M #1126 Prodigy - All Keys Play the Same Note

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erratanote
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Joined: Sun Jul 24, 2016 2:22 am

M #1126 Prodigy - All Keys Play the Same Note

Post by erratanote » Sun Jul 24, 2016 3:10 am

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?

synthchaser
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Re: M #1126 Prodigy - All Keys Play the Same Note

Post by synthchaser » Sun Jul 24, 2016 9:25 am

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

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thealien666
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Re: M #1126 Prodigy - All Keys Play the Same Note

Post by thealien666 » Sun Jul 24, 2016 12:16 pm

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).
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latigid on
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Re: M #1126 Prodigy - All Keys Play the Same Note

Post by latigid on » Sun Jul 24, 2016 12:26 pm

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).

erratanote
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Re: M #1126 Prodigy - All Keys Play the Same Note

Post by erratanote » Sun Jul 24, 2016 6:35 pm

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!

erratanote
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Re: M #1126 Prodigy - All Keys Play the Same Note

Post by erratanote » Sun Jul 24, 2016 10:50 pm

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?

spiders
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Re: M #1126 Prodigy - All Keys Play the Same Note

Post by spiders » Mon Jul 25, 2016 12:17 pm

+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.

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