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.

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

I suspected Z31 and Z53 on Upper Voicing Board, but before purchasing new ones, I did the following test: disconnecting P17 and testing all TDA0470 one by one by connecting the output to P17 pin 1. Surprisingly, all chips were fine. I then tested all the inputs of P17 (same process), and they all were good as well. What's going on? After days of head scratching, I finally noticed...P17 was simply connected in the wrong way!

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:

• Syntaur
• Synthesizer Keith
• Markus Fuller

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)

Other issues:

• 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...

Update (Oct 2020)

All issues fixed!

• Strange behavior of ADSR and String Attack: actually, one of the 2 BREL capacitors (flat green) on the String Control Board had a broken leg. Replacing fixed the issues.
• I took the opportunity to replace all the sliders with the Precision LED Slider Kit from Synthchaser. Really a must-have!