Moog Music Forum

Hello,

I have an old ( first model ) moog prodigy with trouble. All is OK except the release in the in the loudness contour. Which switch release in position ON and Glide to zero, when I release a key, the tune is not keep with original tune, it goes down .

If I positioning glide to 5 it seem the trouble disappears. If anybody have an idea, I have schematic and service manual.

Hi Karen-

I just pulled up a schematic on line. It's a pdf divided into 3 pages but you may have a full hard copy all on one page. Are you capable of troubleshooting? The glide circuit is roughly in the middle-top section of the all-in-one hard copy or page 2 of the PDF; just below and to the right of the 32 note keyboard depiction at the top.

There is a trimmer (potentiometer/adjustable resistor) for the glide and you might start by trying to determine if the glide is operating as it should. It sounds like it is not and it could be as simple as adjusting this trimmer. Otherwise my best guess is the problem lies with one of the components in this circuitry for the glide function. I didn't find anything on line where someone had a similar problem so without having the unit on my bench this is about all I can suggest.

Let me know if you have any further specific questions.

When you press a key on the keyboard, the voltage created is stored (like a battery)by capacitor C21, and the capacitor is then isolated from being allowed to drain off this voltage by an FET that's a part of IC14 when the key is lifted.
IC12 is a high impedance opamp that buffers this cap, also helping to keep it from discharging, and keeping the note steady. If it is drifting, you will hear this change with longer volume envelopes, which expose the problem.
If either of these ICs are not functioning correctly, then the charge on this capacitor can be drained, and the pitch of the note will change, usually drifting down.

This drift can also be caused by contamination of the circuit board by dust, dirt, and especially by flux left behind from when the board was manufactured many years ago. This flux hardens over time and becomes more conductive, helping to drain the charge held by C21 when a key is released.

Have a tech clean this area of the PC board very well with flux remover or pure acetone, removing these parts and cleaning both sides of the board thoroughly and then replacing them and cleaning off the new flux again. These 3 parts are cheap and available, and it should be easy and inexpensive to just replace them.
The glide pot is a clue; adjusting it halfway is helping to isolate C21 from the output of IC12B and the parts around there.
This failure bue to board contamination or leaky components is fairly common in older synths.

Thanks for your insights Synthguy. What you say makes a whole lot of sense. I am wondering if you have a different schematic version from this one: https://www.synthxl.com/wp-content/uplo ... mation.pdf

It looks like C23 is the capacitor you called out as C21 and maybe Q20 the FET? And my apologies to the OP; hope my questions aren't creating confusion.

Yep, looks like there are at least two different schematics out there, but the operation is the same, just different parts as the the board was revved during production.
The capacitor C23 on the other scematic is a filter for the keyboard voltage but the cap that actually holds the voltage when the key is released is C25, the one on the input of U12A (C21 on the other version of the scematic).

If you have the unit with the discreet FET E112, these are hard to find now, but can be replaced with just about any N-channel FET (J110 or J111 should work and is easier to find, just make sure that any replacement you use is installed correctly, as the pinouts vary across different devices).
Chances are pretty good that none of these parts are bad, and the problem is just caused by contamination of that area of the circuit board after all these years. I would try removing these parts and really cleaning the board on both sides, reinstalling them, cleaning again, and seeing if that solves the problem.
Good Luck!

Two different schematics out there and they both have different component designators.

The one service manual out there refers to only one of them. If you attempt to read the text describing designators while following the wrong schematic, your head will spin!

MC wrote: ↑Mon Oct 28, 2019 6:46 pm Two different schematics out there and they both have different component designators.

The one service manual out there refers to only one of them. If you attempt to read the text describing designators while following the wrong schematic, your head will spin!

...let alone reading the text from another synth's description! Some of what you were describing Synthguy sounded vaguely familiar and I went back to the Mini's service manual which has wonderful circuit descriptions with similar circuits to that of the Prodigy. The linked service manual I provided above has no descriptions - not sure if the other version you guys are referring to does?

Anyway thanks again guys - always enjoy learning or periodically refreshing that which I will soon forget .

Btw just discovered the link I provided above which came from this site also has the single page schematic.

https://www.synthxl.com/moog-prodigy/

Selecting "Schematics" gets you this:
https://www.synthxl.com/wp-content/uplo ... matics.pdf

I'm having the opposite Release switch problem. When i move the Release switch to sustain, and play a note, on release, a higher-pitch note is initially triggered. Upon each playing, the pitch comes down a little.

Here's a link to it doing it on Youtube:

https://youtu.be/58taNheu3v0

Any clues as to what could be causing this?