There’s gotta be someone out here who knows a bit more about schematics to help me out here.
I’m trying to diagnose an issue with a Micromoog I just acquired. The issue is that as soon as I turn on the synth, and have it set to Tone, I can hear the tone but then it slowly drifts and goes quiet. I tried replacing a few ICs and that didn’t work. I wonder if this is being cause by a leaky capacitor. I just ordered some caps but I’m wondering if anyone can point me to how I can best diagnose this issue. Any ideas?
Are you sure that “TONE” modus should keep the tone constantly loud or audible?
As I understand the schematic, the envelope is still active on the filter, even in “tone” mode. And even if you bridge the VCA, the filter keeps following the envelope, so anything can happen to the “tone”.
According to my experience there is no infinite envelope. There are looooooooong envelopes possible, but not infinite. If you want to keep filter behavior constant to avoid any “tone” changes you must bridge the filter envelope, you must apply a constant voltage to the current sink IC 304 AND bridge the VCA.
I just mean that when I first tune on the unit, and let’s say it’s set to Bypass for an ‘always on’ Tone, I will hear it peak at the initial boot up, but then it slowly decays and become inaudible with some intermittent audio crackles. Envelope is at full sustain in this case so it should just be a constant Tone. I’m just wondering what component could be failing that would make it do that. The Oscillator is solid and is working as it should, it’s just the Filter tone that is behaving like this currently.
I have it set up as they are suggesting in the Service Manual.
I. VCF not passing signal. Reason undetermined.
Set controls according to Figure 5-1
Insure the input signal of roughly 40 mV p-p appears at base of Q501. > (PIN 4)
This same waveform should appear (40 mV p-p differentially) at top of ladder across emitters of Q509 and Q510. If signal is present here, then problem lias in the gain recovery amp
(Q514, IC501, Q512 or Q5I3). If signal is not present here:
a. Check VF at P3/4. It should be about + 80 mV DC ; 25 mV. If it is far off spec, (i.e., negative instead of positive), the filter will be perpetually shut down. The defect is in the VCF’ summing network. If Vf is correct, then:
b. Remove IC304 from socket. Short socket pin 14 to ground. If filter now passes the input signal, the current source is faulty (see Step F ). If it does not,then the ladder itself is defective. "
No. Setup the board as it would be for normal use. Then measure the voltage at pin 3 of the filter ladder (IC 502) over time, from starting the note until the tone disappears.
That’s interesting for the further analysis, because pin 3 determines the filter cutoff. It is connected to the current sink (misleadingly tagged as “CURRENT SOURCE” in the service manual) IC 304, another 3046 array. If the voltage changes there over time until the tone is lost, we can assume that the filter array itself (IC 305) is ok, as its behavior just follows the current sink.
Alternatively / additionally you can measure the voltage at pin 9 of the current sink (IC 304), as this directly determines how wide the current sink is open which determines the filter cutoff etc. etc.
Thanks for your help with this! Can you clarify what you mean by setting up the board for normal use?
Also to clarify, the Tone is completely inaudible after 3-4secs of the synth booting up. So it’s not like I can initiate it again once it disappears, it’s just gone from the audio signal. To trigger the tone in the audio signal, I have to turn the synth off and on again, but I don’t know how useful my measurements would be since that’s when the synth is booting/warming up (not super stable yet).
Update: R514 was acting up and VCF Balance was improperly set. Fixed R514 trimmer and adjusted the voltage to where it should be and there you go, Filter Tone is back. Thought I’d share the resolution to this problem.
