ARP Quadra (yet again) two new problems....

  • Thread starter Thread starter Blue Jinn
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Blue Jinn

Blue Jinn

Rider of the ARPocalypse
Well, I love this keyboard, but this is probably the 5th or 6th issue I've had with it since I bought it in 1988.

So now.

Problem 1, one key not firing, probably an easy fix, I haven't looked too closely at the schematic yet,

Problem 2, the string section comes out distorted, the 4' string and poly are out of tune on and "warbling" on several notes notes. The string/poly section is based on the Omni, and there should be a lot of info out there on fixing those. Also, the string attack isn't working as it should.
 
Mmmmh. Low output and distortion on the strings goes away after a "warm up" or after on/off cycling a few time. There are three notes more or less that are out of tune with teh others on teh four foot string and poly section. AFAIK the string and poly section use the same master tone generator and divide down. The schematic on this part is a little confusing, so I'm still looking into this. I haven't done any serious troubleshooting yet, but will post here as I progress.

I have a schematic somewhere for the Omni, (the Quadra from what I know used existing technology from the Omni and the Solus, with micoprocessor controls from the Centaur project...)
 
I have no clue about the tuning problem, but the distortion/low output thing might be a capacitor issue. 'Course, it might also be a bad solder join if it's related to thermal expansion.
 
I'm guessing capacitor too, but the dead note now is alive, in that it won't shut off. . . Guessing mechanical issue there, which isn't necessarily a good thing either, just easier to identify. I'm going to start with the bad key and see if it's mechanical. There is an organ repair shop here in town that has a limited supply of Pratt-Reed parts (assuming it's a Pratt-Reed keyboard, which is more likely than not..)
 
Doing research. I've been searching a lot on OMNI rather than Quadra, as the Quadra is more rare. Stuck on OMNI keys seems to be a capacitor issue. However, this one, was first dead, and then stuck on. Also, it is stuck on for every section of the Quadra, lead synth included, in addition to sending CV for that note out the interface jacks (connected to an Avatar. -- when this is working that equals a 5 octave ARP Odyssey...)

That leads me to think it is either a shorted contact or a problem on the keyboard interface board and not the Omni-ish voicing boards (as far as the stuck key is concerned) there are still voicing problems which is probably caps, I should just recap the damn Omni boards and be done with.
 
Looks like the dead (or maybe undead -- it came back to life a few times stuck on...) is not mechanical. The J-wire looks intact, and a simple continutiy test shows it working. (The buss bar could use a cleaning but otherwise all is in order there)

The keys have a series of CMOS buffers (? -- this part I don't really grasp) that create a matrix for the CPU to decode. I was studying the schematic last night, and I'll need to study up on what they exactly do. I'm not sure if it is related, but all the high three B notes have the same tuning problem, and they all share a common IC on one of the voicing boards, so that could be part of hte problem. Sadly none of the chips are socketed so a quick change isn't.

On a side note, there was aguy selling a Quadra PSU on Ebay not too long ago, and I should have bought it to rebuild, Oh well....
 
Can't believe it's been almost two years, and I've not had a chance to really troubleshoot this more. Just posting an update here. I did come across a recent post on the Moog Music Forum, from the tech (I'm pretty sure it's the same guy) who did a lot of repairs for me in the mid 90s. When I get my act together I may post over there for some advice!
 
I'm not sure how similar the Quadra is to the Omni-2 in terms of control boards etc, but I know the string section is supposed to be taken right from the Omni-2. If the string section has tantalum caps, I would replace them all. I had all kinds of problems and open notes on my Omni and relacing the caps fixed all those issues. If the key isn't firing, I would very carefully re-seat the ribbon cable from that section of keys to the board. I actually broke a pin on mine; they are very delicate. But I was able to seat the pin back in the socket and get the connector to touch it just enough that the note works now.

Finicky f'n machines though...Quadra looks dope, and I've always wanted one, until I heard it was basically an Omni-2 with a Solus synth section. With buttons that fail. Hope you get it working.
 
WhinyLittleRunt said:
I'm not sure how similar the Quadra is to the Omni-2 in terms of control boards etc, but I know the string section is supposed to be taken right from the Omni-2. If the string section has tantalum caps, I would replace them all. I had all kinds of problems and open notes on my Omni and relacing the caps fixed all those issues. If the key isn't firing, I would very carefully re-seat the ribbon cable from that section of keys to the board. I actually broke a pin on mine; they are very delicate. But I was able to seat the pin back in the socket and get the connector to touch it just enough that the note works now.

Finicky f'n machines though...Quadra looks dope, and I've always wanted one, until I heard it was basically an Omni-2 with a Solus synth section. With buttons that fail. Hope you get it working.
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Hey, thanks. I have a SM for the Omni too. The boards are referenced as Upper Voicing and Lower Voicing like on the Omni. Also, there is mention of Omni on one of the boards in the Quadra service manual. I'll look at the ribbon cables, I've been in there recently (another thread here) to replace the RAM. The cables for the microprocessor board were absurdly hard to reseat. I don't exactly understand the keyboard logic mechanism, there are a bunch of logic chips involved, and it may be a capacitor associated with one of those. The RAM issue was easy to fix, as the RAM was socketed. Everything else is soldred into place which is a huge PITA.

Managed to get a pre-programmed MPU from a guy in England, AFAIK that's the only part which couldn't be replaced off the shelf.

I actually always wanted one too. Saw one in a record store way way way back when and a few years later found another one for sale, a guy who worked at Guitar Center had one, and it just came up in idle conversation he wanted to sell it. One of the good things about being as old as I am, that analog keyboards were completely out of fashion when I picked this thing up and a few other gems. (Wish I still had the Odyssey and the Crumar DS-2 I used to have....)

As to tantalum caps, it's rife with them. Had to replace one on my Avatar, that was shorting a +15 rail to ground. The Solus is another rather rare beast. I'd like one of them as the keys don't hang over the edge like they do on other ARP's. But if I can get the damn Quadra to keep running having an Avatar slaved to it, is an Odyssey with a five octave range.
 
Blue Jinn said:
But if I can get the damn Quadra to keep running having an Avatar slaved to it, is an Odyssey with a five octave range.
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Good point. I'd try to get the Quadra working, personally. It's just such a hard-to-find synth these days that if you could say you have one, you look that much better :) If you really run into trouble, Wes Taggart @ Analogics may know his way around this one (he's helped me with my OB-8 in the past as well as some other stuff for a friend).
 
WhinyLittleRunt said:
Good point. I'd try to get the Quadra working, personally. It's just such a hard-to-find synth these days that if you could say you have one, you look that much better :) If you really run into trouble, Wes Taggart @ Analogics may know his way around this one (he's helped me with my OB-8 in the past as well as some other stuff for a friend).
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Thanks for the name. I do know a guy here locally that i've used before. He'd cut traces, which scares the hell out of me, but he was factory trained by ARP, and has always come through.

Yeah, it is kinda fun to be the only kid on the block with a Quadra and Avatar. :D

I broke the pickup for the Avatar (I think it would still work, just broke one of the places you screw it down) Oddly enough I've seen 3-4 pickups on Ebay never bothered to get a new one. I know a guy who actually had an Avatar back in the late 70s, (I imagine he also had a nice big wallet too) who used it as a guitar synth, said it was cool, but the guitar to cv interpreter was very unreliable from a musical standpoint. I have an old ARP catalog that has Pete Townshend messing around with one.

Supposedly only 300 Avatars were built.

You have a nice axe collection though. Always wanted an Oberheim, I remember somebody making fun once of the "OB method."
 
Yeah the OB is cool but it is starting to have issues. Sounds good though, I mean the OB-Xa is a little thicker sounding but once you get working and using it it doesn't matter. I like it just the same.
I know most people who have Avatars just cv them to a controller and play it as, well, basically and Odyssey.

I can't talk about these things; I get bad G. A. S easily :-D
 
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