Moog Music Forum

Hi all!

I have a 1981 model D S/N 11000+, so originally it comes with the "new oscillator board" (uA726 board). The Moog is fully functional with this oscillator board.

I have bought an "old oscillator board" (CA3046 board) from about S/N 5500 that I want to use in this Minimoog (read carefully: retrofit an OLD board in a 11000+ Model D). This old board was of unknown functionality so I had to do some repairs. These repairs were succesful (replacing a 741 to inverse OSC3 CV signal, cleaning edge contacts and reflowing all solder joints to improve pitch stability). Now OSC3 works fine, OSC2 as well and both are stable.

HOWEVER... Before I did the work, OSC1 was functional albeit a tiny bit unstable at times. After I did the other repairs, OSC1 is dead. I don't think it's a bad contact at the edge connectors. There are two possible causes I can see: either I did something wrong when reflowing all solder joints, though I've inspected it carefully and I doubt it. Or, and this is more likely: when reseating the CA3046, I accidentally installed the top one backwards. I have since replaced both CA3046, but I'm afraid I've killed another component when the synth was powered on with the IC2 CA3046 in backwards.

OSC1 seems to be oscillating nowhere in the circuit: when measuring IC2 pin 5 (directly connected to capacitor C1) I see no waveform. What I do see there is a negative DC that increases negatively with pitch. Something like -2.70V at LO setting lowest note to -5.63V at 2' setting highest note (I might have to check the exact values). I also see this DC fluctuating at OSC3's rate when I enable pitch modification. So the oscillator appears to get the CV signals and output something related on IC2, but it appears to be unable to oscillate. -4V, -10V and +10V are all available at IC3.

I hope anyone here can help me find which component(s) died. I have a multimeter, an analog oscilloscope, soldering iron, desoldering iron, Chinese component tester, limited electronics knowledge.

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(related info: for this retrofit, I would have to ADD the 90.9 ohm R58 at the OSC1 range panel pot. I have not yet done this, but from my pre-disaster tests it appeared to not need this to generate sound. Another small issue I have with this board, but of low priority unless related to the main problem: the frequency panel pot of OSC2 is way too sensitive/has a way too large range. I don't know if that's also related to the missing R58 or something else is going on there. It's fine with the new oscillator board, but with the old one way too much range. I can't remember if this was already acting like this pre-disaster. OSC3 frequency panel pot works as espected.)

By the looks of it, something bad happened to the E402. Its top looks a bit different from the other 2. The others are plain black, the one in OSC1 shows some gold cracks. You can't feel them, only see them. I'll replace it when I get around to it. Hopefully the E402 took one for the team and no other components are dead. C1 doesn't look blown, I can also try to check it in-circuit. I'm starting with E402 and C1 because I understand this is the heart of the oscillator. I don't known if other transistors (eg Q9, Q10) are critical to be able to oscillate at all.

As for the overly sensitive frequency panel pot of OSC2, this is a bit of a mystery to me. Looking at the circuit, could this perhaps be caused by a dead R70, R94 or R87 that causes too much of the "OSC2 TUNE" signal to enter the CV mixer? I don't see other relevant components for this problem, given that the oscillator itself works fine CV wise (tracking across the keyboard, range switch...) and the frequency panel pot works but too sensitive.

R94 seems dead measured in circuit, so that will likely solve the osc2 freq pot issue. Unless something else is dead too.