I have a late 1970’s Prodigy. Originally, just had an audio out jack. I made factory recommended mods to the unit 30 years ago to include sync in, Gate in/Gate out/ S-trig, audio in, Keybd in, and others. The unit worked back when I did the mods, but over the years of not using it and moving number of times, I no longer get sound out of the audio out jack. If I plug a jack into Sync In, I get sound that I can control pitch via the oscillator controls and keyboard. The problem is that I have no filter control: the sound doesn’t shut off and there is no way to adjust attack , sustain or decay.
So, I suspect that the filter section is being bypassed, but no clue how to work forward from the oscillator section to determine where the dead spot is. Any help would be appreciated.
Open up the housing and clean all connectors and sockets thoroughly, also the jacks with high perceted alcohol. If youre skilled, ICs placed in sockets you can remove carfully and put them back. But if you intend to clean pots, you must be even more careful as pots often are lubricated inside and the lubricant shouldnt be removed.
Next step could be recapping completely.
When you say that thing hadnt been used in the last years oxidations are most likely the culprit, but if this doesnt help dont bother about, its a pretty simple constructed machine…
Thanks I will look into cleaning the connectors. I don’t recall if the ICs were plugged into sockets or hard soldered, but will investigate that too. Most of the chips seem to be relatively cheap if they have to be replaced.
Since you say that the Prodigy worked fine after you did the mods, 30 years ago, but is not working properly today after being in storage and moving it several times, I would do what Megavoice suggested and open up the unit and inspect the wiring for broken/desoldered wires, loose connectors, oxidized or corroded contacts on jacks, switches, potentiometers, chips on sockets (if any), connectors, etc…
I would not go in with a large hammer like mega<>. Troubleshooting should always be: first understanding the circuit, visual inspection, thoughtful troubleshooting, component replacement.
First, I would check the “Audio In” jack by plugging any audio source (phone/ipad/etc) in. If you get sound, its not the filter.
I would then open it up.
As alien said, check for obvious loose wires, etc.
I would then start checking with an audio probe. If you don’t have an audio probe, you can use headphones or a speaker with - to ground and + connected to a wire as a probe. (or you can use a jumper to the + or the signal out jack).
First, check where R104 and R105 intersect. This is where both OSC enter the filter. Place the wire on R104 and/or R105. If the OSC are working, you will hear them (loudly!).
If that works, using the probe, place it on R125. If that works, check pin 6 of U17. If you have sound, check pin 1 on U164 (4558 OPAmp). If you have sound, its the volume pot or wires/connector to the signal out.
Thanks atk and alien. I will check it out when I’m off spring break. A week from now. Going to look for my audio jack converter and try plugging into audio in first. Will let you know how I make out.
But Robert:
I talked yesterday to my tech and he confirmed first what I wrote above and added that he knows that Moog suggested somewhere in the early `80s by their own to have a look at the pin sockets of the connectors and resolder every spot carefully if they dont look good or if the whole thing is wiggling. Take this as important
??? Who said that ???