Ampeg svt pro7 power supply problem

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View attachment SVT7PRO MAIN BOARD & PSU 0032543-XX SCHEMATIC.pdf
I am working on an Ampeg svt pro 7. A tech I know gave up on this after replacing U2, the 2a165 with a 2a265. I’m not sure it was even bad.
When I started on it I found the 5 and 15v supplies “pumping” 0-4v as if it was. Faulting out most of the time. Voltages on U8 optocouple would support that. I found that the supply that feeds pin15 of U5 was also on and off. For a short while the voltages came on and I noticed pin 16 of U5 (vref) was not at 5 volts even though the supply seemed ok at that time. I did not measure a short to ground on pin16 so I replaced U5. Now the voltages created by U2 are solid, but I do not have the main 55v rails. I can see some sort of switching at Q1 and Q2 but they are different from each other. The +55 rail measures 7v and the -55 measures zero. The rectifiers test ok.
Pin 10 of the U5 measures 3.5v, the data sheet makes it sound (to me but what do I know?) like that should shut it down but that also appears to be the voltage that turns on relay 2A. I believe that needs to be on to work. Any help understanding where to test from here and any theory or ideas that might help me get to the bottom of my issues would be appreciated. Thanks, Fred
This is the classic Mackie style smpsu. Cheap and cheerful.

They get high ESR in the smoothing capacitors and rarely anything fails apart from the odd secondary diode which must be replaced with shottky types of the same rating.
You would know if U2 had failed as they always explode! Check the feedback voltage on pin 2 of U2, if it stays low, then there is possibly an over-current issue or a low value electrolytic capacitor in the secondary side. The + - 55v rails will not run until the + - 15volt rails are OK.
When U5 gets its power and is running, the main supply will start producing + - 55volts.

Don't over analyse the issue, keep an open mind and don't forget, it is only a power supply, nothing complicated.
My 15v supplies are working good, pin 2 of U2 is low and happy. Are you saying that I may have high esr on the 55v caps C77, C79, C80-C85? Does that stop the 55v from working almost completely? I have taken this job on because I am hoping to become more comfortable with switching supplies. God knows they aren’t going away ��. Thanks for your help!
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