[sdiy] prophet 600 weirdness

[klosmon]

Yo-
changed the muliplexers as a matter of course;  all the analog switches
check out ok.

Didn't think about flakey zeners, however;  someone also suggested a
"bouncy" power supply on startup -- will check them both out.

Thanks -- and "wild" guesses were what I was after.

~GMM

At 03:00 PM 9/15/2004 -0700, you wrote:
>A bad analog switch or multiplexer? Or possibly a bad reference voltage
>somewhere (a flakey zener regulator?). 
>
>Just a couple of guesses.
>
>
>
>Tim (that's MISTER Wild Guess, thank you) Servo
>
>"Imagination is more important than knowledge." - Albert Einstein
>
>
>
>
>> -----Original Message-----
>> From: klosmon [mailto:klosmon at earthlink.net]
>> Sent: Wednesday, September 15, 2004 11:21 AM
>> To: analogue at hyperreal.org; synth-diy at dropmix.xs4all.nl
>> Subject: [sdiy] prophet 600 weirdness
>> 
>> Cross-posted (and hopefully not echoed...)
>> 
>> Hola--
>> 
>> I've come across a bizaare problem with a P600;  have done about a dozen
>of
>> these, and never dealt with this particular quirk...
>> 
>> The problem is tuning.  When first powered on, the machine goes through a
>> mandatory tuning routine for all 12 oscillators & 6 filters.  If it
>> encounters a problem (or what it PERCEIVES to be a problem), it will scale
>> the particular oscillator(s) out of range.  OK, that's routine.  But on
>> this particular unit, it always comes up with all 12 oscillators scaled
>out
>> of range.  Pressing Tune once might bring one into range;  pressing it two
>> to five more times will finally get all oscillators functioning correctly
>> -- until the next time it's powered up.
>> This problem is specific to the voice board:  I've switched a working CPU
>> board & Voice board with this unit, and the problem follows the voice
>> board.  I've scaled all the oscillators & filters per service manual
>> instructions, with no change.  I've traced all the signals to & from the
>> DAC (even tried the DAC in another unit, and tried another DAC in this
>> unit) -- no change.  I've gone through & replaced all opamps & comparators
>> in the tuning path -- no change.
>> I've removed, tested, cleaned the pins & sockets of the CEM chips &
>> reinserted them -- no change.
>> Can anyone direct me to something I'm missing here?
>> 
>> Thank you, and my (increasing) grey hair thanks you.
>> 
>> ~GMM
>> 
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