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VCO problem, PWM CV leaks into pitch. I'm stumped!
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author VCO problem, PWM CV leaks into pitch. I'm stumped!
After a decade of flawless operation, one of my early issue "One Of Three VCO"s has developed a problem where sweeping pulse width from 0 to 100% causes an approximately semitone change in the VCO's pitch.

I have swapped out the 074 and 2903 that share common amplifiers in the PWM and pitch circuits. No change.

I can't for the life of me figure out how the PW inputs are leaking into the oscillator's pitch. Anybody have any ideas?
Dr. Sketch-n-Etch
Sounds like a possible shorting wire to me. Have you had a good look at all the panel wiring connections?
It could be a problem with the 0V connection to the module. But, I'd also check for contamination on the circuit board's surface. Sometimes 'water washable' solder flux can remain if the board hasn't been cleaned properly which can become conductive after a long time. But grubby fingerprints and spilt beverages are sometimes to blame.

Guinness ftw!

ok, I found the problem. This turned out to be one of those strange and difficult problems that required a some time to diagnose.

I was pointed in the right direction when I broadened the scope of my search from focusing on the PWM circuit to the whole board and noticed that the 10V supply was being modulated ever so slightly by sweeping PWM. This behavior was observed even when U6 was removed, which was unexpected.

Then I noticed that pin 9 of the LM723 (10V regulator) although unused in the VCO design, would fluctuate greatly with PWM modulation.

Finally swapping the LM723 with a known good part revealed that indeed the LM723 voltage regulator has marginally failed.

Interestingly, the PWM circuit must present a heavy load to the supply, because sweeping the other pots and inputs didsn't budge the supply like sweeping PWM did.

And we are done!

Thanks for the tips Tony and Etch-n-Sketch.
Excellent news Keith. I'm glad you got it sorted.

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