Yamaha DG100 Hiss

[Mars Amp Repair]

Hello folks,
does anyone have any experience with this Yamaha DG100 digitally processed amp from the 80's-90's?
This one has a prominent HISS in the sound that is independent of the tone and Gain & volume controls. It will be quashed when the Output pot is turned down. FYI, the Output pot is post the Digital effects. This tends to confirm the Hiss is being created on the digital board.
I have replaced the D-A IC's 122 & 123 as the best I could trace the Hiss seemed to be created in them as well as they are notorious for going bad in older Yamaha console keyboards. That did not change the issue.
Any ideas would be appreciated. Thanx, glen

[glebert]

Is it just hiss on top of the expected output or is the signal garbled or unexpectedly distorted? Did you verify the hiss is generated before the ANR module?

I would probably start with cleaning any ribbon cable connectors, effects loop jacks, the effects level pot, stuff like that, just because it is simple.

[Mars Amp Repair]

OK, I found the issue.
I had audio signal traced for hiss in the opamp stages that preceded the A-D's & did not find what I thought was any inordinate Hiss. Well apparently that was inaccurate. It turns out the one of the two opamps IC105 & IC104 were producing the Hiss. I'm not completely sure which one caused the issue as I actually just replaced all 4 OpAmp IC's when the A-D's didn't fix the issue.
I used the original opamps NE5532P which are low noise opamp, but believe any TL072 would work fine.
Glen