ARP Omni 2 - slide pots?

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commodorejohn
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ARP Omni 2 - slide pots?

Post by commodorejohn » Fri Mar 13, 2020 8:01 pm

So I picked up an Omni, which I'm quite happy with, but unfortunately it's seriously in need of TLC - all the sliders are damn near completely seized up, and some of the cheapo plastic shafts are breaking on top of that. Additionally, some of the switches are unresponsive. I'm considering strategies for sorting these out; I could try removing and doing a deep clean on the aforementioned components, but particularly with the sliders I'm not sure it's worth the trouble. The buttons are listed in the service manual as DIGITAST-ST, which Mouser appears to have in stock, so that's probably viable. Unfortunately, the sliders appear to be manufacturer-specific - the "ARP part number" in the service manual is identical to the "ARP/Mfg. Number," and Syntaur lists them as "ARP-style" (and out-of-stock, natch.) Can anyone weigh in on whether these are readily replaceable with any more standard part? I suppose I can always measure the dimensions and look through the listings on Mouser/Digikey for matching parts with the proper configuration/values, but I thought I'd check and see if anyone else has prior experience with this...
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Re: ARP Omni 2 - slide pots?

Post by Bartimaeus » Fri Mar 13, 2020 11:38 pm

as far as i know there's no modern slider that's a drop-in replacement

there's a ton of good info in this video, including how to repair old sliders (it's a LOT of work):

synthchaser has originals but they're pretty expensive: https://synthchaser.com/product-category/sliders/

but they also offer populated pcbs that work as swap-in replacements: https://synthchaser.com/product/synthch ... rp-omni-1/

the pcb is the most expensive, but it's also going to be the easiest by far

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Re: ARP Omni 2 - slide pots?

Post by commodorejohn » Sat Mar 14, 2020 12:33 am

Yaugh! You ain't kiddin'!

Thanks for the info...gonna have to have a think on strategy here. At those prices, almost seems like it'd be cheaper to rig up a stripboard with modern replacements and compatible wiring...
Computers: Amiga 1200, DEC VAXStation 4000/60, DEC MicroPDP-11/73
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Re: ARP Omni 2 - slide pots?

Post by Revok » Sat Mar 14, 2020 3:02 am

It's not a fun process by any means but it isn't too difficult to disassemble, clean, and lubricate them.

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Re: ARP Omni 2 - slide pots?

Post by KSS » Sat Mar 14, 2020 9:14 am

commodorejohn wrote:
Sat Mar 14, 2020 12:33 am
At those prices, almost seems like it'd be cheaper to rig up a stripboard with modern replacements and compatible wiring...
Modern sliders on intermediary transitonal PCBs is one way to deal with this. At least a few versions have been offered for sale -that's what the Cirroco LED slider upgrades are- and it's not too hard to come up with your own if you're PCB CAD capable.

Against this, if you can rebuild -or have them rebuilt- then you'll find that these original sliders have a nice feel. Be especially careful how you remove the end clips. Easy to bend or break them. if that happens, it 'can' be dealt with. But you're *far* better off not having to do it.

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Re: ARP Omni 2 - slide pots?

Post by commodorejohn » Sat Mar 14, 2020 1:13 pm

The plastic shafts are the main argument against bothering to clean/repair them, I think...at least one is already disintegrating, and none of the others feel too sturdy. But I see that you can get those individually, so maybe it's more economical to just replace those and salvage the rest while I'm at it...hmm.
Computers: Amiga 1200, DEC VAXStation 4000/60, DEC MicroPDP-11/73
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Re: ARP Omni 2 - slide pots?

Post by JimY » Sat Mar 14, 2020 2:47 pm

I feel your pain. Even knocking up a replacement fader pcb with perf or strip board can be problematic since the new slide pots may not be pinned out on an 0.1"matrix. Synth style caps (can they be knobs?) are not so widely stocked, but you may have more luck than I with being in the US. In the UK, everybody stocks those big scooped mixer fader caps which are useless to me - and don't even fit on the few styles of slide pots they sell particularly well!

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Re: ARP Omni 2 - slide pots?

Post by commodorejohn » Sat Mar 14, 2020 3:23 pm

commodorejohn wrote:
Sat Mar 14, 2020 1:13 pm
But I see that you can get those individually, so maybe it's more economical to just replace those and salvage the rest while I'm at it...hmm.
...he said, having recalled seeing that Syntaur stocked them but not what the pricetag was...jeezus. Yeah, at these prices it looks like I may as well either bite the bullet and get made-to-fit replacements from Synthchaser or just try and hack up a homemade replacement with industry-standard parts :/
Computers: Amiga 1200, DEC VAXStation 4000/60, DEC MicroPDP-11/73
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Re: ARP Omni 2 - slide pots?

Post by commodorejohn » Mon Mar 16, 2020 2:02 am

After some consideration, I'm just gonna pick up the made-to-order replacements from Synthchaser. A look under the hood this afternoon showed that all the boards are in good shape, so there's no sense dropping an extra $80 on the LED/PCB upgrade kit, and the switches are all functional but dirty, so some contact cleaner and exercise should do the trick there. Gonna pick up the re-cap/filter mod kit as well, though - get it all tweaked into good shape :)
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Re: ARP Omni 2 - slide pots?

Post by oldgearguy » Mon Mar 16, 2020 9:12 am

FWIW, I've repaired a lot of ARP sliders over the years by carefully taking them apart, cleaning all the pieces, adding a dab of lubricant and reassembling. Unless the track is completely worn, it's a successful but tedious task.

The only times where the sliders were not salvageable was when someone had sprayed some cleaner (usually a heavy amount of DeOxit D5 - the red gives it away) and over time the slider wiper freezes to the carbon track and you either can't move the slider at all, the track snaps under pressure, or the slide shaft snaps off.

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Re: ARP Omni 2 - slide pots?

Post by commodorejohn » Sat Mar 28, 2020 7:29 pm

Well, having a quiet weekend with nothing on the docket thanks to ongoing world events, I started in on recapping the Omni this afternoon. Thing I like: single-layer PCBs with plain solder traces means it's much less nerve-wracking to work on than fancier boards. Thing I hate with the fury of a thousand suns: the fact that the filter board is attached to the synthesizer board by means of big chunky standoff wires soldered directly in, which is going to make it hell to remove and to replace. God damn it, ARP...
Computers: Amiga 1200, DEC VAXStation 4000/60, DEC MicroPDP-11/73
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Re: ARP Omni 2 - slide pots?

Post by KSS » Sat Mar 28, 2020 8:41 pm

commodorejohn wrote:
Sat Mar 28, 2020 7:29 pm
Thing I hate with the fury of a thousand suns: the fact that the filter board is attached to the synthesizer board by means of big chunky standoff wires soldered directly in, which is going to make it hell to remove and to replace. God damn it, ARP...
Those 'wires' are actually Mill-Max .040 dia pins. They may be used with Mill-Max sockets designed for .040 pins. You have to drill out the PCB to fit the larger diameter sockets, and you need to clean the VCF pins well of their solder so the socket and pins mate well. You also need to address the potential for separation between the VCF and mian board which wasn't a problem when they were soldered together. But after all this you have a pluggable VCF.

A vacuum de-solder tool or GOOD FRESH de-soldering braid will make the pins departure from the PCB easier. Be extremely careful not to overheat the main PCB. And it's a good idea to heatsink the leg you're de-soldering. And again when you re-solder it.

edit: If you're replacing the pots, please don't throw them away. They'll be appreciated by someone who doesn't mind cleaning them up. /edit

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Re: ARP Omni 2 - slide pots?

Post by commodorejohn » Sat Mar 28, 2020 9:06 pm

Thanks - I thought they seemed familiar, but having a name to put to it gives me something to go on - might have to have a look at Mouser. Pluggable would be good, although I'd also have to rig up some kind of little slip-in mount for the board on the bottom of the case. Room enough for it, though!

Fortunately, I got the board removed without too much trouble, though doing the filter mod is going to be a little finicky; it's much more cramped than the Omni's main boards. Still, should be doable. And yes, I'm not tossing the pots; they're gummed up pretty good, but if someone wants to take a stab at cleaning them, they're more than welcome to.
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Re: ARP Omni 2 - slide pots?

Post by KSS » Sat Mar 28, 2020 9:17 pm

If by filter mod you mean changing R2,7,12 and 17, you can just put another four 4k7's on the VCF PCB solder side. 4k7's paralleled being 2k35 it's fast and easy. Don't even need to de-solder the VCF from the main board.

edit: While you've got it open, be sure to clean the sockets for and pins of Q1 and Q2 on the PSU. /edit
Last edited by KSS on Sat Mar 28, 2020 9:27 pm, edited 1 time in total.

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Re: ARP Omni 2 - slide pots?

Post by commodorejohn » Sat Mar 28, 2020 9:22 pm

Well, I've already got Synthchaser's recap/mod kit, so I might as well do it the more conventional way. Nice trick, though!
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Re: ARP Omni 2 - slide pots?

Post by commodorejohn » Sun Mar 29, 2020 5:48 pm

KSS wrote:
Sat Mar 28, 2020 8:41 pm
Those 'wires' are actually Mill-Max .040 dia pins. They may be used with Mill-Max sockets designed for .040 pins.
Just wanted to run this by you since you seem to have a little more familiarity with the subject than I do. Based on the datasheets, it looks to me like this socket and this pin should be compatible, correct? The recommended drilled hole size on the pin is closer to 0.047", but a cursory measurement of the holes in the PCB seem to indicate that it's around that size.
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Re: ARP Omni 2 - slide pots?

Post by KSS » Sun Mar 29, 2020 9:11 pm

Socket is fine, but overly expensive if you're willing to re-drill the submodule mounting holes to .073" . The extra material needed for the tail on the one you linked which allows mounting in the original main PCB hole makes for extra cost. Otherwise it's ok. It also raises the height of the SubM PCB more above the main PCB if you're using the original pins, so be aware of that.

Pin you linked is IMO a no-go. It may fit the socket - I didn't check- but it's not what you want elsewise. The 3123, 3124, or 3140 all "-2" for an .060 PCB thickness are what you want for the submodule.
But I don't recommend replacing the pins. They're swaged at the top before soldering and you have to be really good at de-soldering and have good general mechanical skills not to ruin the SubM PCB traces. Because first you need to remove the swage with a grinding, filing or spinning countersink type tool. Then you have to get rid of the extra solder. Then you have to press out the old pin and re-solder the new one. And not rip the traces from the PCB substrate in the process! DON'T try to press out the pin without removing the swage top first!
Each of the prior three processes adds heat to the PCB, and ARP PCBs are single sided and don't take extra heat well. Though they are of epoxy and so higher quality than some other storied synth brands.

Original ARP pins were 1.00" but mill-max no longer offers these. The 3122 at .880" are the longest still made in buy it now qty. There's no magic in the length, and you can decrease the distance between main and sub PCB without ill effect. Don't over-do it, and don't do it if you don't need to. Look at all the DOA's (Discrete Op Amp) using these same kinds of pins and sockets in studios worldwide for more clues and resources.

edit: ARP used long pins to have room for soldering. They used soldering over sockets to save money. /edit

You don't need gold pins and sockets but you don't want to mismatch them either. So if one has to be gold due to availability, the other needs to be also.

Mill-Max are not the only supplier for these kind of pins and sockets. Harwin is another mfr.

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Re: ARP Omni 2 - slide pots?

Post by commodorejohn » Mon Mar 30, 2020 1:25 am

Thanks for the warnings ahead of time. Curious about the PCBs, though - as far as I can tell it pretty much looks like single-sided bare solder traces sitting atop the PCB, so I'm unclear on how it would be possible to "ruin" them (as compared to more complex boards where you can pull up the traces from under the coating, or where there are transition points between layers.) Worst case you just run a patch wire from point A to point B, no?

In any case, I'm going to have to replace at least one pin, as it snapped off in the process of de-soldering from the synthesizer board, and the others are in similarly old/brittle condition, so not doing it isn't really an option.
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Re: ARP Omni 2 - slide pots?

Post by Revok » Mon Mar 30, 2020 9:51 am

The traces in mine lifted quite easily. It was as if the adhesive had decayed in such a way that the tiniest bit of heat completely freed the copper from the board. It's been years but I recall using something, maybe a chopstick, to hold the trace down as I worked on it.

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Re: ARP Omni 2 - slide pots?

Post by KSS » Mon Mar 30, 2020 7:59 pm

commodorejohn wrote:
Mon Mar 30, 2020 1:25 am
Curious about the PCBs, though - as far as I can tell it pretty much looks like single-sided bare solder traces sitting atop the PCB, so I'm unclear on how it would be possible to "ruin" them (as compared to more complex boards where you can pull up the traces from under the coating, or where there are transition points between layers.)
As Revok said, the heat lifts the traces completely from the PCB substrate by degrading the glue between the original foil and board. So you can be left with a free-hanging copper foil trace with the pad still attached at its end-in a good case- and in the bad cases the foil separates and you're left with a repair job. It is possible to re-glue the foil down using epoxy, but it's FAR better to be careful with overheating in the first place.

Two-sided PCBs with plated though holes PTH are harder to mess up since the pad on top and bottom works with the plating in the hole to make a 'rivet'.
This rivet is only attacjed to its 'heads' by a very thin foil thickness so care in soldering still applies.

The second way the PCB is ruined with heat is when the epoxy filling of the substrate gets too hot for too long and 'liquifies' leaving whitish glass fibers. You'll know it if you ever see it. The greenish or tan tint of the ARP substrate material is gone from the overheated area. This weakens the PCB, but this is not generally a factor for synths. It's ugly and embarrassing though. And it does allow the possibility of water/moisture getting into the board between the fibers which then comes with a new set of problems.
Worst case you just run a patch wire from point A to point B, no?
Yes. But it's not always so easy to do that in a way that is robust and reliable over time. Again, epoxy should be used to aid the mechanical aspects of this kind of repair. Some recommend and use CyanoAcrylate super glue, but that's not a great idea. CA isn't really intended for use in high-heat applications.

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Re: ARP Omni 2 - slide pots?

Post by commodorejohn » Mon Mar 30, 2020 8:06 pm

Okay, good to know.
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Re: ARP Omni 2 - slide pots?

Post by Jim the Oldbie » Mon Mar 30, 2020 8:10 pm

KSS wrote:
Mon Mar 30, 2020 7:59 pm
...Some recommend and use CyanoAcrylate super glue, but that's not a great idea. CA isn't really intended for use in high-heat applications.
Man, I can't +1 that enough.

Many years ago I was doing a re-repair on a board that had some traces glued down with Super Glue. I didn't notice it was there until the heat from the iron vaporized it, and it went right up my nose. For a few seconds, I Could. Not. Breathe. It was not fun.

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