[sdiy] Prophet 5 question

[Karl Ekdahl]

Hi list

I found the culprit to be a single last tantalum i had not seen earlier, of course it was the one that was dead.

Right now the synth makes sound, but it's weird; the keys are working very on/off, different keys will not work at different time just to start working all of the sudden. I've checked the mechanics and they all measure fine, i've also reseated the keyboard ribbon cable several times. I'm starting to wonder if there's something up with the keyboard scanning circuitry.

Another problem i have is that oscillator A is very faint on all voices, i can hear it if i turn down oscillator B all the way - and it's the same for all voices. I'm guessing there's some mixing circuitry common for all voices that i need to look at.

Karl

--- Den ons 2009-04-01 skrev Barry Klein <Barry.L.Klein at wdc.com>:

> Från: Barry Klein <Barry.L.Klein at wdc.com>
> Ämne: RE: [sdiy] Prophet 5 question
> Till: synth-diy at dropmix.xs4all.nl
> Datum: onsdag 1 april 2009 21.13
> Wow, I guess all P5's I worked on
> were 2.2's or just my memory retention is
> flawed - I don't remember that strip.  No, I would not
> remove it.
> 
> 1. remove all socketed IC's that take 15V (maybe one of em
> is the culprit)
> 2. get an ohm meter that is good at reading low resistances
> - maybe an ESR
> meter for caps...?  See if you can find the short by
> finding the lowest
> resistance path.
> 3. Get a lab supply with current limit capability
> function.  Short its
> output and set current limit to 2A.  Connect to +15V
> input and increase
> voltage from zero until you get the 2A or so.  Feel
> for warm traces or
> parts.  Or go exotic and take a picture of it with IR
> film(!).
> 
> Barry
> 
> 
> 
> 
> -----Original Message-----
> From: Karl Ekdahl [mailto:elektrodwarf at yahoo.se]
> 
> Sent: Wednesday, April 01, 2009 11:42 AM
> To: Barry Klein; synth-diy at dropmix.xs4all.nl
> Subject: RE: [sdiy] Prophet 5 question
> 
> 
> > have a photo?
> 
> http://www.sdiy.org/knas/temp/IMG_3828.jpg
> 
> > sure you didn't put a cap in backwards.... :-)
> 
> Yeah...  it didn't work once powered on and it's
> apparently been sitting in
> a closet for 10 years so after noticing it not working i
> immediately found
> that the PSU was being shorted by this PCB so i replaced
> the caps... no
> backwards caps.
> 
> Karl
> 
> 
> > 
> > 
> > -----Original Message-----
> > From: synth-diy-bounces at dropmix.xs4all.nl
> > [mailto:synth-diy-bounces at dropmix.xs4all.nl]
> > On Behalf Of Karl Ekdahl
> > Sent: Wednesday, April 01, 2009 11:01 AM
> > To: synth-diy at dropmix.xs4all.nl
> > Subject: [sdiy] Prophet 5 question
> > 
> > 
> > Hi list, working on a faulty Prophet 5 which has a
> short on
> > the +15V supply
> > somewhere on the CPU board - i notice that there's
> some
> > kind of white
> > power-rail distribution tab that runs about 2/3 of
> the
> > upper PCBs, question;
> > is it a bad idea to desolder this entire thing? It
> seems
> > like it'd be so
> > much easier to find the faulty parts if the different
> parts
> > were separated
> > this way. And yes, i have replaced all Tantalum caps
> on the
> > board and
> > reseated all ICs.
> > 
> > Karl
> > 
> > 
> >      
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> 
> 
>      
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