Roland E-70 keyboard with noise on right channel

[frisc0]

I have a Roland E-70 keyboard that is working great, however while not playing the right channel/speaker is constantly producing (digital) noise. The other channel/speaker is decently quiet. The noise on the right channel is audible on both the speaker, headphones and line out.

I've looked at the service manual (https://www.synthxl.com/roland-e-70/) and I suspect the muting circuit, around Q6, but I have no idea how to diagnose this.

Attached is an audio sample of hitting a key and recording (phone mic near right speaker) the noise.

Any suggestions on how to diagnose/pinpoint the faulty part(s)? Thanks in advance for your help!

(
note : pushed this message to the wrong subforum, already got this reply from user moffy:
"I opened the noise file in Audacity and zoomed in and it had a repetition frequency of 120Hz. So I assume you are somewhere that uses 60Hz mains frequency and that the noise is power supply related. Nice that you provided the noise file. It isn't the mute, you wouldn't want to hear that noise when playing. Best bet is that some electrolytic capacitors in the PSU have lost their values."

In my country mains is on 50Hz, so can it still be the PSU? And the noise is only on 1 channel, any suggestion on how to further diagnose? Caps seem okay on the PSU/Amp board. I can get a scope to check in more detail, but need some help on where to start..!
)

[Audiorepair]

Does the noise go up and down with the master volume pot, or is it independent of it?

If the master volume pot does control the noise then it is likely a DAC issue, or the circuitry between the DAC and the master volume pot (a couple of electrolytic caps and one op-amp)


And yes, that IS 120 Hz noise, so unlikely to be power supply caps in 50 Hz world.

[marinedalek]

It sounds like a grounding problem. What happens if you pull CN2 on the PSU board? If the noise stops, it's coming from the GES board (DAC possibly?).
The signal to the speakers has a low impedance path around Q6, so I wouldn't expect Q6 to have that much of an impact on the speaker output. However, have you plugged an amp/audio interface into the Right line output on the back? If that output has the same digital hash on it (and pulling CN2 didn't make a difference), then the problem's around IC1. If it doesn't have any noise on it, then the problem's further down in either the filtering stage or one half of the TDA7350 power amp.

[frisc0]

Wow thanks guys, why didn't I come up with that...

I pulled CN2, disconnecting the GSE board from the amplifier and the noise stopped! So it must come from this board and not the PSU or AMP boards. And like audiorepair suggests, the issue must be somewhere in between the DAC and CN4 (going to CN2 of the amp board). I don't think the DAC itself since it does produce a clear piano sound on both channels. I'll check this GSE board in more detail (somewhat harder to reach than the other boards).

Btw isn't it strange that the volume is controlled on this GSE board, so on the unamplified signal? That means the power amp is always on on full power? I would expect the volume to be managed on the power amp side of things?

[Audiorepair]

Disconnecting CN3 going to the master volume pot will break the chain and narrow down the search somewhat.

[frisc0]

Problem solved! Cap C3 on the GSE board was leaking, see attached picture... replaced it and the hum/noise is gone!

Thanks again for helping diagnosing this!