Vintage Synth Repair

The story with mine is different. I got my Rev 3.2 with no powersupply
and bottom/read panel. I DIYed a powersupply for it and had a few
issues, among them dropping voltages. I found that I used tantalum
caps of voltage rating which is too low for that circuit (although
they all rated 25v), and that the wires going from the synth to the PS
rails had bad contacts. Once I solved those problems, mine seems to be
stable for two days now. BTW, I use a small 5cm CPU fan on the heat
sink to keep it cool, ran by a separate power supply. The leds do
flicker when rotating knobs, but afaik this seems to be a known issue
on P5's.


If yours is equipped with MIDI, then you probably will need to raise
the +5.6v rail voltage a bit so the +5v rail on the main board doesn't
drop too much. An article that I've found explains that the MIDI
circuit brings the +5.6v rail to it's limit. It suggests adding
another diode in series with the diode on the power supply which
connects the 7805 regulator to the ground, thus rising the headroom
voltage from +5.6v to +6.2v, resulting in higher voltage on the +5v
motherboard rail. 

I tried that on mine and got +5.5v instead of +5v, which seems a bit
too much, but anyway,  once I realized that the voltage problem
occured as a result of bad contacts, I didn't need that anymore. But
yes, expect the MIDI equipped P5's to have voltage issues otherwise
they are work-around somehow. 




--- In vintagesynthrepair@yahoogroups.com, "easterleggs"
<milotoll@...> wrote:
>
> It has remained perfectly stable since the fix.  Are you having a
> similar problem?
> 
> Thanks,
> Stephen
> 
> --- In vintagesynthrepair@yahoogroups.com, "gil_we" <gil_we@> wrote:
> >
> > Hi there.
> > 
> > Is your P5 still stable after the fix you describe here ?
> > 
> > Thanks!
> > 
> > 
> > --- In vintagesynthrepair@yahoogroups.com, "easterleggs" 
> > <milotoll@> wrote:
> > >
> > > My Prophet 5 display flicker/lockup problem has been solved!  When
> > > replacing the LM337 voltage regulator I noticed that the screw and 
> > nut
> > > that connects the regulator to the large heat sink would not tighten
> > > beyond a certain point.  I thought it was snug enough and proceeded
> > > with my power supply cap replacement.
> > > 
> > > Upon second inspection the threads of that screw had worn away so 
> > the
> > > LM337 regulator (negative supply) was not making good contact with 
> > the
> > > heat sink.  Following the recommendations of the service manual, I
> > > replaced all three regulator screws with new #4 3/8" round head
> > > machine screws.  I cleaned the heat sink, mica tabs and regulators 
> > of
> > > any old heat sink compound.  I reapplied heat sink compound to the
> > > back of the regulators and to the mica tabs.  I fastened the
> > > regulators to the heat sink using the nylon washers, lock washers 
> > and
> > > nut with new screw prepared with a blue, medium duty threadsealer
> > > compound.  I tightened the screw until the nylon washer started to
> > > deform.  
> > > 
> > > The Prophet 5 now does not exhibit the initial flicker/glitch noise
> > > problem I had before changing ALL power supply caps NOR does it 
> > jitter
> > > and lockup after an initial 15-20 minute warm up.  Lesson learned:
> > > Don't neglect the power supply in the Prophet 5!
> > > 
> > > Thanks for all the suggestions!
> > > 
> > > 
> > > --- In vintagesynthrepair@yahoogroups.com, "easterleggs"
> > > <milotoll@> wrote:
> > > >
> > > > My Prophet 5 had an issue where the LED's would occasionally 
> > flicker
> > > > slightly and introduce a glitch noise into the audio output.  I've
> > > > since replaced all  power supply caps (tantalums replaced with
> > > > electrolytics) and the LM337 voltage regulator.  After some 
> > initial
> > > > tests it seems to have remedied the glitch noise problem (even 
> > though
> > > > the LED's will flicker a bit when adjusting the FILTER CUTOFF
> > > > pot...?), BUT (isn't there always a 'but'?!) a new issue has
> > > > manifested.  My Prophet is a rev 3.3 with MIDI.  When in preset 
> > mode
> > > > on any patch ending in the number 8, some LED's on unused buttons
> > > > flicker on and off, the numbers on the panel will scramble a bit, 
> > and
> > > > occasionally, the synth will skip into TUNE mode.  The problem 
> > doesn't
> > > > happen when I press the PRESET button to take it to panel mode nor
> > > > when the patch number ends in anything other than 8.  Is this a 
> > RAM
> > > > (6508 CMOS) problem?  Anyone dealt with this before?
> > > >
> > >
> >
>