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