Moog DFAM issue / VCA Decay Bleeding, Noise

System 55 • System 35 • Model 15
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Chabbles
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Joined: Thu Nov 24, 2022 8:43 pm

Moog DFAM issue / VCA Decay Bleeding, Noise

Post by Chabbles » Thu Nov 24, 2022 8:58 pm

Hey guys,
I've run into an issue with my Dfam which i bought back in august, its basically making terrible popping/clicking noise related to the VCA Decay, the decay is bleeding very badly. When the decay knob is open past maybe 30% the synth sounds broken with a constant clicking popping sound when only one step is active in the sequencer. Full patterns just sound way off.

Here is an example of one hit in the sequencer
https://www.youtube.com/watch?v=qWjc_tU_E_c

Here is an example of the "Tubular Bell" preset. Sound "A" is from my Dfam, sound "B" is as is should sound.
https://youtu.be/66-rXHBYy6E

Any ideas ??

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analogmonster
Posts: 263
Joined: Wed Nov 24, 2010 9:50 am
Location: Germany
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Re: Moog DFAM issue / VCA Decay Bleeding, Noise

Post by analogmonster » Tue Nov 29, 2022 4:00 am

I'd say the envelope creation and / or processing is down. It is the opposite of percussive. Seems that the VCA doesn't close or fall down as it should, what could be an issue of the VCA itself or of the ADSR (EG). According to the DFAM signal flow chart there are envelopes for filtering, amplification / attenuation and VCO modulation. The popping you hear comes from the sequencer steps, I assume. So the VCA seems to be open all the time. If the circuit is OTA based (LM13700?) the OTA might be dead, or the envelope is constantly high. I had this combination problem in my Moog Prodigy and had to replace the timer IC of the EG and the VCA OTA, if I remember correctly.

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