Vintage Synth Repair

Hello, all.  I'm new to the group.

I just got a Moog Prodigy of Ebay that looks like it's been sitting in
a garage for 20 years.  It plays, but it needs work, and I plan on
replacing some parts.  Primarily, I'm trying to figure out how to tune
it.  Specifically, the keys are not in tune with each other.  That is
to say, if I play a scale, the low C and high C are a half step apart!
(and octave + half step)  When playing melodies, it sounds terrible
because some notes are ok and others are flat/sharp.  Just wondering
if anyone had any suggestions/knowledge on what would control
individual note/key pitch, and how I can repair/correct this problem. 

On a side note, which components on a Prodigy (or any Moog synth,
really) tend to fail or falter over time?  My point is, I'm wondering
where to start, as far as replacing components goes, that I may see
improvement in overall sound quality and playability/reliability.

Thanks!

Dustin

dustin-
a prodigy is a good place to start if you plan on learning how to
maintain an analogue synth, since they're quite straight-forward inside.
the problem you have there is a simple adjustment, so long as the
tuning error you get is stable & not changing with time, temperature
or vibration.
switch off the osc sync & set the interval controls (octave switches &
interval knob) so that the oscillators are in tune when you play the
lowest note. 
does the tuning discrepancy seem to be the same for both oscillators
or do they drift apart as you play up the keyboard?
is your prodigy a later model with cv in/out already fitted? if not,
you will need to get under the hood & find the test points on the
circuit board. keyboard cv is marked. it may or may not be tracking
correctly at 1V/octave.... there are adjustments for the key volts,
also for the cv buffer gain & for the tracking of each oscillator, +
high-trims for each aswell.

but before you go adjusting anything, make sure the keyboard connector
(carries voltage from the keyboard to the main board) is good. I
removed the plug & socket in my older prodigy & hard-wired these
connections for better reliability.

after this, you are going to need schemos & the adjustment procedure;
these are all over the internet & may even be in the files section
here. some of the adjustments can be reached from outside the case,
depending on the serial number of the moog, but you need a long narrow
flat-blade screwdriver with an insulated shaft.

let us know how you get on- we're here to help!

duncan.

Thanks, Duncan.  It is an older model, without the cv in/out.  I found the user's manual and the service manual, but haven't yet found the adjustment procedures within them.  I did see 4 flat head adjustment pots, as you mentioned, but will likely un-mount the board for easier access.  (I won't have access to the synth until tonight, after work)  I'll try some things tonight and post again tomorrow with my findings.  For arguments sake, what if the tuning errors are NOT stable, IE, changing with time or temp as you mentioned?  

Thanks,

Dustin




________________________________

From: duncan <ferrograph@...>
To: vintagesynthrepair@yahoogroups.com
Sent: Thursday, December 4, 2008 1:50:57 PM
Subject: [vintagesynthrepair] Re: Moog Prodigy out of tune...


dustin-
a prodigy is a good place to start if you plan on learning how to
maintain an analogue synth, since they're quite straight-forward inside.
the problem you have there is a simple adjustment, so long as the
tuning error you get is stable & not changing with time, temperature
or vibration.
switch off the osc sync & set the interval controls (octave switches &
interval knob) so that the oscillators are in tune when you play the
lowest note. 
does the tuning discrepancy seem to be the same for both oscillators
or do they drift apart as you play up the keyboard?
is your prodigy a later model with cv in/out already fitted? if not,
you will need to get under the hood & find the test points on the
circuit board. keyboard cv is marked. it may or may not be tracking
correctly at 1V/octave... . there are adjustments for the key volts,
also for the cv buffer gain & for the tracking of each oscillator, +
high-trims for each aswell.

but before you go adjusting anything, make sure the keyboard connector
(carries voltage from the keyboard to the main board) is good. I
removed the plug & socket in my older prodigy & hard-wired these
connections for better reliability.

after this, you are going to need schemos & the adjustment procedure;
these are all over the internet & may even be in the files section
here. some of the adjustments can be reached from outside the case,
depending on the serial number of the moog, but you need a long narrow
flat-blade screwdriver with an insulated shaft.

let us know how you get on- we're here to help!

duncan.

Prodigys are easy to tune. There s 4 little holes in the back that each go through to the tuning pots for each oscillator. Set all the tune pots on the synth

>>Prodigys are easy to tune.
> 
> There's 4 little holes in the back that each go through to the
tuning pots
> for each oscillator.
> 
> Set all the tune pots on the synth to their central position.....<<

jez, you're right, but only if the keyboard cv & buffer are behaving
themselves. otherwise, dustin could get the synth in a right mess &
maybe even wreck the trimmers... I have seen this happen to prodigies
& rogues where the real problem was the keyboard connections...
first thing to do, as part of the warming up process, is make sure
that the tuning errors are not varying in a way that suggests a bigger
problem.

duncan.

>>For arguments sake, what if the tuning errors are NOT stable, IE,
changing with time or temp as you mentioned?<<

check the connector from the keyboard to the main pcb. the part that
is soldered to the pcb is under some mechanical stress & the solder
joints can fail.
if both oscs are out by the same amount, it could be the keyboard
scaling buffer. see if the filter tracks properly (make it
self-oscillate & use a guitar tuner). 
measure the keyboard volts & adjust them to 1V/oct if necessary.

if the oscillators are both out /by the same amount/, then don't start
adjusting their tuning or scaling to remove an error that's being
introduced by something else.  

d.

Update:

I've been able to tune both oscillators together, but I can't seem to
get the synth to be in tune with true C.  That is, even using the
tuning pot on the front panel, I can't get the synth to be in tune
with another keyboard, a guitar, or another instrument that is already
in tune.  This will be an issue if/when I plan on recording.  Any
suggestions, other than continuing the knob twiddling? (which i have
done a lot of)  Thanks.

Dustin

hi,
long years ago i owned a prodigy, and the problem was the pitchbend pot, which did not reset exactly, so i changed the pot and it was ok.
regards,
rich


--- ramdust <ramdust@...> schrieb am Mo, 15.12.2008:

> Von: ramdust <ramdust@...>
> Betreff: [vintagesynthrepair] Re: Moog Prodigy out of tune...
> An: vintagesynthrepair@yahoogroups.com
> Datum: Montag, 15. Dezember 2008, 16:06
> Update:
> 
> I've been able to tune both oscillators together, but I
> can't seem to
> get the synth to be in tune with true C.  That is, even
> using the
> tuning pot on the front panel, I can't get the synth to
> be in tune
> with another keyboard, a guitar, or another instrument that
> is already
> in tune.  This will be an issue if/when I plan on
> recording.  Any
> suggestions, other than continuing the knob twiddling?
> (which i have
> done a lot of)  Thanks.
> 
> Dustin
> 
> 
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