[sdiy] Prophet 5 rev2 conundrum

Adam (synthDIY) synthdiy at adambaby.com
Wed May 3 07:50:50 CEST 2023


Is the CPU jumping to the end of the process prematurely? What determines the end of the process? Does the CPU receive a signal form the voice board?

A

> On 3 May 2023, at 1:50 pm, g m montalbano via Synth-diy <synth-diy at synth-diy.org> wrote:
> 
> I've been looking for that -- the problem here is that the tune process won't even start.  Usually, on powerup or pressing Tune the front panel goes dark, the CPU section enables the 8253 programmable interval timer and the individual oscillator inputs to the two multiplexer chips are read through the LM311 comparator back to the CPU board.  But NONE of that is happening. Pressing Tune causes a split-second blink of the front panel, and nothing else.
> ~G
> 
> On Tue, May 2, 2023 at 7:49 PM brianw <brianw at audiobanshee.com <mailto:brianw at audiobanshee.com>> wrote:
>> 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.
>> 
>> Brian
>> 
>> 
>> 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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