[sdiy] Prophet 5 rev2 conundrum

The SynthiMuse synthimuse at gmail.com
Wed May 3 20:16:18 CEST 2023


It seem like this might be a job that a 'Tracker' could help with.
The 'Huntron Yracker' is a commercial version of this idea.

It's an audio transformer that's isolated from ground, driving 1mA at 3V
p-p with a 1khz sine wave. This is the same voltage and current you get
from the diode checker range on a DMM.
You set up your ( preferably analog) scope in X-Y mode and set the gains
and offsets so a short and an open circuit are centred and scaled on the
screen.
Once you connect it up, you should expect to see the waveforms described in
the attached image.
The idea is that you set two voice cards side-by-side on the bench,
unpowered and you compare the waveforms between points on all the
pins/components of the boards.

I usually just connect the black probe to a ground line and move the red
probe from point to point.
You'd be amazed at the subtle differences that can be shown with this setup.

1kHz sinewave is an OK frequency but it will show a fat electrolytic as a
short circuit. For that situation,  you can change the frequency to 50Hz
and you'll see the impedance of the fat cap. Conversely, you can crank up
the frequency towards 20KHz to show smaller value caps.
I hope all the above makes sense.
As I say, a 'Tracker' is most useful on the very rare occasions you have a
couple of boards, one good, one bad.
Hope this helps.
Gerry


On Wed, 3 May 2023, 17:32 g m montalbano via Synth-diy, <
synth-diy at synth-diy.org> wrote:

> Correct, there's no change on pins 6, 9, 10 and 11 of the 4051s.  Have
> swapped out all four of them on the voice board with no effect.
>
> And yes, with the voice board disconnected, the CPU board bypasses the
> tune routine both on powerup and when Tune is pressed. This is exactly
> what's happening here with the voice board connected; and it's only this
> one voice board (swapping in another voice board works just fine).
> I'm going to go with Brian's suggestion of finding a high-res recording
> scope to
> a) observe all the steps on a correctly functioning tune routine and
> b) compare it to the failing board
>
> Thanks to everyone who's replied. I'm going to put this thing aside for
> while to search for a suitable scope & work on some of the other work
> that's piling up here.
> ~GMM
>
> On Wed, May 3, 2023 at 1:45 AM NT <nathan at idmclassics.net> wrote:
>
>> 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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