Issues with biasing a Mackie M2600 power amp

[psycle_1]

I have a Mackie M2600 power amp on the bench. Channel 2 was going into protect for no reason. Somehow that cleared up on it's own, but when checking the bias test point for 12 mv on the 170 board I get -5 mv. When sweeping the trim pot outside of -5 mv the channel goes into protect. Swapped boards with channel 1's 170 board, problem follows the board. I've checked for shorted transistors and open resistors. I can't seem to figure out what is causing the issue. Any help would be greatly appreciated!

Mackie-M2600-08 pwramp.pdf

[The Dude]

Ignoring the polarity. Are you able to get + or -12mV?

[psycle_1]

Okay, I feel like an idiot now... I'm now able to get the amp to bias correctly, albeit the trimmer is super sensitive. It wasn't doing this earlier when I had it disassembled on the bench. Interesting to say the least. Will report back tomorrow if I run into any more issues.

[The Dude]

It could be the bias pot itself is intermittent.

[psycle_1]

Yeah, possibly. I kept the board that was giving me a fit swapped on top so if I need to replace the pot it's a lot easier to get to.

[psycle_1]

Board is giving me another issue now. Output meter on channel 1 is pegging and was getting a lot of pops and crackles. Now that channel is going into protect right off the bat. Grrrr...

[psycle_1]

Now I can't get -115V on the 170 board. I've traced the circuit as best I can and can't for the life of me find what the issue is.

[The Dude]

I'm not seeing the PS in the schematic you posted, but just thought I'd mention that these things (like all Mackie gear) are notorious for having ribbon cable problems. Can you read a revision number on any of the ribbon cables?

[Jazz P Bass]

Full Schematic.