Roland Jupiter 4 - Noise

[Galmz]

I finally managed to find a Jupiter 4 after months of wanting one.

However, a day or so after getting it home, I noticed some continuous unwanted noise when ever the key is pressed. I would like to emphasize that of course the noise switch is off. This is really irritating gritty noise which I know for a fact shouldn't be there. The noise is also there when pressing a key with the waveform switched off.

It kind of reminds me of that mechanical vinyl sound - sizzling, crackling, dusty. If anything it has got worse and over powers the sound if the LPF is open. The noise is also there if the 'hold' button is engaged after pressing a key. However, there is no idle noise on the outputs which seem fine. A few of the presets (especially ones which close down the LPF such as bass) sound betterish - even though you can still hear the noise mixed in.

Trying to isolate the problem before spending any more money; it already emptied my savings Any ideas?

Galmz

[Z]

Is the Ensemble turned on? If so, you might be hearing noise form the Ensemble's circuit.

[Galmz]

Hi Z

The noise is there whether the Ensemble is on or not.

[nathanscribe]

My Jupe does something similar - tiny glitches in the CVs in what it sounds like, and after a while, sometimews those glitches suddenly go ape and the synth does whatever it wants. On mine at least, I suspect the logic associated with the key mode selector switches, as part of the symptoms involve intermittent failure to recognise changes in mode despite the switches registering correctly. I haven't investigated this fully, but it look like faulty CMOS. I know there is other faulty CMOS in mine too.

Yours could be something as simple as dirty sliders, which would be a much nicer fault than dying CMOS.

[Galmz]

Hi Nathan

Sorry to hear your having similar trouble. Interestingly, I spoke to someone who suggested that the noise signal could be leaking through the noise generator itself. This could be true since it does resemble that kind of noise...

It's mixed in with every single patch, and again present with the waveform switched off. Additionally, closing the envelope down cuts the noise off to just an audible 'click' . So to me this suggests that there is definitely a signal leaking somewhere before the VCA itself. Again the outputs themselves seem fine.

Just food for thought..

[Dr. Phibes]

There's a lengthy calibration procedure in the service manual. It's worth going over those. Sometimes excess noise can be due to out-of-whack biasing on things like the VCA.

[Galmz]

Recorded a video of the very sound:

[nathanscribe]

Yep, it's nothing like I'm suffering. It does sound like white noise, so I'd check the signal path from that through the synth. If the filter cutoff affects the noise, then the fault is before it, so I'd be tempted to first check everything from the noise source to there.

EDIT: service notes show a test point at CP31 on the Module Controller board. At that point you should get a 2V p-p white noise signal. There's a trimmer, VR11, which sets the gain of the post-noise-source amplification.

ALSO EDIT: I wonder if the switching FET (Q23) or associated parts could be faulty?

[Galmz]

Just tested that and the filter does indeed affect the noise. Also tested functionality before the noise and everything works fine. So might be able to isolate the problem based on that. Excellent thank you for providing that info!

[Dr. Phibes]

ALSO EDIT: I wonder if the switching FET (Q23) or associated parts could be faulty?

Yeah that was exactly what I was going mention after I (finally) found the noise generator circuit in that mess of a service manual.

It does sound very much like white-noise bleed through rather than excessive background static.

If you're comfortable with desoldering you might want to remove Q23 and do I diode check between the gate and the source.

[oldorado]

Galmz,

Did you ever resolve this issue? My JP4 has been almost perfect for ages (my grandad bought it new in 1978) but has recently developed the issue you described. It works fine for 5minutes or so after being powered off overnight but once the noise starts, powering off then on does not reset it.

It is the same on all sound generator cards (using Poly1 to test) and is affected by the filter. I find the Bass preset seems to work almost as normal just like you described.

I'm hoping for a quick inexpensive fix before i take it somewhere for a service with all the money I don't have!

Oldo

[freiburg]

Exactly the same on my unit. Seems to be temperature dependent......

Christian

[zoomtheline]

Not seen this thread. My JP4 came with many issues that I still haven't sorted out yet. It does have a similar noise issue too. I'll keep an eye on this thread and If I get any info i'll post it.

[freiburg]

Ok, decreasing the noise level (vr11) resolves the issue. I will replace the fet, to see whether it leaks...
christian

[mkho]

Did replacing the FET Q23 or the NPN Q24 do anything to the perceptible noise leakage? How about the diode D11?