[sdiy] Prophet 5 rev2 conundrum

g m montalbano montalbanogm at gmail.com
Thu May 11 19:07:58 CEST 2023

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.


On Wed, May 3, 2023 at 8:32 PM Michael E Caloroso <mec.forumreader at gmail.com>

> 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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