice is appreciated.
Cheers

[mojotronix]

Thanks for replying.
Initially, I was working from the schematic on the website which was incorrect. Called Traynor, and they sent me the correct one right away.
The one attached is the correct one.
Cheers

[g1]

If you were going to replace the opto anyway, you may also want to do the same with D13. You can check it with a meter for forward drop, but it won't test whether it's functioning as a zener. Same as Q12, the blown Q8 may have stressed all the semi-conductors in that standby/regulator circuit.

[mojotronix]

Correction: R100 is a 1 ohm (fuse) resistor, not 100 ohms. My bad.
That said, isn't D13 acting as a protection diode? Yes, it did test good on forward voltage. Didn't think about it not being able to function as a zener. I've always found them to be shorted.
I'll replace that and D12. Not much else it could be, I guess.
Need to study up on regulated power supplies.
Thanks for the information.
Stan

[52 Bill]

I've fixed a few of these and as far as I know, there is no real voltage regulation of the 410 vdc supply. All of the circuitry is there just to turn on and off the high voltage for standby use.

If you've got 410 volts at C55 the first main filter cap, then you need to find out why the mosfet Q8 isn't turning on enough to pass the voltage to the output transformer and to the 300 vdc supply.

Everything from the opto to R100 are suspect and should be checked. And remember that none of those components are grounded.

[Chuck H]

Not saying that all this fussing over the HV supply isn't where the answer might be, but what about the old school consideration of excessive current draw caused by bad power tubes or a failed bias supply.?.

EDIT: Or bad power supply caps, a short, shorted transistor, etc.

90V more on the power amp when the preamp current is removed seems like a lot to me. I'd be looking for problems or failures that cause excessive current.