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ARP Omni repair - trouble with ADSR

I purchased my Omni totally non functional in 2012. I came across all the issues: oxidized contacts and sliders, bad caps, dead CI, bad solder joints. However, I still have an issue with the envelope generator.

The envelope generators

The ARP Omni has 2 sections, each one with an envelope generator.
  • The string section has an AR (Attack-Release) envelope
  • The synth section has an ADSR (Attack-Decay-Sustain-Release) envelope
Both envelope generators are triggered with the keyboard. One important thing: the AR envelope drives the overall volume. In other words, a short release on the AR will cut the volume of the synth section, no matter how long is the Release on the ADSR. The

The issues

String's AR envelope: Attack is always short, Release is OK. Synth's ADSR: some combinations of settings don't produce the expected results.
I then draw a table of the combinations of A, D, S and R I've tested (with VCF FREQ half, VCF ADSR full, other sliders down). The idea was to find a pattern, but no luck, unfortunately.

The Fix

After some time on the workbench, I finally noticed that monitoring pin 1 of Z4 with my scope changed the behavior of the ADSR! Z4 is a quad NAND gate in CMOS (CD4011UDE). CMOS are sensitive to ESD; could this be a bad gate? I changed both Z3 and Z4 and ... the problem was gone!

I've checked both chips on breadboard, and yes, both had one bad gate.

Conclusion

Always have a couple of CMOS in spare.

Update

I noticed after some time the issue with the Synthesizer envelope was not gone. I suspected the sliders (which I had already cleaned up, but were still pretty stiff) so I decided to install the Precision Kit from Synthchaser. When I install the new sliders on the String Control board, a green capacitor (BREL) fell down: one leg was unnoticedly broken, the second just broke. Once the capacitor replaced, everything worked perfectly!

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