Vintage Synth Explorer Forums

hey folks, got one of these on the cheap, as it "didnt make sound", according to seller. its a 203a model. the problem im having now is that across the keyboard, it is RANDOMLY inconsistent. depending on preset, some or none of the 1st 2 octaves work, but max 2 notes. octaves 3/4 same deal, but more notes trigger, maybe 5-7. octaves 5/6 are also primarily dead, max 2 notes. as the notes that trigger will change from preset to preset, im VERY confident this isnt a j-wire/KEY trigger issue.

here's what i did already:

-went over PSU, its now putting out what it should.
-took apart manual, cleaned all contacts.
-went over the 2 "programmer boards", 002 and 003. these are the boards that connect to the underside of the control section. i cleaned all pots, buttons, and went over all board traces w/ light alcohol solution and cloth.

what im GUESSING is the source of the problem is that the 003 board has a lot of white, corrosion-looking stuff on a lot of the solder joints. strangely, the 002 board, right next to it, looks clean as a whistle. anyway, there's a lot on the board, and shotgunning all the parts would be a nightmare. i scrubbed as hard as i could with the solution and cloth, but it didnt change anything. once it dried, it looked just as it did before cleaning.

any ideas? any of you either seen A) white corrosive s**t on solder pins, or B) know these polymoogs well? thanks for the help.

i just got a polymoog that has just one key not working, so i'll be opening it up, hopefully soon. i know jack about them, but will send you pictures of its insides if you'd like.

These are one of the most difficult synths to work on. Here are some inside pictures...

Space6oy, The problem is probably a modulator card. Find the modulator card that corresoponds with the bad note and just reseat it. Chances are it will come back to life. If not there could be a problem with the card itself. The three top boards are all on hinged joints to give you easy access to the modulator cards below. These can be seen in the third picture.

rjd2, your problems are more likely chip based. There are certain presets that use different waveforms and will cause certain issues that are different from preset to preset. You could also have some divider chip issues.

hfinn wrote:Space6oy, The problem is probably a modulator card. Find the modulator card that corresoponds with the bad note and just reseat it. Chances are it will come back to life. If not there could be a problem with the card itself. The three top boards are all on hinged joints to give you easy access to the modulator cards below. These can be seen in the third picture.

thanks! any idea how those cards are organized? i know nothing about them... don't have mine in front of me, but from your pics it looks like there are 24. 71 keys, know how they're split?

There are three top boards. You can see the three boards in the first picture. All three top boards have modulator cards running underneath. In this picture you are only seeing the first 24 keys. They just go from left to right. Meaning first key is the first card, etc. etc.

ohh - gotcha. thanks again!

unfortunately it's not the card causing the problem, checking it out now. that key has a slightly looser bounce than the others, i'm guessing it's simply not triggering. these things are a jungle inside...

editing once again...found my problem. brace yourselves for my incredibly formal technological terminology...
underneath that key, the plastic tab that the rubber strap hooks to isn't pulling it forward far enough to make the wires touch the rods. awesome that the panels on the bottom can be removed to check this stuff out, b***h is that i'm still going to have to pull the keys out of this thing to see if that's fixable.

scratch that - found the source...all it is, a missing rubber cap that stops the hammer beneath the key. without it the hammer (or whatever that tiny part is called) pushes too far in the off direction and not far enough in the on direction when the key's pressed.

seriously rjd2, sorry i diverted your thread. just chance that we're both working on polymoogs at the same time...though you've gotta know 10x more than i do!

thanks, heath. youre right-divider card issues. i took some V measurements, and the 24 IC's on the divider board, MM5823 on the schem, have some duds among them. the 2 cards that sit in sockets on top are fine.


right now, it looks like the only place i can source them is this cat chris at flatkeys.com. gonna be expensive.....

Sucks. I have some MM5824's. I doubt they are interchangeable. You could also contact Dave at [email protected] He might have some spares laying around. Not sure if they will be cheaper though.

heath