Moog Music Forum

I'm looking for a little help from the experts. Beside the fact that all keys play the same note, all other functions appear normal. I checked the contacts and they are not an issue. I checked all connections between the keys and the circuit board and all appear normal. Any suggestions on what to check next?

U11 and U12 would be the first places I'd look.

Hello and welcome.

Measure the voltage at the "Kybd Out/In" jack on the back with a voltmeter. It should vary according to the key pressed (from 0 to roughly 2.5 volts DC).

If not, and if there is no mechanical contact problems with the keyboard, I would suggest replacing U12 (LF353 dual J-FET op amp).

I would also have a look at the Glide potentiometer and check it for proper operation with an ohmmeter (power off on the synth of course).

Did you try using the external CV input? Check for tarnishing on that jack too (if it carries the KB voltage through a normalised connection).

Thank you for your replies. My Prodigy is an early model that only has a High Level Audio Out, just like this linked Prodigy: http://www.vintagesynth.com/moog/moog_prodigy_rear.jpg
I'll check the Glide pot. Is there anything else you can think of before replacing U12? Thank you!

Thank you for the responses. I have a model from 1979 so it doesn't have the CV input or the keyboard out/in jack. I'll take a look at the Glide pot, as suggested. Anything else you can think of?

+1 on U11, U12. I had this problem with my mini. Your issue is probably with the CV that is being sent on by these chips. The CV is stuck, essentially. Giving the schematic a stare U12 seems more likely than U11 to be the cause of the problem.