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DISCLAIMER: I AM NOT A TECH (see power supply smoking episodes below for proof). The following principles of ARP Omni repair are based on my own experience (which consists of getting my Omni back into playable, but not yet optimal condition), and a consensus of opinions from private email, public email lists, and other web resources. Always let the ARP Omni Service Manual be your guide.
Many people believe that the first step in getting an Omni back into working order is to replace all tantalum capacitors and CD40xx CMOS integrated circuits. Each gating circuit in the Omni contains a 22uF tantalum cap which controls note release time. In addition, tantalum caps are used in the bass gating circuit, which contains one 10uF tantalum cap for each of the 20 keys on the bass end of the keyboard, and they are also used on various circuit boards for power supply decoupling and the like. However, tantalum caps are prone to degradation with age, and should be replaced. I replaced all of the tantalum caps in my Omni's gating circuits and bass gating circuit with like-valued aluminum electrolytic caps. I addition, I replaced most power supply decoupling caps with new tantalum caps of the same value since I wasn't sure if the non-polarized electrolytic caps would work in that situation. Replacing the caps was tedious but was worth the effort since it fixed some long-standing problems I had with leaky notes.
CMOS chips are also prone to failure due to age. Experts also suggest replacing all CD40xx chips on the Omni. These chips are used on all circuit boards in the Omni so replacing them can be time-consuming. I replaced all of these chips on the upper and lower voicing boards, and string control board of my Omni. The other boards containing CMOS chips seem to be working properly (e.g., the phaser board is phasing as beautifully as ever) so I have not replaced any CMOS chips on them.
As mentioned above, replacing these components can be tedious and time-consuming. I removed the ICs by cutting each pin at the junction with the IC with a utility knife, then using a soldering iron and needle-nose pliers to remove the pins from the circuit board.
Experts suggest using IC sockets instead of soldering ICs directly into the circuit board. Furthermore, they suggest using the slightly more expensive machine-tooled IC sockets. I have mixed feelings about using IC sockets, but if you use them, I wholeheartedly agree with using the better quality ones. The advantages of using IC sockets are that it's easier to replace components that are plugged in as opposed to soldered in, and you don't have to worry about heat or static damage to components during the soldering process. On the other hand, it's not that hard to unsolder an IC, and soldering the pins of an IC socket on the component side of a circuit board can be difficult since there's not much clearance. Also, if you use a heat sink and grounded soldering iron, you can minimize the possibility of heat/static damage. I used IC sockets on the upper voicing board of my Omni, but I soldered the ICs directly into the lower voicing board, so I can go either way on this issue.
For more information on the topic of replacing tantalum caps and CMOS ICs, see the following web-based resources:
Here are some problems I came across with my Omni, along with their solutions (if they have been found).
This was caused by an soldering error when attempting to replace an LM1458 op-amp (Z50) on the lower voicing board. The solution was to replace the op-amp (without the cheap IC socket that I attempted to install), and to replace the smoked 4.7 ohm resistor on the power supply.
While wiggling one of the keyboard DIP connectors (P1 on the lower voicing board), the power supply again emitted a plume of formaldehyde-scented smoke. The same 4.7 ohm resistor was toasted like a burnt marshmallow. I replaced a few different components on the power supply to fix this, including the 4.7 ohm resistor (I'm glad I ordered extras the first time), the IN4001 diodes, an LM301 op-amp, and the 723DC voltage regulator. I'm not positive, but I think replacing the voltage regulator was what did the trick.
The keyboard worked only when the low F key was pressed. Pressing other keys (while holding down that low F) would result either in an incorrect pitch, or no sound. The solution was to plug in the DIP connectors that connect the keyboard to the circuit boards THE RIGHT WAY. I was investigating this problem when I smoked the power supply the second time. The cables seemed to be plugged in correctly according to colors shown in the service manual (the drawing is kind of confusing) and according to the way the shapes of the plugs matched the shapes of the sockets. However, using a continuity tester to follow the signal from the bus rod to the pin that it allegedly corresponded to showed that they were plugged in backwards. In the backwards state, the bus rod was charged only when the F key was pressed and of course, other keys would trigger either the wrong pitch or no pitch at all.
Solution was to replace a 3.3K resistor network on upper voicing board.
More to come....in progress...