Roland JC120 sounds farty if played hard

[Jay Mann]

And what the output looks like on the scope

[glebert]

Have we actually measured power supply voltages? That seems like it needs to happen.

[The Dude]

See post #10.

[glebert]

See post #10.

Thanks, I thought he had but then missed it when I scanned through the thread. It seems very strange that the distortion happens in both channels and both speakers, I'm wondering if one of the rails could be sagging when it has to deliver significant current, but that is just a guess. Other than that I would think you would have to start probing through the signal chain while it is pumping out the distorted signal.

[eschertron]

Thanks, I thought he had but then missed it when I scanned through the thread. It seems very strange that the distortion happens in both channels and both speakers, I'm wondering if one of the rails could be sagging when it has to deliver significant current, but that is just a guess. Other than that I would think you would have to start probing through the signal chain while it is pumping out the distorted signal.

Were the power supply voltages measured only at idle? A measurement at load would be telling...

[The Dude]

I think I said this earlier, but I would turn the amp up to clip and start scoping from the input to see where clipping starts. It may not even be the power amp that's at fault.

[BrookfieldTubeAudio]

And what the output looks like on the scope

Did you ever solve this? I’ve got a JC60 with seemingly the exact same issue. I also replaced the power supply filter caps, which reduced hum, but the farting out clipping remains.