PPI AX400 Art Series advice

[azvrt]

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

[marko]

yes, PPI liked to slap loads of thermal paste on back in the day!!

[azvrt]

Obviously, yes

[azvrt]

Also, does anyone know at which locations to measure the bias on this amplifier ?
And how much mvdc it is supposed to be set to ?

[pergo]

the DC offset changes with temperature. It may drift.

You have to keep it under +-50mV, better near 0mV obviously.

[Perry Babin]

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.

[azvrt]

Thanks Perry.
Any idea where to start looking for the cause of the 3 volt DC offset ?

[DREW028]

Which tutorial for DC offset are you referring to?

[azvrt]

Perry's repair tutorial which can be purchased on bcae1.com

[Perry Babin]

What is the DC voltage on pins 3 and 4 of the driver board?

Do you have the same supply voltage on pins 1 and 15 of the driver board in both channels?

Are the 100 ohm resistors (under the driver transistors on the driver board) within tolerance?