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ARP Omni Rebuild

ARP Omni

ARP Omni

ARP Omni

ARP Omni

The Omni was one of the best selling synthesizers that ARP manufactured. It incorporates both a string synthesizer, and a tranditional synth section with an ADSR and low pass filter. Both sections are fully polyphonic, with some limitations due to the lack of fully dedicated envelopes for each key.

Polyphonic keyboards like the Omni have more in common with home organs, than with traditional analog synthesizers. Rather than having 1 or 2 (or 3!) oscillators for each key, the Omni uses a single master oscillator. The square waves from this master oscillator are then divided down into the individual frequencies required for each key.

For a more detailed description of this scheme, see my article on the Korg Lambda.

Omni Power Supply Woes

The ARP Omni is the most common synthesizer that my clients bring to me for repair. Over time, I've come to recognize the most frequent failures. However, this Omni arrived completely dead. I had to start at the beginning: the power supply.

Power Supply Before Recapping

Power Supply Before Recapping

Power Supply Before Recapping

Power Supply Before Recapping

This photo shows the power supply before I did any work on it.

The first thing I did was replace the old capacitors, and turn on the power supply without connecting it to the synth. It looked good, and gave a clean +15 and -15 volts out. However, if I connected the power distribution cable to the other circuit boards, the -15 volt rail dropped almost to 0 volts. Yikes, what's going on?

Burnin' Out

Burnt String Control Board

Burnt String Control Board

Burnt String Control Board

Burnt String Control Board

It took me a while to find it, but I traced a short in the -15 volt rail to the string control board (under the front panel). One of the tantalum bypass caps had burnt out, and took a few other components with it!

String Control Board Fault

String Control Board Fault

String Control Board Fault

String Control Board Fault

While cleaning up the mess, and fixing the failed components, I found the culprit. The strain relief on the power connector had snapped off, and the ground and -15 volt wires had broken off. They were flapping about, causing havoc.

String Control Board Repaired

String Control Board Repaired

String Control Board Repaired

String Control Board Repaired

Here is the board with a new cable-tie (in orange), replacement caps (in blue and yellow), transistor, and other components.

Phaser Board Recap

Phaser Board Recap

Phaser Board Recap

Phaser Board Recap

While I was at it, I replaced all of the tantalum bypass capacitors, on every board. Here is the phaser board, with the new caps (in yellow). The process is tedious, but adds a lot to the reliability of the synth.

Capacitors, Capacitors, Capacitors

Bad Capacitors On Voice Board

Bad Capacitors On Voice Board

Bad Capacitors On Voice Board

Bad Capacitors On Voice Board

Even before I opened the Omni, I knew that I would be spending a lot of time replacing capacitors.

The release time of the string sound is fully polyphonic. To achive that, there is a single capacitor for each envelope (all 49 of them). The original design used tantalum capacitors. They were chosen as they are very stable in value over a large temperature range. This is important, as the release times of each envelope should be the same. Unfortunately, they do not handle voltage spikes very well, and the design of the Omni causes a little spike every time you press a key. Eventually the tantalum caps fail in a dead short, and you get a dead key.

In the above photo, you can see that some enterprising repair tech had tried to fix a few faulty capacitors in the past. However, the only way to permanently fix the problem is a wholesale replacement of all of them.

New Capacitors Lower Voice Board

New Capacitors Lower Voice Board

New Capacitors Lower Voice Board

New Capacitors Lower Voice Board

New Capacitors Upper Voice Board

New Capacitors Upper Voice Board

New Capacitors Upper Voice Board

New Capacitors Upper Voice Board

Capacitor technology has come a long way since 1975, and standard aluminum electrolytic capacitors are now more than stable enough to use in the Omni envelope generators. They are less likely to fail, and are less expensive as well.

In these photos you can see the neat rows of blue replacement caps. They are split between two boards, one for the upper voices, and one for the lower voices.

Bad Interconnects

The designers of the ARP Omni never expected that it would attain classic status, and musicians would be playing them 40 years later. They also didn't put much thought into high-quaily cables and interconnects, either.

The following photos show broken or damaged interconnects that I had to repair or replace.

Bad Ribbon Cable Interconnect

Bad Ribbon Cable Interconnect

Bad Ribbon Cable Interconnect

Bad Ribbon Cable Interconnect

The main boards in the Omni are connected via ribbon cables, and rather flimsy DIP assemblies. In this photo, one of the pins on the DIP header has broken off into its mating socket.

Bad Power Interconnect

Bad Power Interconnect

Bad Power Interconnect

Bad Power Interconnect

Bad Signal Interconnect

Bad Signal Interconnect

Bad Signal Interconnect

Bad Signal Interconnect

Power and signals are routed between boards via 0.156" spaced housings, and long pins soldered directly to the PCBs. Other than the solder, there is no other mechanical support holding the pins in place. After the connectors have been removed and re-inserted a few times, the pins tend to break off.

Clean As You Go

Dirty ARP Omni Faders

Dirty ARP Omni Faders

Dirty ARP Omni Faders

Dirty ARP Omni Faders

Wow, break out the gloves and solvent! It's amazing how much dirt and grime can get embedded into faders, pots, switches, and keys. Many common vintage synth issues can be resolved by simply cleaning all electrical and mechanical contacts.

Clean ARP Omni Faders

Clean ARP Omni Faders

Clean ARP Omni Faders

Clean ARP Omni Faders

The tools of the trade are isopropyl alcohol, contact cleaner, brushes, and toothpicks. If the crud is really imbedded, a knife to scrape with is often handy. Having a fan to blow across the work surface is also required, as lots of nasty stuff can become airborne.

Sometimes the mechanical cleaning takes as long as the electrical repairs.

Cleaning ARP Omni Faders

Cleaning ARP Omni Faders

I have probably spent more time cleaning ARP faders, than any other type of pot, fader, or switch. The basic idea is to de-solder them from the board, break them apart (gently!), clean out all of the grime with a cotton swab, and re-assemble them. I go through the process in detail in my article on Potentiometer Cleaning.

If you want even more photos, check out my article on restoring an Arp Odyssey.

Cleaning ARP Omni Keys

Cleaning ARP Omni Keys

Cleaning ARP Omni Keys

Cleaning ARP Omni Keys

While everything is apart, it's a good idea to remove all of the keys, and clean them as well. I usually just soak them in warm water with a bit of dish soap. Then I rinse them, and let them dry.

ARP Omni Keyboard Assembly

ARP Omni Keyboard Assembly

ARP Omni Keyboard Assembly

ARP Omni Keyboard Assembly

Here is a photo of the keyboard being re-assembled.

Power Supply Heat Control

Power Supply Heat Control

Power Supply Heat Control

Power Supply Heat Control

Power Supply Heat Control

The power supply has two power transistors, one for each of the +15 and -15 volt rails. They generate a lot of heat, and are bolted to an aluminum heat-sink. The heat-sink is then bolted to the back of the case.

Heat will transfer through the mechnical contact of the metal surfaces, but it is much more efficient if there is thermal grease (heat-sink compound) between the parts. The original grease had mostly dried up, and was flaking off. In this photo you can see where I've liberaly applied new grease. In fact, I probably went overboard on this!

Here is a video I made, giving a technical overview of the ARP Omni.

ARP Omni Tech Overview

ARP Omni Tech Overview

17 comments on ARP Omni Rebuild

1 2 > [last]
Jonathan
HI Keith - can i ask how you repaired the broken pin on the DIP plug head? Did you deal with it as a 'field repair' - or just order new parts? i just did the same thing (feeling stupid because i felt it probably could have been avoided)... thanks
December 17th 2015 02:45 EST
Keith
Hi Jonathan,

I had some header/housing parts from a previous repair, and they fit the 16-pin DIP pattern on the board. I cut off the old connector from the ribbon cable, desoldered the DIP socket, and replaced both parts.

-Keith
December 17th 2015 08:50 EST
Jonathan
Thanks! I ended up soldering a piece of half watt resistor wire onto the end (butt joint) and smoothing the splice out - good enough for an insertion for sure - but not sure if it will survive the next extraction. Do you do any consulting?? Remotely? I would be more than happy to pay for some help ... Read More
December 18th 2015 02:53 EST
Mike Thompson
Keith, I just got finished re-capping the entire Omni-2 (previously did the 22uf caps on the voice boards about 7 years ago); I realized between having a release problem, and lots of noise and sticky sliders, I decided it was time to finish the job. So I replaced every tantalum with electrolytics ... Read More
December 27th 2015 01:16 EST
Keith
Hi Mike. The Omni has a simple linear power supply. Unlike a switching supply, you can just measure the voltage output without connecting a load. Remove the power connectors that go from the power supply board, down to the other boards, and turn on the synth. Arp usually uses red/black/purple wire ... Read More
December 27th 2015 13:54 EST
Mike Thompson
Keith, So it turns out, I am getting power to the boards from the supply; I embarrassingly forgot to connect a ribbon pin connector to the board. But- a couple of things... While I'm getting power and all buttons and switches seem to light as normal, I'm getting no audio except a pop when I ... Read More
December 27th 2015 17:50 EST
Mike Thompson
Keith, Well it turns out that the screw was grounding to the case- in my Omni it's the one directly underneath the power supply connector on the lower voicing board. That also fixed the heat issue on the power supply. However, my original problem is not solved which is the release time of the ... Read More
December 27th 2015 19:03 EST
Troy Stains
Hi Keith, I'm fixing an Omni 2 for a friend. Of course it has the release time issue in the string section. But the bass section 8' and both synth section LED's are constantly lit, won't turn off when the button is pushed, AND those voices don't make any sound whatsoever. The ... Read More
February 15th 2016 18:38 EST
Keith
Hi Troy, Audio signals don't pass directly through the buttons on the Omni. Instead, those buttons control CMOS switches. Four switches are combined into one chip (CD4016). The audio signals pass through the CMOS switches. There are also some additional transistors to turn the LEDs on and off. ... Read More
February 15th 2016 20:10 EST
Troy
Hey! Thanks for the quick reply. Yeah, once I actually got in there with the multimeter I realized what was going on with the buttons/switching circuit. I swapped some "bad" buttons with good ones, and realized they're all actually fine, so yes, I'm definitely suspecting the ... Read More
February 16th 2016 00:40 EST
Troy
Found some suspect caps actually. All good now. Just waiting on cmos chips to arrive now. Thanks for the help!
February 16th 2016 22:10 EST
john west
Good Day,

When I try to play my Arp Omni 2 there is a droning sound that makes it difficult to play but sort of goes away when you play but when I left my fingers off the keys it comes back. It was a faint drone for several years but now interferes with playing.

Thank you so much for your time.
May 9th 2016 15:11 EDT
Keith
Hi John,

This is almost certainly caused by bad capacitors in the envelope circuits (there is one capacitor per key). The best bet is to completely replace all of the old capacitors with new ones. This should be done on every old Omni.

-Keith
May 9th 2016 19:38 EDT
T
HI,

Ive replaced all tant caps and have a string release issue. does not decay at all, same with synth section. any tips?
also i only have the lower 3 bass notes play. the others dont sound
cheers
June 29th 2016 23:54 EDT
Herbie
Hi! This page is wonderful!! I will buy a arp omni II working from a Ebay, and I would know if the problems, generally, are fix easily... Like the replacement of caps, etc. I'm traumatized, for example, with my Juno 106! his chips are very hard to find in Brazil. The arp has any problem very ... Read More
July 20th 2016 02:34 EDT
1 2 > [last]
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