PPI AX400 Art Series advice

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Just purchased my first PPI, an old AX400 from the Art Series.
I hooked it up to my power supply, it idles around 1.2 ampere.
Checked the DC offset, which was very good on one of the 4 channels (less than 5 mvDC).
On another channel it was not good nor bad, around 20 mvDC.
On another channel it was dramatic, around 200 mvdc.
On another channel it was worse than dramatic, around 3.3 volts !

Checked Perry's tutorial and found some very interesting information.
I had already noticed 4 pots in the outputs, presumably for the bias, and 4 pots in the input section, according to Perry's tutorial for DC offset. Very handy ! Just like my Adcom.

Anyway, I should be able to get the 5 and 20 mvdc right, I might or might not be able to get the 200 mvdc right, but the 3.3 volt channel must be in bad shape, as pointed out in the tutorial.

Anyone have the schematics for this particular amp ? Any advice ?
I'll return to my father's place tomorrow and adjust the DC offset where possible.

I must say that it was very variable. For instance, on the 200 mvdc channel, it actually was 330 at first, later it was 180. Same for the other channels, while the amplifier was warming up the offset would change quite a bit. Is this normal ? Seems like a lot of difference, 180 / 330 mvdc.

Here are some pics of the board :) LOTS of thermal compound, got it all over my clothes :eek:

An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.
 
The voltage across the emitter resistors is the way to determine the idle current on most amps. If you don't have the bias specifications, on most amps, 0.001v across the emitter resistors is close enough.

Remember, the transistors MUST be tightly clamped to the heatsink when you do this. If they are not, the bias won't be the same after you reassemble it AND there is a great risk of blowing the outputs. You'll need to make clamps to hold them down.
 
See attached.
 

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