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Here's a Sunday electronics puzzle...

I'm having trouble finding the culprit on my Gallien-Krueger 2000RB power amp board. If anyone has a suggestion on what might be affecting the highlighted sections on the schematic, it would be much appreciated!

Notes:

• Running at 75V on my variac...

• Outside of highlighted areas, voltages seem to match the board on the other (good) channel.

• Yellow area is all reading approximately 50mv above the other (good) board.

• Blue area is lower VDC than good board

• Q535 & Q538 get real hot real quick but voltages are very close to good board.

• Turning up variac too much overheats R528

• I've replaced Q586 (wondering if that could have thrown bias off too much if it's not now a "matched" set)

• Bias measurement at P541 (supposed to be 5mv) is reading 0 regardless of adjustment (R524)

• R460 is reading 99.6k (out of circuit) instead of 100k. (not sure if that's enough to throw everything off)

• Replaced Q534 & Q539 but quality of chips used is suspect (possible counterfeit chips)

Thanks for any help. Still advancing my electronic skills.

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  • \$\begingroup\$ writing all the node voltages around the troubled zones on a copy of the schematic with a balloon should reveal the answer from the excessive Vbe \$\endgroup\$
    – D.A.S.
    Commented May 15, 2022 at 20:51
  • \$\begingroup\$ Thanks! I’ll look into Vbe… \$\endgroup\$
    – MtnMann
    Commented May 16, 2022 at 4:43
  • \$\begingroup\$ Then trace backwards to see why Vbe is excessive \$\endgroup\$
    – D.A.S.
    Commented May 16, 2022 at 13:30

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