Jupiter 4 Noise Issue

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JFunc
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Gear: Juno 106, Juno-6, JP-4, A-Juno-2, DX21, CZ-101, 01/Wfd, SY85, X3, XV3080, M1, M3R, D110, MPC1k, S950, Prophecy, Plugiator, TR-8, MFB-522, Volcas

Jupiter 4 Noise Issue

Post by JFunc » Sat Feb 06, 2016 1:28 pm

Howdy folks!

I was lucky enough to get a hold of a JP-4 a while back. She was in a terrible state - near dead.

I managed to restore the power supply (all electrolytics and regulators changed), I cleaned the key contacts, solved a bizarre grounding issue on the control boards, desoldered, opened up and cleaned EVERY fader, checked the track resistance (just to be sure), resoldered the faders. And finally did a full calibration... Magic!

Now when I turn off the VCO Sub, and turn off the VCO Waveform, and play any key, I get a low level 'vinyl' like random 'scratching' noise (and what appears to be bleed-thru from the voice cards at the pitch played). The Level slider is halfway up. When pushed all of the way up the noise/bleed increases in volume, but stays at the same ratio.

These noises are just about masked when the Sub AND Waveform are on, but audible when Sub OR Waveform are on...

I have removed, cleaned and reseated all of the chips/boards that can be done, without desoldering.

I'm kinda hoping it will just be some of the bypass electroytics that are on their last legs, or possibly some cold joints, but I thought I'd ask you guys for any advice you may have, or have you experienced this before?

Look forward to hearing from anyone!

All the best!

Andy.

Mooger5
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Re: Jupiter 4 Noise Issue

Post by Mooger5 » Sun Feb 07, 2016 3:14 pm

This is where a can of freezing spray comes handy. Freeze the suspect components in the area and the crackle stops. Spray a bit more to cover them all with the dry ice or whatever it is and as the culprit warms up it will be the first to show up. Then use a common hairdryer, not a hot air gun, to evaporate the dry ice.

The crackling becoming louder tells it´s not a bad volume pot. Another good clue is the crakle only happening when the keys are played. It could be something in the VCA circuit or before it, not after. Find out which voice is being affected. Use a scope and headphones to watch for spikes above the noise floor while monitoring the audio, for locating the area and the freezer to pinpoint it.
Herrare umanum est.

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synthroom
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Gear: Fairlight IIx(!), JP-8, D-50, S-50+550, S-760, JX-3P, JD-800, EII, Emax II, Mini, ARP 2600, P-5 Rev.1, Pro-One, Performer, K1m, K5m, few other things.
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Re: Jupiter 4 Noise Issue

Post by synthroom » Sun Feb 07, 2016 6:21 pm

The stuff you see frozen to the part is most probably not dry ice - the freeze spray is probably made of tetrafluoroethane but what you see frozen to the part is water ice that condenses from the surrounding air. But it evaporates quick so it's not a big issue.
Fairlight IIx (Mid-Life Crisis - cheaper than a Corvette!)
Roland JP-8, D-50, S-50, S-550(2x), S-760(2x), JX-3P, JD-800
EII, Emax II, Minimoog, ARP 2600, P-5 Rev.1 (broken...), Pro-One, Crumar Performer, K1m, K5m, MS-2000B, Virus KC, a few other things.

Mooger5
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Re: Jupiter 4 Noise Issue

Post by Mooger5 » Sun Feb 07, 2016 9:04 pm

That´s good to know, thanks!
Herrare umanum est.

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