[sdiy] Prophet 5 rev2 conundrum

g m montalbano montalbanogm at gmail.com
Wed May 3 01:20:38 CEST 2023


Interesting, thanks, but I can't see similar circuitry here.
The rev2 P5 is problematic in that each oscillator & filter has a number of
opamp, CMOS switch and resistor inputs that can cause wild differences
between voices ( that fabled "analog warmth" that some people think makes
the rev2 superior to the rev3, but for service purposes is a pain in the
balls).
Will attempt to swap out all oscillator chips and monitor all CMOS and
opamps to see if anything is happening there.

~G

On Tue, May 2, 2023 at 3:20 PM Luís Marka via Synth-diy <
synth-diy at synth-diy.org> wrote:

> Not sure if it helps since the electronics are different, but I had an
> issue with autotune in 2 voices on my Memorymoog clone. Turns out the
> Glide time trimpots for those voices was adjusted too high, and the CV
> injected into the VCOs was showing a slope when switching between the 0
> / 5 / 10V CV during the autotune routine, messing up the frequency
> counter and making autotune impossible. Readjusting the trimpot solved
> the problem (in the Memorymoog, each glide trimpot affects 2 voices).
> Maybe you could investigate how that particular voice reacts to
> frequency CV during autotune?
>
> Cheers!
>
> On 2023-05-02 2:11 p.m., g m montalbano via Synth-diy 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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