Alas, that wasn’t it. Solid connections there, and when I measured the output of the ALT buffer I made sure to probe at the input pin of Z46 in the timer circuit, so I know the signal was making it that far.

Past that I’m not sure I understand the tuning routine enough to know what I’m looking for. Could it be something in that timing circuit?

e

The most likely culprit is a bad connection to the output board. When

tuning, the signal from each channel is routed to a different line (ALT
OUT) to the output board, where it is squared up and fed back to the main
board on the SYNTH ZCD line. It could even be a bad ground to the output
board, which could account for the audio noise, too.

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Ciao, Paul D. DeRocco
Paul mailto:pderocco@...