PCM-42 Won't Modulate

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Toneseeker504

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Aug 1, 2021
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Hello + I hope this finds you well! Thought to ask about a bit of a riddle with a PCM-42. Have combed through it with the service manual and schematics, the unit works perfectly – except – it will not modulate. Can’t think of anything else to check. It's been recapped, pots are good, and all healthy otherwise. Sounds perfect, other than not being able to modulate. Wondered if anyone had ever seen this anomaly before, or had any ideas what part(s) to check on the board or which part is controlling the modulation. Thank you in advance for any leads!
 

radardoug

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Oct 10, 2009
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If you mean the lf oscillator is not modulating the delay time, do standard testing on that. Is the oscillator running? Tip, is the led blinking? Do you have output from the depth pot?
Do you have it in input mix? Do you have no delay time set?
 

Toneseeker504

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Aug 1, 2021
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Thank you @radardoug ! The rate led stays on, can't get it to pulse. The depth knob changes the display time, but the time does not oscillate as I turn up. If I put the waveform on center indent, the time will change on input signal. But nothing I can find will turn off, or unstick the led on the rate.
 

radardoug

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So the oscillator is not oscillating. Find the schematic, and centre your efforts around the oscillator section. We cant do it for you, you have the hardware in front of you.
 

Toneseeker504

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Aug 1, 2021
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@radardoug -- I'll take another look around that section. When going through it, everything checked out on the board. Had hoped someone may have seen this before. I understand I'm missing something, it's been a mystery for sure.
 

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