ARP Omni repair

Hi there. Long time.
Last summer, I bought a non working ARP Omni.

Powered up: no sound but a small glitch. Here is what I've done to fix it.
1- Deep cleaning. Vacuumed first, then I pull all the board off and bath them. Yes, in water with soap and an old tooth brush to gently scrub. Rinced and thorough dried them with common a hair-dryer.
2- Cleaned the Pratt-Reed keyboard.
3- Cleaned the casing. Sanded the wood ends and sprayed some Pledge. Looks nice now.
4- Check the PSU; every thing was OK in this department.
5- At that time, I had no scope. My first guess was a dead output op-amp. Replaced, but still dead.
6- Suspected the 4007 on upper board; replaced it. Still no sound.
7- Got a small digital oscilloscope (116 $ incl shipping and taxes).
8- Upper Voicing Board. Check the output of main osc: OK, outputs of MK50240: Ok. Checked the 4069s and found 2-3 dead gates. Replaced the dead 4069.
9- Ground was shorted to -15V on Upper Voicing Board. Once forced to 0V, could produced some notes. The ground default was due to bad tantalums. I replaced them. Upper Voicing Board now OK.
10- The dividing logic on Lower Voicing Board was bad. Once again, due to faulty tantalums (nearly half of them). I replaced them all. I could save some cash by replacing the 10uF tantalums by equivalent modern electrolytics.
11- That was pretty much it. Still have a faulty capacitor on the String Control Board (attack always short), but not a big deal. I will replace it later.

ARP Omni a quiet a good synth, rather pleasant to play.


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