|ARP Omni ^||4075 Filter Mod >|
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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: firstname.lastname@example.org, (Jeff Gage) Wrom: FDULHPQQWOYIYZUNNYCGPKYL cc: email@example.com, 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 firstname.lastname@example.org 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