Loud hum 100Hz

[olddawg]

A picture of the board would be nice. Frequently the filter cap leads become disconnected from the pads on the board after years of vibration. I would resolder everything in the power supply area first. A floating filter cap displays the same symptoms as a bad one.

[g1]

I think your meter can not measure AC when DC is present (inexpensive meters often can't). You would need a cap in series with the positive probe, something like a .1uF 600V.
But I also agree with the others that the main filter cap is probably bad. The DC voltage is too low like Pedro said, even 380V there is too low.

[J M Fahey]

Like G1 said, your meter can not measure AC if mixed with DC so you can not measure ripple (you still can measure mains and transformer windings though)
So "800VAC" means nothing in this case.

Please repeate 2 tests:

1)

c) pull Phase Inverter V4 and repeat tests ... any change in hum/buzz?

2) with V4 pulled, measure VDC at +V , which is the positive end of the first filter cap.
I am guessing itīs open/unsoldered/pad or track cracked/"it is not there", but guesses are checked by measurements.

Hint: if a cap is open, a meter set to DC reads about the RMS voltage fed there by the transformer while if the cap is doing its duty it will read about the peak voltage.

The second is about 1.4 times the first.

[InDecember]

The new filtercaps came in today, I installed them and the hum is gone. This was prolly a standard repairjob so sorry if I made it look like a massive problem, it's because of my lack of knowledge of these amps. Anyway, I learned alot, things to keep in mind for the next time, thx for helping me troubleshoot guys. Cheers

[J M Fahey]

Cool.
So you were straight missing your first power filter.
That explains both the massive (unfiltered) hum and the 30% lower "DC Voltage" ... although it was not real DC but unfiltered full wave rectified AC ... not the same.