acoustisonic problem

[nicewrk]

Having a little trouble diagnosing distortion on intrument input side of a
acoustisonic jr. The channel with Mic input sounds great. I first thought it was the pot at the notch filter so I cleaned and later reatplaced it. Seemed to work but the problem returned. I noticed some heat marks on the circuit board
at the 2 resistors r124-125 next to the large electrolytcs. I dont suspect the resistors though because they are not in the preamp section and the one channel works fine. Any ideas?

Thanks

[Mars Amp Repair]

Well,
It looks like the Inst_ch & the M/L channels all meet just before the digit pcb as well as the send/return processing where they blend at the op amp U10 & U21. hopefully the issue is isolated to the inst_channel before all this mess as it looks rather complicated past this point.

You can find the schemo for free at www.schematicx.com

Personally I would signal trace at U-2 pin 7 at C-10 to determine if the distortion is back from that point or forward. If you don't have a audio signal tracer you could start with measuring all the in & out voltages on the opamps in the bad channel. On a closed loop opamp the input & output DC voltages should all be the same except on that goofy CKT used in the string dynamics ckt U6.

this could be a tough one...glen

[Enzo]

OK, one channel is clean the other distorted. If it were power supply, then both would be distorted, I'd say, so look to the channel itself.

You have an insert jack that serves only that channel. Connect that insert jack to another amp and listen to it there. Is that also distorted?

In that channel, I see at least 11 op amps - 5 and a half ICs. I would start by looking at the output pins - 1 and 7 - for unwanted DC offset. We are talking several volts now, not a few millivolts.

[drewl]

I had one with the same problem last week.
Reflowing a bunch of the pc board connections and cleaning the pots jacks and switches cleared it up.
Unless this is the same one!

[nicewrk]

I am wondering if the IC's themself are suspect? I am going to bypass the
pots on the notch filter and feedback. All that could be wrong with one of the ics is that they are stuck open , correct?

[nicewrk]

Drewl ,

This isnt the one you worked on its been messin with my head a while now

Thanks

[Enzo]

The ICs themselves are the op amps I referred to in post #3.