[sdiy] Prophet 5 rev2 conundrum

brianw brianw at audiobanshee.com
Fri May 12 00:49:40 CEST 2023


Thank you for sharing the solution.

I've definitely had multiple repairs where a stranded wire had a break (that would be fixed at random when moving the board) or a trace that was cracked (that would similarly change at random when flexing the board - either when probing or just disassembly or reassembly). Another common failure is solder joints that have cracked over the decades - these benefit from a quick reflow (in a few cases, I never found the cracked solder joint, but just reflowed them all with success).

As for whether it's interesting, we don't have the luxury of exciting repairs. Hopefully, the excitement is saved for after the instrument is repaired! The repairs themselves can be boring and tedious, but they're worth the pain.

Congratulations,

Brian


On May 11, 2023, at 10:07 AM, g m montalbano 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 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 brian 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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