[sdiy] Prophet 5 rev2 conundrum

NT nathan at idmclassics.net
Wed May 3 10:45:23 CEST 2023


I am pretty familiar with the Rev 2 but haven't had any notable tuning
issues on my own. It's always been my assumption that with the voice board
disconnected that tuning aborts instantly because it gets nothing from TUN
MUX when it initializes tune as decribed in 2-20 - this only lasts a few
milliseconds and counts off each osc before it goes into the individual
voice tuning where it does a lock up, and would do the same if it can't get
past that stage with a voice board installed.

Am I misunderstanding and you're getting no activity at all on pins
6/9/10/11 on u423 and 428 at boot or when pressing tune? To me this would
be a total failure to initialize tuning. If there's data there, the 4051
outputs, on the comparator output when hitting tune, then it's failing
during that tune init window for some reason and something mentioned in
that paragraph is off.

I wouldn't rule out 4051 failure on any used for pitch info that haven't
been replaced yet even if they appear to work fine - one of the last
Prophet 5s I worked on everything was great until you set certain pots to 9
o'clock and those parameters would glitch out. The 4051 they all shared
didn't like how they were set for whatever reason.. Something like that
happening on VCO bias (where all outs are set to the same level) could
easily cause tuning init to fail before it gets to goes into the voice
tuning.

re: firmware as mentioned by Brian - Riku probably has the most experience
with the firmware and would be able to give more insight into aborting the
tune procedure vs failing while tuning.

Nathan

On Wed, May 3, 2023 at 2:45 AM g m montalbano <montalbanogm at gmail.com>
wrote:

> These are all good points.  However ---
> All oscillators are sounding; all voices are playing; the two 4051 mux ICs
> are receiving their correct timing signals; all signal paths on the voice
> board are functioning.  The tune procedure is just not starting.
> On the rev2, if one or more of the oscillators is bad, the tune procedure
> will hang up; but in this case, the procedure is not starting at all.
> The rev2 CPU board will boot without the voice board connected to it; it
> comes up immediately, without tune -- this is the effect I'm seeing now,
> even though the voice board IS connected & otherwise functioning correctly.
>
> On Tue, May 2, 2023 at 11:22 PM NT <nathan at idmclassics.net> wrote:
>
>> On the first paragraph of page 2-20 in the service manual it says if any
>> oscillator is missing from the tuning mux it will abort. Greg - you haven't
>> mentioned it but do you get signal from any/all voices? Or presumably you
>> the SSMs are tested and you're still hitting this issue?
>>
>> If the SSMs, mux, comparator, and path to connectors are all good, i'd
>> start looking earlier than the VCOs. paragraph 1 of 2-20 says that auto
>> tune engages all S&Hs that control tuning, so I would check each VCO to
>> make sure they're getting set to the required 300hz at tune start. Even if
>> they're working OK outside of the auto tune procedure, something could be
>> causing one to go way out of range during tune, so all 4051s and
>> opamps before the VCOs should be looked at.
>>
>> Service manual for everyone's reference:
>> http://www.synfo.nl/servicemanuals/Sequential/SEQUENTIAL_PROPHET-5-REV2_SERVICE_MANUAL.pdf
>>
>> Nathan
>>
>> On Wed, May 3, 2023 at 1:50 AM Adam (synthDIY) <synthdiy at adambaby.com>
>> wrote:
>>
>>> 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> 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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