Vintage Synth Repair

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?
>