[sdiy] Re: [AH] prophet 600 weirdness - resolved

[klosmon]

OK, cross-posted for the record...

Apparently the problem was two-fold:  for some reason, the tune comparator
circuit was whack - replaced the IC & each individual component, checked
the circuit traces, was all good... but it didn't work.  Wired up the same
circuit on perfboard, tied it into the voice board, and it would tune -- on
the third or fourth try.

Finally got down to IC4 -- the CMOS IC that level-shifts the signals that
control the on/off switches for the sawtooth waves of the oscillators;  it
was working, but NOT IMMEDIATELY -- when first powered up, it would kill
the sawtooth signals, impeding the tuning process;  when it finally came on
full, everything worked ok.
Replaced that chip, and all is now well (and this is the MOST time I've
ever spent on a p600).

Thanks to everyone who chipped in with suggestions.

~GMM

At 03:49 PM 9/15/2004 -0700, you wrote:
>Hi Greg,
>
>Wow, what an interesting problem.  You've addressed the problems I would
>have considered.  I'll take a look at the schematics and see if I can
>dream up anything.  This is a "good one."  If you solve it, would you
>please drop me a line.
>
>
>
>Thanks and good luck, Mike
>
>
>> 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
>> 
>> 
>> 
>
>_______________________________________________________________________
>Save over $140/ year on your Internet access
>Juno Platinum only $9.95/ month - less than 1/2 the price of EarthLink!
>Visit http://www.juno.com/connect to save money today!
>