moog prodigy strange problem

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chrisxofxdeath
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Joined: Thu Dec 24, 2015 12:41 am

moog prodigy strange problem

Post by chrisxofxdeath » Thu Dec 24, 2015 1:01 am

I'm trying to fix a friend's moog prodigy with a strange issue that doesn't come up in any google search I've tried. When turned on, it immediately starts playing a note, and all of the functions seem to be working except the VCA and VCF sustain knobs. The keyboard will play whatever note is pressed down, but it will latch to that note, and the note will never be released (meaning that it just keeps droning ad infinitum until a different note is played). My uncle is a retired electrical engineer, and I'm currently in school for electronics, and from looking at the schematics we think that the 4007 chip responsible for triggering the release of the 556 is having some kind of problem. We also got a few weird voltage readings on the scope when looking at various pins on the 556 chip. A few of them were unreasonably high voltages, though I don't remember at the moment which pins were associated with the high voltage. It may have been pin 9.

If anyone has any suggestions as to where we should be looking, it would be very much appreciated. We think it has something to do with the oscillator chip because the VCF and VCA circuits are discreet signal paths, but both of them display the same latching symptoms. But I'm no expert on synthesizers, and my uncle isn't used to working on audio circuits (especially complex waveform manipulation circuits), so it may be that there's just something don't know about the functionality of the synthesizer.

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HicoMe
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Re: moog prodigy strange problem

Post by HicoMe » Mon Dec 28, 2015 5:12 am

Hello,

maybe simple, but ... have you checked the keyboard-circuit? For me it sounds like a problem with the gate/trigger-signal from the keyboard.

Best regards ... and good luck.
Heiko
My current Moogs: Sub 37
My former Moogs: Minimoog Model D, heavily selfmodified Prodigy, Satellite, Etherwave Theremin

chrisxofxdeath
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Joined: Thu Dec 24, 2015 12:41 am

Re: moog prodigy strange problem

Post by chrisxofxdeath » Tue Dec 29, 2015 10:46 am

Thanks for the reply. We will try and check that, though we were originally thinking that the keyboard isn't at fault because the problem remains even if the keyboard is removed from the circuit.

kenzie25
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Re: moog prodigy strange problem

Post by kenzie25 » Mon Feb 15, 2016 2:43 am

if I were you, I would bring it to near service centre to avoid broken part.
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hanksterman00
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Re: moog prodigy strange problem

Post by hanksterman00 » Sun Mar 06, 2016 10:02 am

Don't know if you got the problem repaired. I can offer some help if you post the schematics. Own a Moog Source that had a similar problem and found that there was a bad OTA IC which is a comparator driven op amp. It could also simply be a normal op amp that is latched high

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