ARP Omni repair
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.
- 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.
- Cleaned the Pratt-Reed keyboard.
- Cleaned the casing. Sanded the wood ends and sprayed some Pledge. Looks nice now.
- Check the PSU; every thing was OK in this department.
- At that time, I had no scope. My first guess was a dead output op-amp. Replaced, but still dead.
- Suspected the 4007 on upper board; replaced it. Still no sound.
- Got a small digital oscilloscope (116 $ incl shipping and taxes).
- 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.
- 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.
- 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.
- 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.
Update (Sept 2018)
I've found another issue with the Omni:
- keys G#4 to D#5 (notes G#5 to D#6) become silent when 4' (only) is selected
- keys E6 to C7 become silent when 8' (only) is selected
- keys E5 to D#6 sounds "richer" than others
Anyway, this gave me the opportunity to fix the remaining issues: scratchy pots and non-working Attack of strings.
Cleaning the pots is time consuming. I first tried with Deoxit D5 + MCL. This was not a good idea, the pots remained scratchy (less though) and gummy. So my next option was to remove and clean them one by one. Many tutorials explain the procedure:
The issue with the Attack on String Control Board is simply an attempt to fix it by a previous owner. A resistor (R31, 100 Ohms) has burned in place, partially roasting the nearby tantalum. The tant was left in place, and the resistor was replaced, but with a too hot iron that unglued the tracks; a nice job indeed!
Next job is to replace the tantalums, replace R31, cut the loose tracks and fix them with wires.
Update (Oct 2018)
- key F#2 inaudible after power on: a dry join found and fixed
- Strange behavior of ADSR and String Attack: the gate signal was missing.
I realized my Omni has a mix of voicing boards: Lower Voicing Board is v1, but Upper Voicing Board is v2. On v2, there is a filter on the gate signal, resulting to a too weak signal sent to ADSR and AD. More to come...