Thank you for your detailed reply
@LBPete
....
If the output transistors, the ones mounted on the big heat sink on the back panel are removed there should not be any voltage at the speaker terminals. It has to go through those transistors to get to the speakers. Since the protection circuit is not detecting that voltage, you have some sort of wiring issue on the power supply board. What I don't understand is why it's shifting from a positive voltage to a negative voltage based on whether or not the outputs are installed.
With the four TO-3 2sc1116 outputs removed, and with the driver board installed, there is (just measured it again) -48.8 VDC at both L&R speaker terminals.
The power supply board (F-2546) doesn't appear to have ever been worked on. I do not doubt that you are correct. But with everything appearing to be un-touched, then could it be that something has failed that is not able to be seen via visual inspection? I am not seeing anything that I would describe as a "wiring issue"...to me that implies something that has either been physically damaged or altered from original. Or perhaps I am not understanding.
I don't know for certain that the voltage is shifting from + to - . I am an ex-electrican, oriented to working in AC where polarity does not matter. From working many years only in the AC world, I have developed a bad habit of looking only at the absolute value of voltages. So I often forget the fact that when working with DC voltages polarity does matter. Those noted directly above, the -48.8 VDC, are indeed negative DCV. But the readings I took yesterday, and reported in my last post, could well have all been negative, I wasn't paying close attention to that particular detail. But I will do so going forward though.
Where are you taking these measurements?
The measurements being referred to here are from the cases of the TO-3 outputs to ground. With the driver board AND all four outputs installed, there was 50-something VDC (polarity not noted) on the two innermost TO-3's and 0 VDC on the two outermost TO-3's. I realize that this is a non-standard place to take a voltage reading, but thinking that a subtle clue might perhaps be contained within that could provide some sort of insight into what might be happening on the driver board.
The driver board is always suspect when you have rail voltage and good output transistors. Fusible resistors and the VD1212 should be replaced regardless and you might get lucky but faults with them would kick it into protection. First step is to figure out why you have voltage at the speaker terminals with no outputs installed.
If the fusible resistors and/or the VD1212s are not causing these particular issues, meaning the near-rail voltage at speaker outputs even when the TO-3 outputs are removed, then yes, I will opt to leave the fusibles and the VD1212's in place for now. As much as I would enjoy bringing this beauty back to full health, I am not in a position to take on the 8080 as a restoration project at this time.
My plan is to pass this unit along to someone else who is able to give it all of the attention it deserves.
But before that, what I would like to do if possible, is just get her working. I would like to hear this receiver. And I would like to be able to go through and methodically assess functionality and determine what other issues may, or may not, exist.
What think I understand based on your last post, is that there could be (or are?) issues on 3 boards: protection board, power supply board, and the driver board...is this correct? Could it be that there is one fault on one of those boards, that could explain all of the observations I have reported? What about shorted driver transistors TR11 - TR14?
EDIT:
@LBPete I have, for a second time, edited the test results reported in post #17. The original post was difficult to follow. It should be much more clear and concise now.
)
