Common failures on theARP Omni and Omni 2

Below is an email I posted on the Analogue Heaven mailing list about the things that frequently fail on Omnis. Hopefully the info will be useful to anyone doing repairs.

To:, (Jeff Gage)
Subject: Re: Arp Omni repairing
Date: Wed, 19 Mar 1997 16:54:02 -0600 (CST)

> Greetings all-
> I've recently obtained a silent Omni mk1. When I got it all of the sliders
> were hidden by years of amazing grime... I de-gunked them and gave them a
> liberal dose of DeOxit-type product, but still no sound. The power supply
> is good, the LEDs light... before I go step thru this thing and test all
> of the switches, I thought I would cast out and ask if anybody has
> experience repairing Omnis (common problems, etc.).
> And yes, Mark, I know about Our God Phil, I'd just rather do the repairs
> myself. :)
> Thanks
> Jeff
> -------
> Jeff Gage

I've repaired several Omni's recently including an Omni I. The first thing
is, of course, get a copy of the service manual. I have one which I could
copy, but it would be a pain since it is bound. I can't remember who copied
it for me, but I can probably find out by looking at old email.

Several things I have noticed that fail on the Omni's:

1. 22 uF tantalum capacitors in the envelope (AR) generators for the
individual keys. These cause notes to be stuck on all the time, and every
Omni I've looked at (one Omni I and three Omni II's) has had this problem
to some extent. I would venture to guess that once you fix the problem
which is preventing any audio output, you'll have some stuck notes. I have
just replaced ALL these capacitors in the Omni's I've worked on, since they
probably WILL fail soon even if they haven't. I replaced them with
non-polarized electrolytic caps rated for 50 volts, rather than the 35
volts of the originals, the idea being that power supply transient over or
reverse voltages over the years caused them to fail.

2. 10 uF tantalum capacitors acting as power supply decoupling caps on the
various boards. Several Omni's I've seen had these caps shorting out the
power supply and preventing anything from working. When this happens, the
power supply's voltages regulator(s) and the caps get REALLY hot. Sounds
like you might not have this problem since the lights work.

3. Bad CMOS analog switch chips. These fail a lot, and could be causing the
lack of audio output. They are the chips which turn the various sections on
and off when you hit the buttons. 

4. Bad 1458 or LM311 op-amps. These could also cause no audio output.

5. I had one Omni where the op-amp on the filter board for the
"synthesizer" section failed, and caused no audio output from the
"synthesizer section.

One other thing I should mention is that the terminology of the outputs is
kind of weird. The "Low Output" is actually the main output of the synth,
and the "synthesizer output" is just the output of the "synthesizer"
section, which is where the tone generators go thru the filter and VCA
without going thru the string chorus. If you were plugging into this
output, and the "synthesizer" section was broken, then you wouldn't hear
anything even if the strings section was working. The guy I sold my Omni I
to called me after he got it and thought it was broken because of this
weird terminology (hi Barry!).

I could go on and on about Omni's, but I'll stop here. Write me a note if
you have any more questions. I chose to start buying broken Omni's and
fixing them because there are a lot of them out there which are all broken,
and you can still get most of the electronic parts! Plus they just sound
cool. Once you fix it you will be able to do the wawawa sound at the
beginning of Pink Floyd's "Welcome to the Machine" from the "Wish You Were
Here" Album!!! 

Mark Smart