Arp Omni II Dead Bass note

I am in the process of restoring an Arp Omni II that I purchased a couple weeks ago at an estate sale for a great deal. I’ve never worked on this particular synth before, but I have decent experience in restoring other keyboards and synths in the past, so I felt I could tackle this one.

So far, I’ve replaced the power supply with a new unit with a trip function, so basically nothing over 14.5 volts can pass into the boards if there is a power issue. I’ve also replaced every tantalum cap with an electrolytic, which was time consuming but worth it. Just finished going through the keybed, too.

Everything functions correctly except for one key (F#1, key number 7) on the synth bass and string bass voices. I know the actual key and key contact work, because that key does play the note in the viola and violin voices perfectly fine, which leads me to believe the issue is on the lower voicing board. When pressing that key with one of the bass voices selected, all I hear is an audible click but no note. The note was working perfectly fine on the bass voices but gradually crackled out and died completely. Every other key on the bass voices works perfectly. This is where I’m stumped. Could this be a bad CMOS chip? I haven’t been able to find anyone else with the same problem or a similar problem, so if anyone has any experience with these synths any help would be greatly appreciated! Thanks

Update on this upon further investigation, I found that Z35 was bad and have fixed the problem by replacing it!

I’ve got this synth mostly functioning now, but the one thing I’ve got left to fix is the chorus/phaser only working on the string section and not on the synth section. Any advice on this?

Welcome to the forum!

Glad you got that other problem fixed. For this problem I would bet it’s Z2 CD4016 on the string control board. This is an electronic switch IC that adds the synth signal to the strings before they get to the phaser. The switch is enabled when the phaser switch is enabled.

Also if you haven’t done the power supply mod, it’s highly recommended if yours has unbuffered ICs. Later Omnis use buffered CMOS ICs and the modification is unnecessary. If a CMOS IC is buffered then it will have the letter B as a suffix, such as CD4016B. If yours has unbuffered ICs they have a high failure rate in Omnis because of the poor peak voltage suppression of the power supply design when it is turned on or off. Below I have linked an Arp service bulletin and some discussion on the topic.

This is very helpful, thank you! Ordered a couple of those chips and we’ll see how it goes. As for the power supply, I found a new replacement power supply that has a voltage spike protection circuit and it seems very stable.

I replaced Z2, but still no luck getting the chorus to work on the synth section. In addition, another issue has popped up where the release on the string section is no longer functioning correctly. The envelope seems not to be cutting off when another note is triggered, and when the release fader is set to minimum there is still a noticeable decay to the note. Any suggestions here? I’ve replaced Z1 and Z4 but both of these problems still remain

I believe I have had that same problem with the note decay after beginning to service an Omni that didn’t initially have the problem. I solved it as follows. On the lower voicing board there is a grey cable that has 3 inner wires, a red one, a black one and one which isn’t insulated. If that uninsulated wire comes into contact with the neighboring pin it can cause damage to Q29 and/or Q28. It may even be able to cause damage while the synth is off if there’s enough charge left to reverse bias those transistors to destruction. You can check them out of circuit with a diode test on a DMM.

When placing the cable back on the posts make sure that uninsulated wire goes up above the post it leads to that way it can’t be bent into touching the other pin.

Now back to the other problem:
Do you have access to an oscilloscope? It would be very useful to verify signal is reaching pin 4 of Z2 on the string control board. Also the LED turns on when the phaser button is pressed, correct?

Finally had a chance to come back to this synth tonight and made some progress. The issue with the string release must have been a result of that bare wire shorting out Q28/29 because they both tested bad. Replacing both resolved this issue.

Back to the phaser… I’ve discovered that if I select a synth voice, and turn the phaser on I hear no difference. However if I move the mix fader over to string, I still hear the synth voice but now with the phaser effect. Selecting a string voice obviously still plays a string voice here, but with the synth mixed in. I’m assuming this is not normal behavior, but it does help narrow down where this problem could be coming from. Still haven’t found the culprit yet though.

Another odd issue I’ve found with this synth as well is whenever I power it up, “Hollow Waveform” automatically lights up. If I hold the button while powering the synth on it does not light up.

The saga continues…

This may be a non-issue. There’s some weird design choices on the Omni and one is that the phaser effect is only present when the mix fader is set to strings. This includes when the synth voice is used in the phaser, the output of the effect is only present on the string signal. This is a limitation of the synth because it’s a single channel chorus effect circuit that both the string and synth voices go through together.

As long as you hear the synth voice through the phaser with the mix knob set to strings then yours is working as intended. This confused me too when I got to working on Omnis but the block diagram in the service manual confirms this is the intended behavior.

Ah, I see. Thanks for the help on that.

Any reason why Hollow Waveform would be turning on automatically upon powering on?

I think Z53 LM301 on the lower voicing board is probably the problem. If not then it could be CR109 or C89.