Roland jc120 hums loudly

[Bigdrums]

I have a JC120 here that hums quite loudly anytime it is turned on. None of the knobs affect the loudness of the HUMM. I thought got to be filter caps. I checked them in circuit with my Peak meter sayin they were 0uf capacitance / in circuit. I ordered & received new ones 3300Uf 50V. When I got the old ones out they measured good, but put the new ones in anyway. Still hums like crazy. Using the variac I can see it looks like its drawing too much current. But not enough to pop the fuses. So with the o-scope I probed the outputs and on 2 of the legs there is significant saw-tooth even at just 25% variac. Any ideas on how to isolate this and the saw-tooth should that be there on the outputs? What should I see there? The rail volts look correct. Serial #560599

SM_JC-120_11052.pdf

[g1]

Did you measure for DC at the outputs?

[glebert]

I recently did a JC120 that had hum in the one channel and big dc offset in the other output channel (with no speaker hooked up). It ended up being that the normal channel preamp board wasn't getting a ground and was throwing off the DC for the PA for that side and then also creating hum where the normal channel cross connects to the effects channel. This was on a ~1983ish model, it was really weird that it had multiple grounds for the normal channel board but they weren't connected to each other so there was a single point failure mode for the ground that connected to the input jack.

[Jon Snell]

There are two amplifiers in these, hum from both?
Insert an empty jack plug into the Main In sockets. Is the hum still there ... if it is, you have an amplifier issue if not you have a preamp issue.
Is there any DC across the loudspeaker output connections? If there is, carry on fault finding Without a Loudspeaker connected. You don't need a load on a solid state amplifier.