Hi, I just picked up an ESQ-1, metal case with the spring-action keys. I’ve cleaned it up and reseated the chips and ribbon cables, did a soft-reset. The firmware version is 3.5… For the most part it’s working well now, just a couple of remaining issues I’d appreciate some help with!
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Biggest issue right now is when notes are triggered they keep playing. Not on a per-key basis, the whole keyboard is responding the same way. It doesn’t appear to me to be mechanical because it’s happening all the time - I have some intermittent issues with some of the individual keys and this is different. Any ideas what might be the cause? I’ve tried placing and shorting a 1/4" cable in the sustain jack and it doesn’t appear to make a difference (I don’t have a pedal to test with).
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Second issue is on some presets (looking at you “Kalimba”), notes are intermittently distorted, sound bad, like fuzzy. When it happens, repeatedly pressing a key will result in it happening reliably for that note. I’m guessing it’s voice-related because of this but it doesn’t always cycle in 8 note sequences so it feels like it doesn’t happen every time. I’ve reseated the filter chips, that didn’t help. I’m reluctant to replace these as they’ll cost more than I paid for the synth! So before I do that, any recommendations on other repairs I can do to fix? I am comfortable replacing caps if that’s recommended, just want to make sure it’s a good direction before I do as I know how fragile traces can be on these old boards.
Quick update… I replaced the transistors in the sustain circuit (Q1 and Q2) and that fixed the sustain issue! Hurrah! I also replaced the electrolytic caps in the filter circuit but that didn’t have any effect on the issue with the voicing. It appears to be related to resonance, the issue almost goes away when I turn the res down so I’m going to investigate that further. Seems taking things apart I also somehow broke velocity sensitivity so I need to look into that too now 
Another update. I pulled the filter chips (CEM3379) and swapped one in to see if I could determine if the chips were bad. What I found was that placing a chip in different slots resulted in the same behavior for that slot… So the same chip that sounded bad in slot 1 sounded fine in slot 2, etc. So I do not think at this point that the chips themselves are bad - it seems that it must be something to do with the signal per-voice? I’m wondering about the op-amps? I will study some more to see how the LM318s are involved. Interestingly enough I metered the voltages on the LM318s and all of them are showing ~9v on their positive / negative inputs, save for 1, which is ~12v. This seems odd but it may be by design.
Ok, another update - posting in case this helps others with similar issues. I believe I found the problem. It is with one of the SSM2300 chips to the left of the CEM filter chips. I used an oscilloscope while playing notes and found that the notes that sounded with issues in the first voice chip had a square wave pattern on the resonance pin, whereas on the second voice which sounded good the resonance pin was receiving a flat envelope instead. So the garbled audio was being caused by an oscillation of the resonance. I traced that back to pin1 of the lower left SSM2300 (same pattern when a note was played). I swapped that chip out with one of the others and the issue disappeared (but caused other issues with the slot I placed it in), so I am pretty certain it’s the chip itself and not any supporting circuitry or components. I’ve ordered a replacement SSM2300 and hopefully that will be all good.
Also, velocity started working again after I did a soft-reset. Seems the keyboard controller is kind of finicky and the soft reset must have cleared that up.
Cheers,
roland
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