[sdiy] Prophet 5 rev2 conundrum

g m montalbano montalbanogm at gmail.com
Fri May 12 03:45:31 CEST 2023


After every other approach had failed, inspection under strong light & high
magnification revealed the crack.
Some experimentation with the probe showed the intermittent nature of the
connection.
It's strange how many intermittent faults I've found in rev2s over the
years -- haven't seen half that many in rev3s or any other synths.
~GMM

On Thu, May 11, 2023 at 11:29 AM Jimmy Moore <jamoore84 at gmail.com> wrote:

> How did you end up finding the crack?
>
> On Thu, May 11, 2023, 11:13 AM g m montalbano via Synth-diy <
> synth-diy at synth-diy.org> wrote:
>
>> Finally solved this -- and I'm sorry to say it's not very interesting.
>>
>> There was a microscopic break in one of the ground traces going to the 60
>> pin connector on the voice board. When pressing the meter probe to it, it
>> made contact and showed as good; with the probe removed, contact was lost,
>> and the tune circuit was lost.
>>
>> Hope to come up with something more interesting & illuminating next time.
>>
>> Thanks to everyone who contributed ideas.
>>
>> ~GMM
>>
>> On Wed, May 3, 2023 at 8:32 PM Michael E Caloroso <
>> mec.forumreader at gmail.com> wrote:
>>
>>> Short the TUNE button contacts.  This will loop the autotune process
>>> then you can start probing on the 'scope.
>>>
>>> I know that a DC offset at the audio autotune summing buss will mess up
>>> the autotune algorithm.  Check each voice audio at the summing buss for DC.
>>>
>>> MC
>>>
>>> On Wed, May 3, 2023 at 9:23 PM brianw <brianw at audiobanshee.com> wrote:
>>>
>>>> Indeed. Tests have confirmed that the EPROM chips (I think there are
>>>> three) have the full and uncorrupted tuning subroutines in them, because
>>>> the same digital CPU board can tune a known-good analog Voice board.
>>>>
>>>> You're lucky to have spare working P5R2 boards for these diagnostics
>>>> (despite the frustration so far).
>>>>
>>>> Brian
>>>>
>>>> On May 3, 2023, at 3:45 PM, g m montalbano wrote:
>>>> > All very true. However, the problem isn't on the CPU board, since
>>>> that board works correctly with another Voice board.
>>>> > The power supply, front panel,CPU board and 60 pin cable are all good
>>>> --the fault is on the Voice board.
>>>> >
>>>> > On Wed, May 3, 2023 at 2:49 PM Jay Schwichtenberg wrote:
>>>> >> For somethings not in the analog realm.
>>>> >>
>>>> >> One of the first things to check in old systems like these after the
>>>> >> power supplies is the reset circuit.
>>>> >>
>>>> >> If I remember correctly there are multiple EEPROMs in it. Could be
>>>> the
>>>> >> tuning code in one of them could of gone bad. EEPROMs don't last
>>>> forever
>>>> >> and if the code is bad the processor would wonder off into oblivion.
>>>> >>
>>>> >> Might very carefully pull the EEPROMs and reseat them to see if a
>>>> socket
>>>> >> connection might of gone bad too.
>>>> >>
>>>> >> These are they types of problems logic analyzers are good for. Big
>>>> pain
>>>> >> to setup but the problem maybe a lot easier to find.
>>>> >>
>>>> >> Jay S.
>>>> >
>>>>
>>>>
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