Prodigy Troubleshooting

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1

Hello,

I have a moog prodigy here that does not make any sound via the keyboard. I have tried various combinations of osc settings, but with no luck. By chance, I was fiddling with the release on/off switch in the filter section. When I gently pushed the switch into a middle position, I began to hear a steady tone which could be modified by various oscillator and filter settings. It was almost as if the thing was functioning, but could not be controlled by the keyboard. I’ve opened it up, and the cable that runs from the keyboard to the circuit board seems to be connected properly, though I have not removed the circuit board to investigate further.

I will add that the fuse blew and the machine wouldn’t power on, but this was remedied with a fuse replacement.

Can you suggest anything I might check that could cause such a strange issue? Is there any way to trigger the synth without the keyboard to check if it’s just a keyboard problem or something more sinister? This is not the version with an input on the back, it only has the main out and a power cable.

Thanks for your help!

2

Hi,

fuse blew is bad, so you don’t know the amount of electrical damage and the original reason for it.

If the oscillators and filter are still running, it’s good. If you hear something when playing around with the release on / off switch of the filter section I would look at the ADSR circuits at first, because a damage here would block the VCA and no key stroke would enable the VCA to let any tone pass to the output.

So I would start here. Check the ADSR output of U18 (4007, Pin 1) via U22A (first half of 4558 OpAmp) to the loudness controlling OTA U17 (3080). If there is no envelope voltage, check keyboard voltage input (pin 10 of U18).

Cheers

3

I agree with analogmonster, a blown fuse is usually an indication that something is very wrong somewhere else in the circuits. Shorted Tantalum capacitors, shorted out chips, or shorted out voltage regulators ?
There must have been a short somewhere to blow the fuse.