1976 Minimoog Unstable Oscillator 1

[dzlvs8]

Hi, I'm struggling with determining what's wrong with my minimoog. I have a 1976 Minimoog that works perfectly for 20 minutes. but after it warms up oscillator 1 starts to have erratic changes to it's pitch. The issue only affects oscillator 1. A video of the issue is linked at the bottom of this post. If anyone has any thoughts on what could be the issue, please let me know. I am also attaching pics of all the boards.

Notes:

- I bought and installed a capacitor replacement kit 7 years ago and haven't had an issue until now.

- I have cleaned the two outermost boards contacts and reseated them in their connector.

- I have messed around with the oscillator 1 trim pot and it doesn't seem to be the issue even though they are original. I bought a replacement but I don't think the trim pot is the issue because oscillator 1 is stable for at least the first 20 minutes of turning on the minimoog.

- Oscillators 2 and 3 don't have the issue.

Here is a video of the issue: https://youtu.be/GFnmRl_juTE

[VCO]

Do you think the caps could be the wrong specifications? Although you said you noticed this problem recently. 47 year old Minimoog could be many things. Maybe it would be best to take it to a tech that has experience with minimoogs. I know if you have the wrong spec capacitors the pitch will drift, although I would assume that wouldn’t be the problem sense you replaced the capacitors 7 years ago. Wonder if that one oscillator might be failing . Good luck hope you figure out what the problem is and it’s an easy and inexpensive fix. I wonder what the most common issues are with minimoogs that old.

[dzlvs8]

Thanks for the reply. I replaced the caps over 10 years ago. This problem didn't start occurring until recently. I have always serviced my own equipment. I own many vintage synthesizers. I don't really want to bring it to a service tech. I was just hoping that someone could point me towards what some of the common causes of this problem were so I didn't have to start testing EVERYTHING on EVERY board.

[VCO]

That’s great you can service your own equipment. Hope you find the cause of the problem easily.I wonder if analog monster would have an idea. He’s here on this forum . He has a lot of technical knowledge..

[ummagumma]

Depending on how audible the problem is, you could try spraying cold compressed air on various parts to identify the troublemaker?

If it is related to temperature

[sdwillingham]

I would look at the voltages in the expo converter section, preferably with a scope. You can compare good oscillators to the bad one for all the various pins of the CA3046 IC. Likely some voltage there shows up as unstable for one oscillator and stable for the others. (You will have to sue the schematics to match-up the right pins. I recommend the scans at fantasyjackpalance.com.)

Once you find some unstable control voltage, we can work outward from there to suss out the cause.

[RL]

[sdwillingham]

Interesting. My post should be corrected: LM3046, not CA3046. Also, they appear to be socketed. I suppose you could swap the chips to see if the problem moves to a different oscillator.