[sdiy] Prophet 5 rev2 conundrum

Randel Osborne randel at redshift-consulting.com
Wed May 3 15:53:56 CEST 2023

It’s a long shot, but look for DC offset.

I’ve seen bad 4501s seem to operate normally, yet introduce a small DC offset in the output.

Audio will pass, and you will see/hear a waveform, but the DC offset may not allow the zero crossing to happen in the auto tune.

I try to leave my scope in DC mode when working on synths for just this reason.


> On May 2, 2023, at 12:41 PM, g m montalbano via Synth-diy <synth-diy at synth-diy.org> wrote:
> I've been working on these for twenty years, and thought I'd seen pretty much everything that could go wrong with them.
> Now, I have one that works perfectly except for the fact that it will not go through the tune procedure, either at powerup or when the Tune button is pressed.
> The confusing part is that the fault has been narrowed down to the voice board: if I swap in a working voice board, the unit functions correctly; if I install this voice board in another unit, the problem follows.
> As far as I can see, the ONLY parts on the voice board that relate to tuning are the two 4051 multiplexers (U423 and U428) and the LM311 comparator (and its associated resistors & caps). All of these components have been replaced; traces and resistors have been checked &  measured. All traces to the 60 pin connector between the CPU and Voice boards are good, as is the 60 pin cable.
> EVERYTHING is there, and everything works correctly except for Tune.
> I've been hitting my head against this for a couple of weeks, and would appreciate any suggestions as to where to look.
> Thanks.
> ~GMM
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