ARP Omni Power Supply Mod

(Please read all instructions before you begin)

Many Omni 1 and Omni 2 keyboards you see for sale are toasted in some way.Apparently this is due to a badly designed power supply which generates transient voltages when you turn the synth on and off. Over time, these power spikes fry the synth's chips and capacitors. ARP later figured this out and sent service bulletins to all their technicians instructing them to perform a mod on any Omnis they repaired. I got copies of the documents from Joachim Verghese, who also suggested using SA15A transient suppressors since the UZ314s specified by ARP no longer seem to be available. The mod can be performed on both Omni 1and Omni 2 keyboards because they use the same power supply board. Here are the official ARP documents:

How to do this mod without frying your synth

I performed this modification on one Omni 2. Unfortunately, there was a problem. Instead of the power supply voltage dropping to around 13 voltsas ARP predicted, it INCREASED to around 18 volts. CMOS chips don't like 18 volts very much, so a couple of them blew up. After angrily changing the blown chips, I disconnected the power supply board from the rest of the Omni and set about trying to figure out whatyou REALLY had to do to get the power supply to go down to around 13 volts. After some experimenting, I hit upon REPLACING R12 with a 41.1K resistor instead of soldering a 68K resistor in parallel with it. This lowered the voltage into the right range so it could be tweaked with the trimpots to exactly 13 volts. So if you do this mod:

  1. Use a resistor value in the neighborhood of the value I used instead of what ARP specified. Other than this resistor mistake, I think the mod is a good one. So far the Omni 2 on which I performed the mod (in 1997 or so) has not failed again.
  2. Disconnect the power supply from the rest of the Omni before turning it back on. Check the voltage to make sure it's correct before hooking it back up to the rest of the boards.

Another Method

I got a note from Mike Morris about how he performed this mod:

Hi Mark,

Thanks for your Arp Omni 2 resources. I've just finished restoration of my 
Omni 2.. replacing all the tantalums and performed the power supply mod (it 
now works as well as the day I bought it in 1977!)

For the power supply mod, I have one suggestion to make component 
acquisition a tiny bit easier. Instead of trying to find a 1% 41.1k 
resistor to replace the 30.1k on the power supply board, I simply lifted 
the lead of one side of the existing 30.1k, and 'tented' a 10k in series 
with it. This places the supply rails well within the ability to adjust to 
exactly +/- 13V, as well as only requiring a very common value (anyone with 
a stash of resistors will most likely already have one. :)

Thanks again for your great resource pages!

- Mike

The way I had the mod described before might have given the impression that it was critical to use a 41.1K's not. As long at the total resistance is in the neighborhood of 40K, it will get you in the right ballpark so you can tweak the power supply to +/- 13V with the adjustment pots. Thanks, Mike for the correction.

Happy Omni hacking.