[sdiy] Prophet 5 rev2 conundrum

brianw brianw at audiobanshee.com
Wed May 3 04:46:17 CEST 2023

The Prophet 5 Rev 2 uses a multiplexer chip to connect only one oscillator at a time to the comparator input. This should effectively remove all other oscillators from the tuning. However, it's theoretically possible that there could be some bleed-through from other oscillators if the multiplexer doesn't have perfect isolation. i.e. crosstalk.


On May 2, 2023, at 5:09 PM, chris wrote:
> I'm not that familiar with Prophets - do they use some funky PWM setup
> to turn off oscillators during tuning, as the Oberheims do? 
> Those can have auto tune problems if the PWM doesn't quite shut off at
> extreme settings.
> Chris
> On Tue, 2 May 2023 16:20:38 -0700 g m montalbano wrote:
>> 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 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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