Vintage Synth Repair

Howdy all, I have recently moved and when I used my Moog for the first time at my new apartment, I noticed that my resonance was very weak. When I have the res turned all the way up, it sounds like it were turned up halfway. The filter used to self oscillate, and no-longer will. Other than the resonance being attenuated, I have observed no other changes. Looking at the schematic (below 4100) for the filter section, there is: a trimmer, a pot and a lot of ICs, things that tend to go bad from my limited experience. I will be trying to fix this problem soon, and I am curious if anyone has any ideas of what I should be looking for. I have the schematic and a DMM, but no o-scope. Any help would be appreciated, and I will certainly share my results, or lack of with the group. Thank You,Ben

hello,
  I don't have any experience with synths that were made before microprocessors were added, which includes all the Moogs, ARPs, Korgs, Yamahas, Rolands, etc...  It's the main reason that I joined this group; to learn.  
  I suspect that the section in question is the EMPHASIS and EMPHASIS TRIM areas.  The chains of transistor pairs are difference amplifiers, I believe. Not unlike the first stage of an op-amp.
  Check for mechanical problems first.  Bent and unclipped leads from components touching traces.  Try resoldering the connections between the components and the circuit board.  I've read that 4000 series CMOS IC made in the 1970s often break down after thirty years.  Try replacing the 4007 matched-CMOS-MOSFET ICs on the board.  These are still available because they are used for footswitches in guitar stompbox effects.

  A possible oscilloscope substitute might be one of the various PC sound card oscilloscope programs available on the web.  They are limited to the general range of the Audio Line input (the blue stereo mini-jack) of the sound circuit of the PC.  That is the signal should be limited to 1-2 volts peak-to-peak (no accidental +12v); AC coupled (there is a capacitor on the input to the sound chip); and audio frequency range since the input signal is sampled at CD rates (44,100 samples per second.) Some newer sound ICs on PC motherboard sample up to 96000 samples per second.  
  These PC scope programs can make long digital recordings of the input signal.  You could use an audio recording program to record ten minutes of dual trace signal at 44.1K, something that a digital storage scope costing less than 10 grand couldn't do.

--- On Sat, 5/16/09, benjamin_garland <benjamin_garland@...> wrote:

> From: benjamin_garland <benjamin_garland@yahoo.com>
> Subject: [vintagesynthrepair] Prodigy Resonance Problem
> To: vintagesynthrepair@yahoogroups.com
> Date: Saturday, May 16, 2009, 11:47 AM
> Howdy all, I have recently moved and
> when I used my Moog for the first time at my new apartment,
> I noticed that my resonance was very weak. When I have the
> res turned all the way up, it sounds like it were turned up
> halfway. The filter used to self oscillate, and no-longer
> will. Other than the resonance being attenuated, I have
> observed no other changes. Looking at the schematic (below
> 4100) for the filter section, there is: a trimmer, a pot and
> a lot of ICs, things that tend to go bad from my limited
> experience. I will be trying to fix this problem soon, and I
> am curious if anyone has any ideas of what I should be
> looking for. I have the schematic and a DMM, but no o-scope.
> Any help would be appreciated, and I will certainly share my
> results, or lack of with the group. Thank You,Ben
> 
> 
> 
> ------------------------------------
> 
> Yahoo! Groups Links
> 
> 
>     mailto:vintagesynthrepair-fullfeatured@yahoogroups.com
> 
> 
>

>>my resonance was very weak. When I have the res turned all the way up, it sounds like it were turned up halfway. The filter used to self oscillate, and no-longer will. Other than the resonance being attenuated, I have observed no other changes.<<

no other changes at all? is the signal as hot as it used to be with the emphasis turned down? 
does the filter roll off at 24dB/oct like it otter?

could be the feedback circuit that the emphasis control forms part of, or it could even be the pot itself. if something had failed in the filter's ladder, I'd expect other symptoms.... has the moog started to sound like a yamaha or do you still get the same filter sweeps at lower resonance settings? :-)

U16 would be a suspect, but 4558's don't usually fail assymetrically, & the other half is the main output amp....
C36 is worth a look, since it's an electrolytic.

& don't forget to let us know how you get on!

duncan. (2 prodigals, 2 rogues, 2 sources, LAMM, signature theremin)