ARP Omni 2 sustain (release) problem on all sections

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ARP Omni 2 sustain (release) problem on all sections

Postby WhinyLittleRunt » Mon Dec 21, 2015 10:28 pm

I'm about to embark on one of my other synths, the ARP Omni 2, and complete the repair I never finished several years ago. Back in 2008 I got the Omni and in 2009 in did the cap replacement across the upper and lower voice boards. I did not do every cap in the unit, nor did I do the power supply. One of the issues that actually crept up after the cap replacements was that the release would not work on either the string or synth section. It actually will sustain very slightly if the slider is maxed, but obviously not the way it should. Before I start just haphazardly swapping caps, which I know needs to be done anyway, could anyone tell me if the release function is tied into any of the chips that are known to go bad? I could trace it out against the scheme but I know these are common problems on the unit so figured safe to ask.
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Re: ARP Omni 2 sustain (release) problem on all sections

Postby WhinyLittleRunt » Mon Dec 28, 2015 2:57 am

Update:

I finished re-capping the entire synth over the last 2 nights and removed / cleaned the sliders. Pics for whatever it's worth:

Image

Image

I even got that row of like 20 tantalums, which for some reason didn't even show up in the service manual in the parts section:
Image

Recapped power supply:
Image

My sliders were nasty:
Image

Dirty slider:
Image

Clean slider:
Image

The Original slider lol:
Image

Heh... anyway - so unfortunately doing all of that work really didn't fix the problem I was having, but I went into it expecting it to be the case. The release is actually stuck on; I realized that maybe the last time I had it apart, I had connected the string control wires/harness to the board with upper bare wire touching the middle peg. I realized this when playing with the connections with the synth on. This is how it should be connected:

Image

The thing is, the play on that wire isn't very good, and it's a tight fit to get it on those pegs, so if you're not paying attention, it's super easy to get the bare wire touching the middle peg. When it's touching, I can't tell if it's shorting something or what, but basically you have a weird release, like it's got a slight bit of sustain to it / not completely abrupt, and gets a little better when set to the max, but when hooked up correctly it's stuck on max release. Attack works fine.

The sustain footswitch seems to work backwards, but when it does it's producing the same effect as when the bare wire was touching the middle peg.

I was reading in the manual something about gate circuits and the transistor arrays (10). I'm not going to even doubt that a chip has gone bad. Just wondering if it had, how to figure it out...
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Re: ARP Omni 2 sustain (release) problem on all sections

Postby Dr. Phibes » Fri Jan 01, 2016 6:02 pm

You see those little metal-can transistors Q28 & Q29? Check those - one of them failed in my omni leading to similarly odd release behavior. Luckily they're both just bog standard 2n2907 PNPs.
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Re: ARP Omni 2 sustain (release) problem on all sections

Postby WhinyLittleRunt » Sat Jan 02, 2016 12:30 am

Dr. Phibes wrote:You see those little metal-can transistors Q28 & Q29? Check those - one of them failed in my omni leading to similarly odd release behavior. Luckily they're both just bog standard 2n2907 PNPs.


Heh... I just replaced the 5 2n3904/3906 transistors right next to it to the right, at the advice of Rob @ NE Analog, which was really cool that he gave me some advice about it for free as he's run into this problem himself on quite a few Omni's... Unfortunately that didn't fix it and I never even thought to examine the metal cans which probably would have blown first... Although he explained to me the same thing I had anticipated would be the case... That bare wire from the string control board was touching the middle pin and shorting, which was probably my fault at some point but hard to avoid... And he said it blows the transistors in that group. But yes, I will run over to the shop and see if we have some 2907s... Want to see if that fixes it. Thanks man!
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Re: ARP Omni 2 sustain (release) problem on all sections

Postby WhinyLittleRunt » Sat Jan 02, 2016 1:54 am

I replaced with 2n3906 because that's what I had on hand and I checked the data sheet and researched to make sure of compatibility... But now the release slider works for fast, but not working for slow. Ugh...

EDIT: I went to the shop and found some metal can 2n2907s and replaced them, still no change... just wanted to make sure. Any other suggestions of failed parts in that section?
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Re: ARP Omni 2 sustain (release) problem on all sections

Postby Dr. Phibes » Sun Jan 03, 2016 4:40 am

Try checking if the the 'pulse drive' and 'ADSR Blanking' circuits working as they should. The pulse drive resets the envelope every time a new key is played (unless the single trigger switch is on). The ADSR Blanking stops the ADSR resetting when the sustain pedal is released.

The easiest way to check these is on the Synthesizer Board, junction 12. Pin 11 is the Pulse drive connection. Normally this should sit at -15V then when a key is pressed it should momentarily jump to 0V before going back to -15V. Pin 8 is the ADSR Blanking signal, when the foot pedal is released this should jump from -12V to 0V and then back to -12V.
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Re: ARP Omni 2 sustain (release) problem on all sections

Postby synthparts » Sun Jan 03, 2016 11:19 pm

You really should replace all the tantalum caps on all the boards. There's about 100 of them...
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Re: ARP Omni 2 sustain (release) problem on all sections

Postby WhinyLittleRunt » Mon Jan 04, 2016 12:08 am

synthparts wrote:You really should replace all the tantalum caps on all the boards. There's about 100 of them...


See my posts above. Done.
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Re: ARP Omni 2 sustain (release) problem on all sections

Postby WhinyLittleRunt » Mon Jan 04, 2016 5:55 am

Dr. Phibes wrote:Try checking if the the 'pulse drive' and 'ADSR Blanking' circuits working as they should. The pulse drive resets the envelope every time a new key is played (unless the single trigger switch is on). The ADSR Blanking stops the ADSR resetting when the sustain pedal is released.

The easiest way to check these is on the Synthesizer Board, junction 12. Pin 11 is the Pulse drive connection. Normally this should sit at -15V then when a key is pressed it should momentarily jump to 0V before going back to -15V. Pin 8 is the ADSR Blanking signal, when the foot pedal is released this should jump from -12V to 0V and then back to -12V.


Are these linked to the string section? I can tell you it's in the "keying" circuit somewhere because I didn't realize that when you hold down one key, the note will not sustain. If I hold down more than one key, and several, and several more, the release will start to work and get longer and longer the more keys I hold down...

I just replaced the (2) Cr4016's in the string board just based on some other reading I did, which didn't solve it clearly...

Still shooting blanks...
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Re: ARP Omni 2 sustain (release) problem on all sections

Postby dstrength » Mon Jan 04, 2016 3:36 pm

In your photos, you still show some tants on the synth board.

But, you need to measure the AR functions and double check some things on the String board:

  • Double check the release slider works, and check caps C17 and C18 (tants).

  • Double check the string board path to ground. One of these paths is through the mounting screws. Release works by dumping negative voltage slowly back to ground.

  • Look at the AR output Z3 pin 7. Attack swoops down...release should smoothly trend up. If not, double check Z3 and the C15.

  • The AR Squelch: Z3 pin 13. Should quickly drop to -15 on press and slowly trend up on release. IF not check Z4 and C10.
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Re: ARP Omni 2 sustain (release) problem on all sections

Postby WhinyLittleRunt » Mon Jan 04, 2016 6:05 pm

Interesting about the grounding through the mounting screws... I actually remounted the lower voice board to the chassis using nylon washers because I had at least the one screw at the upper left near the power supply connector grounding out and shorting the board way back when I worked on it the first time... I'll check this. I think it was just hitting a connection on the part side of the board so I used the washers instead. Only on that board though.

Also, my picture of the sliders is older than the ones above it. I have recapped the entire synth, all tants are removed including on the 4075 filter board. I ran train on that thing...
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Re: ARP Omni 2 sustain (release) problem on all sections

Postby Dr. Phibes » Mon Jan 04, 2016 7:25 pm

WhinyLittleRunt wrote:
Are these linked to the string section?



Indirectly. The reason why I asked is that since both the synth & string release is acting up I suspect there's still a fault with the foot pedal release or the suppression trigger - pulse drive & adsr blanking rely on these to work properly.

You could try checking if the suppression trigger generator directly but I think it's hidden behind the phaser board - EDIT: actually no, this should be detectable on pin 3 of P10.

The suppression trigger generator sends a quick spike of +15V to the keying circuits every time a new note is pressed to stop the previous note's release hanging on (this is a divide-down instrument after all). Maybe the trigger itself itself is stuck on?
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Re: ARP Omni 2 sustain (release) problem on all sections

Postby WhinyLittleRunt » Tue Jan 05, 2016 10:49 pm

That's a good point... I'll check this tonight. I did finally plug a sustain pedal into the jack, and what happens is when it's plugged in, the release goes back to long. Step on the pedal, and it cuts. Is this just a property of the way the sustain works on these? The pedal itself is just a standard Yamaha piano sustain, so by default it should work the other way around..

Also replaced the string control wire and heat-shrinked it, etc.
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