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 Forum index » DIY Hardware and Software » The Repair Shop
Arp Omni - Attack Release Troubleshooting
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facon



Joined: Jan 14, 2015
Posts: 22
Location: Michigan, USA

PostPosted: Tue Jan 20, 2015 9:55 pm    Post subject: Arp Omni - Attack Release Troubleshooting Reply with quote  Mark this post and the followings unread

I just finished recapping my Arp Omni MKI. All tantalum caps were replaced with electrolytics. I also cleaned the sliders using Deoxit, canned air, then some fader lube. Before the recap, the attack and release in the string section didn't seem to do anything, so I was hoping it was a cap issue.

The attack on the strings works occasionally. It's hit or miss. Usually when it works, it's after being on for a minute. The release has never done anything. The release on the synthesizer also does nothing. The ASD seems to be working fine on the synth, but the ASDR slider only works right when it gets full up.

I dont have the footswitch and can't find any wiring diagrams for it. I did take TS and TRS plugs and attach alligator clips to each spot to see if it would make it sustain. I didn't have any luck figuring that out.

Any direction on how to start troubleshooting this? I don't know if I'm hunting down a single component/connection, multiple components, or if I need to figure out how to wire a footswitch.

http://www.synthfool.com/docs/Arp/ArpOmni/ARP_Omni_2480_service_manual.pdf

Last edited by facon on Wed Jan 21, 2015 10:36 pm; edited 1 time in total
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facon



Joined: Jan 14, 2015
Posts: 22
Location: Michigan, USA

PostPosted: Wed Jan 21, 2015 3:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

I have an update. I realized that I had one of the connectors backwards. Flipped it around and now the release on the higher strings works. However, the attack on the string section doesn't work.

I'm also noticing that when I'm not touching any keys, there is still sound coming out when the Viola and/or Violin preset buttons are pressed in. It sounds like I have a bunch of keys pressed down at once. It's low volume and is clearly going through the phaser.

The Bass and Cello are not doing this. But the attack and release both do not work on the Bass and Cello and it is not polyphonic. I'm assuming this is normal, but there is no users manual to be found anywhere for this synth.

Key 18 does not work. I'm not sure if this has anything to do with the other issues.
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facon



Joined: Jan 14, 2015
Posts: 22
Location: Michigan, USA

PostPosted: Wed Jan 21, 2015 10:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

I fixed the problem with the sound being generated without keys being pressed. The solder joint of the Q28 emmiter on the lower voicing board was broken.

I went through the circuit a bit with a DMM. On the string control board, I'm getting a 1V difference on pin 2 of Z4B while pressing keys, but I'm not getting anything that changes on any other pins of Z4. This is confusing me since the AR suppression and squelch are clearly working on the release.

The voltage doesn't change at all when measuring voltage on the gate and trigger outputs. However, they seem to be working internally in the synth section.

That's it for tonight. I have about 20 hours into attempting to bring this synth back to life. Feeling like I got in over my head a bit.
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facon



Joined: Jan 14, 2015
Posts: 22
Location: Michigan, USA

PostPosted: Mon Jan 26, 2015 6:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

I replaced Z4 with no immediate change. So I went through again and reheated all of the solder joints. Now I can get the attack to work, but it builds very slowly. When the attack is slow, the release doesn't work. When the attack is all the way to the left, the release works, but it works strangely. It drops down quickly, then takes a long time to release. This synth is the biggest pain in the ass!
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ringroad



Joined: Feb 28, 2012
Posts: 38
Location: Coventry, UK
Audio files: 1

PostPosted: Sat Jan 31, 2015 1:51 am    Post subject: Reply with quote  Mark this post and the followings unread

Dodgy timing capacitor, perhaps? Might be worth replacing just to rule it out.
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facon



Joined: Jan 14, 2015
Posts: 22
Location: Michigan, USA

PostPosted: Sun Feb 01, 2015 12:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

Replaced the cap again with no change. There seemed to be some intermittent issues with P9 and P10, so I cut the connectors and wired them direct. I'm getting closer.

Now the attack works most of the time, but the release is strange. Starting with nothing pressed, the attack works as it should. When I let go of all keys, the sound cuts immediately. Regardless of the release settings. Now, when I'm holding multiple keys down and I release one of them, the release works on that key. Until I let go of all keys, then the sound cuts right out again.
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facon



Joined: Jan 14, 2015
Posts: 22
Location: Michigan, USA

PostPosted: Sun Feb 01, 2015 3:53 pm    Post subject: Reply with quote  Mark this post and the followings unread

Of course I just turn it on and the Attack no longer works, the the release works properly again. This is so strange. Nothing would moved or changed. An IC going bad?
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Blue Hell
Site Admin


Joined: Apr 03, 2004
Posts: 22884
Location: The Netherlands, Enschede
Audio files: 239
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PostPosted: Sun Feb 01, 2015 4:05 pm    Post subject: Reply with quote  Mark this post and the followings unread

Confused bit of a nightmare eh ... just had a quick glance at the service manual ... it is a bit hard to find stuff there ... it is the string AR?
_________________
Jan
also .. could someone please turn down the thermostat a bit.
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facon



Joined: Jan 14, 2015
Posts: 22
Location: Michigan, USA

PostPosted: Sun Feb 01, 2015 5:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yeah, its pretty difficult to read the schematics. It is just the string AR. It appears that the problem should be isolated to the String Control board, but I'm not sure. The gate voltages on the Lower Voicing board (3 of 3) read fine.
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facon



Joined: Jan 14, 2015
Posts: 22
Location: Michigan, USA

PostPosted: Sun Feb 01, 2015 6:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

I made some more progress. The ground connector for P10 on the lower voicing board wasn't properly connecting to the circuit. So, I just manually wired it to the next ground point.

So, the attack and release now work! However, there is something strange about it. When I press a key while a note is fading out, it completely cuts the release and starts the new note. I don't know if this is normal from the synth, but I doubt it. If I'm pressing a key down the entire time, the attack and release function normally.
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memetix



Joined: Jan 01, 2013
Posts: 6
Location: Denver, CO, USA

PostPosted: Thu Feb 15, 2018 10:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

i thought i'd necropost here to bring up that i see the LM301 opamp in these has hit end of life in production.
presumably there are equivalents, but purists here may want to pick up a few for that project waiting in the closet, now.

mouser has about 1700. (as of this writing)
digikey is out.


thanks guy, for posting your repair process, mine is mostly working now.
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