ARP Omni MK1

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ARP Omni MK1

Postby tomorrowstops » Thu Mar 27, 2014 4:41 pm

Got a great deal on a 'for parts or not working' Omni MK1. The previous owner's complaint was a broken slider and 'stuck notes.'

So it came in today, I fired it up, and sure enough there's got to be 4-5 notes constantly sounding. I noticed that its a J-wire keybed - ah ha - I've dealt with these before, it must be some stuck j-wires. Well, there was one, but I fixed it and everything else looks hunky dory. I plug the ribbons back in (theres 4 running from the keybed to the voice boards), and viola, no triggered notes! Until I touch a key. Bam, stuck notes come back.

I experimented with pulling the ribbons, and discovered that when the ribbon farthest to the left gets unplugged, the stuck notes fade away, like a super long release setting. When I plug it back in, nothing. Until I hit a key. Any key, high or low. Then the same group of stuck notes fades back in. Manipulating parameters doesn't seem to change anything, other than changing the voice sounds.

As it sits now, all of the sliders need to be pulled, taken apart, cleaned and lubed. Since that needs to be done regardless, I'm going to start there. Everything else component wise visually looks ok, no burnouts or leaks, etc.

Any thoughts?
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Re: ARP Omni MK1

Postby rhino » Thu Mar 27, 2014 5:12 pm

Locate one of the keyers for a sustaining note and change the 22uF cap. If that works on that one note, change them all (49)
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Re: ARP Omni MK1

Postby tomorrowstops » Thu Mar 27, 2014 6:45 pm

Cool, that'll be the first thing I do after the cleaning. I thought I had read somewhere about the caps for the keybed end of things being common suspects.

Here's what I found when I pull the ADSR board.

Image

Image

Image

I can just clean that off with a little isopropl and a soft bristle tooth brush, right?
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Re: ARP Omni MK1

Postby HideawayStudio » Thu Mar 27, 2014 10:10 pm

tomorrowstops wrote:Cool, that'll be the first thing I do after the cleaning. I thought I had read somewhere about the caps for the keybed end of things being common suspects.

Here's what I found when I pull the ADSR board.

Image

I can just clean that off with a little isopropl and a soft bristle tooth brush, right?


That's fine for cleaning the pcb but -DON'T- get it in the sliders as it will ruin them!

...and although I'm not one for gratuitous recapping - you really should replace ALL of the blue tantalum caps in this thing - the Omni-1 and Omni-2 are full of very poor quality caps that more often than not fail dead short circuit and many are directly across the supply rails. There are around 70 of them in total - it's a horrible job but thankfully if you do it yourself the parts are very inexpensive. My advice is don't replace them with tantalums - use electrolytics and make sure they are correctly polarised. The plate-thrus are weak/non existent in this design so make sure the top and bottom traces are still showing continuity after replacing each cap.

I cannot stress this more strongly - the Omni is just one of those synths that will make your life a misery if you only replace the caps that are currently giving you trouble. I love this synth - it has formed the basis of some amazing samples for me over the years - it definitely deserves a little TLC and love as far too many of these are ending being trashed.

Also beware that the BBD chips in the triple chorus are starting to get a little difficult to source and that the CMOS multiplexors around them are rather prone to failure - fortunately the MUX chips are really cheap and easy to get hold of.
Last edited by HideawayStudio on Thu Mar 27, 2014 10:18 pm, edited 1 time in total.
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Re: ARP Omni MK1

Postby synthparts » Thu Mar 27, 2014 10:13 pm

See all those light blue caps? They like to short out...
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Re: ARP Omni MK1

Postby tomorrowstops » Fri Mar 28, 2014 12:23 am

Awesome, thanks for the advice guys. I'm starting to really get into this electronics fixin' stuff.

Hideaway - I thought it was ok to clean the sliders with alcohol? When I was working on my Opus that's what I used, based on several recommendations.

I see that New England Analog has a full electrolytic cap kit available for the Omni, so I'll just order one up. Unless of course, I wuss out and just have them do it (they're only a half hour away from me)!

I'm thinking this board is going to be a keeper for me, even just listening to this dissonant mess of a chord its spitting out at me - the thing sounds pretty magical!
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Re: ARP Omni MK1

Postby HideawayStudio » Fri Mar 28, 2014 1:04 am

tomorrowstops wrote:Hideaway - I thought it was ok to clean the sliders with alcohol? When I was working on my Opus that's what I used, based on several recommendations.

The problem with cleaning any pot/slider with alcohol or switch cleaner is that it almost always ends up washing out the grease which is there not only to permit the wiper to move smoothly but also to keep moisture off the track.

There is a bit of a knack to cleaning pots and sliders in the correct manner but it usually involves dust/debris removal followed by some form of oxidation removal (eg Deoxit) but it must be followed by the application of a suitable lubricant after this otherwise the pot will rapidly loose its smoothness and more importantly the oxidation will come back with a vengeance in a short while. Its the last bit that so many miss out and many a pot has been ruined this way. Conductive plastic types need to be treated with care too as many cleaning fluids/sprays will destroy the track - in fact some manufacturers (eg. P&G) state a distilled water based cleaning process.
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Re: ARP Omni MK1

Postby tomorrowstops » Fri Mar 28, 2014 1:40 am

Gotcha.

On the Opus sliders, I actually relubed them with petroleum jelly. So far so good!
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Re: ARP Omni MK1

Postby error_trials » Fri Mar 28, 2014 8:38 am

as others have said, taking the time to re-cap everything will save you headache and maybe solve some problems you don't know you have yet! I've used panasonic electrolytics in mine - some will recommend this and others will tell you they're not the same as tantalums, which, they're not. so far so good though.

I've also had some key wire issues that were causing keys to not trigger - the contact had become dirty and I needed to carefully clean the wire and connecting rod, used alcohol and light sand paper.

good luck!
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Re: ARP Omni MK1

Postby EmptySet » Fri Mar 28, 2014 3:13 pm

I've read pages on the web suggesting to clean the sliders by putting them in a soapy bathtub. I haven't gotten that far on my omni yet. I'm curious what others think about this idea. mine sliders feel like they had maple syrup poured in them.

I'm about half way done with my omni cap replacement. Like others have suggested, I am also using electrolytics. So far, so good (despite not being identical to the tantalums). The interesting thing about this old board is that you can swap out a handful of caps, power it up, and see what part of the synth came back to life. Maybe not the safest idea, but I've had no problems testing this way.

The other thing to watch for is that these boards suck. It is very easy to damage traces on boards when you desolder the old caps. Take it slow is the only advice I have. If you wreck a trace, I suppose you could hardwire it somehow, but I try to avoid this.
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Re: ARP Omni MK1

Postby tomorrowstops » Fri Mar 28, 2014 6:58 pm

Yeah, I'm thinking this might not be the board to take on for my first recap job. :D NEA's rough estimate for the job seems reasonable enough. I'll just focus on the sliders and general cleanup!
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Re: ARP Omni MK1

Postby tomorrowstops » Thu Apr 03, 2014 9:52 pm

Small update:

I was able to take just the circuit boards down to NEA. They're going to replace all the caps, chips, test the power supply, and finish cleaning the sliders properly.

Super cool shop, and super nice guys. Lots of cool old synths hanging around in various stages of repair. Even a few I recognize (mmp!)
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Re: ARP Omni MK1

Postby dstrength » Fri Apr 04, 2014 1:52 pm

I'm about finished restoring one of these. You might want them to look at the power supply and apply the field service mod to it. Without it, the power supply goes to +20 volts on startup, then back to 15 or so = No good for CMOS. They recommend installing some transient suppressors, replacing a resistor, and trimming the entire thing down to 13.5 volts.

http://www.marksmart.net/gearhack/arpomni/PSMod/PSMod.html
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Re: ARP Omni MK1

Postby tomorrowstops » Fri Apr 04, 2014 2:01 pm

Cool, nice piece of info. I think whats actually going to happen is they're going to replace the original power supply altogether with one of they're custom built ones! I guess that just drives home the point further that the original ARP power supplies are junk!
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Re: ARP Omni MK1

Postby dstrength » Mon Apr 07, 2014 5:44 pm

Yeah I think a new PSU altogether is the way to go. This one is for resale...if I were keeping it, or if it were going to a friends studio, I'd go ahead and get a modern supply in there. That Mark Smart's site has some other nice info. The VCF filter fix is a good idea as well.
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