Modular 4 voice Oberheim massive repairs

[chromium]

I have been restoring a four voice and the keyboard interface is missing the two 1120A boards. I think my chance of finding these boards is pretty slim so I suspect I will have to create new ones. The schematics available for this board are not very legible and there are many parts I cannot determine the exact part number. Does anyone have a detailed parts list for the 1120A board and what parts may be difficult to get?

Thanks

Harold

Hi - just wandering back in here and happened to see your message. If it helps, Kevin Lightner shared a good photo of that board with parts (chips) list here: http://www.synthfool.com/fvsstuff/fvskeyio1120a.jpg.

My board was labeled "1120" as opposed to "1120A", and I have a hi-res photo of that here: http://i87.photobucket.com/albums/k125/ ... CF9005.jpg

You might check with Ebay seller "onkloud9" - they've had several FVS keyboard modules up for sale recently. In fact, there is one on there now.

[chromium]

I have a question that I was hoping to get some feedback on...

VCO2's frequency control on one of my SEMs had no effect in the middle 2/3-or-so of its travel. I finally got time to scope it out, and traced it to......... this









So........ I'm thinking about trying to find a new 50K donor pot (maybe CTS?) that I can use to scavenge the resistive element and restore this freaky fine-tuning-ball-bearing pot. Have any of you been down this road? Any words of wisdom?

Almost done with this one! I've recapped the entire programmer (the "programmer" part works great), replaced all the power caps elsewhere, and this thing is sounding mighty fine! Just have this and the memory store/recall quirks to sort. I can live with the memory issues for a while... heck, I live with them everyday!

Cheers

[Voltor07]

If you can't fix that pot, it's going to cost a small fortune to replace.

[chromium]

If you can't fix that pot, it's going to cost a small fortune to replace.

The good news is that a replacement pot at least comes with a bonus SEM attached

[Kevin Lightner]

I agree that if you can locate the same CTS resistive wafer and rebuild the pot, it should work.
I have a bunch of CTS pots in various values, but not sure I have that particular value.
It may not matter much regardless- a 25K or 100K may work.
You may need to change the resistor that pot feeds, but it would probably work.

I'd also suggest checking the condition of the other VCO pot and filter pots.
Other pots may not have much life left in them.

Fwiw, I DO have that exact dual concentric pot in stock.
NOS, never used condition, but only one or two left and so it won't be cheap.
Contact me via PM if interested. Thanks.

[chromium]

I'd also suggest checking the condition of the other VCO pot and filter pots.

Roger that. I'm kinda scared to look, but now's the time.

Thanks for the reassurance on the "transplant" approach. I'll definitely be in touch if that doesn't pan out.

[Technician Larry]

Wow - I started this topic back when and have not been back here to look for quite some time. Lots of views and comments.

The keyboard module boards on the unit in my shop had all the numbers sanded off the chips. I did not have much trouble though as the IC designation #s correspond with the docs. I stocked up on all those old CMOS logic chips from Mouser - all the ones I needed and did not have were available there - cheap. 50 cents or so each in small quantity.

That keyboard logic is an excellent example of old school Boolean algebra logic design. That work was done before they started using microprocessors to streamline the implementation of such workings.

[chromium]

Wow - I started this topic back when and have not been back here to look for quite some time. Lots of views and comments.

The info in this thread has been a big help to me. I come to the Moog forum now for all of my Oberheim advice! (I do have an old model D that I cherish, FWIW). I only do this as a hobby, and working on a beast like this was a big stretch for me. I really appreciate all of the generous information sharing here.

Re: the pot rebuild(s), there's a seller currently on Ebay with a bunch of CTS 50K pots with PCB mount prongs - 5 unit lots for $5/ea. I had a similar CTS pot to those in the parts drawer (different value), and it seems like everything fits together okay... so I stocked up. The donors will be here next week, and I'll share back the results.

Cheers

[dougt]

Hope those work out. I found some 100K CTS pots but no 50K. So you're selling this back to me when you're done, right?

[chromium]

So you're selling this back to me when you're done, right?

Heck, I was hoping to add the fvs expander next! Thought one of those enclosures might make a nice home for the seq and some patch panels someday...

Hey, so the pot transplant worked out... well, mostly.

The substrate/resistive-element piece was a direct swap:


The challenging part was the physical connection to the outer lug pads on the PCB. The new pot's prongs were positioned differently and couldn't be made to reach. You could always fly wires to them, I suppose, but I did it the hard way. I drilled out the rivets and removed the prongs from the original unit, pried open the mounting tabs on the replacement unit's prongs, seated the original prongs into those and clamped them back down, and then tack soldered them together for good measure. End result was this:


It's not the prettiest solution, but the pot fits back into the board now, it doesn't mess with my OCD too badly , and the pot seems to be functional again according to the DMM:


Now I need to go inspect the other freq, cutoff, etc. pots and determine whether those saw an equal amount of action...

If anyone needs 'em, these are the donor pots that I'm using:
http://www.ebay.com/itm/ws/eBayISAPI.dl ... 0852807776

[MC]

I got a FVS (not mine) on the bench that triggers SEMs but no audio.

A studio owner got this from a buddy of his in this non-working condition.

Also included is the Anvil case with rotted foam.

And a DS2a.

A tech tried to fix the FVS before it landed in my hands.

I found a missing power rail on one of the connectors. When I restored that, the FVS started making audio. Enough to make a list of what works and what doesn't.

As with any old Oberheim, there's a LOT wrong with this. I forwarded a report of the ills and potential work to him. He is primarily a guitar player and doesn't want to put much $$$ into fixing this, so he offered it to me for a trade - a working polysynth of some sort.



It may not be mine, but it soon will be. I don't have anything I want to part with (my OBX is a keeper, sorry). I know how much he paid for the FVS so I got a chance to land a handyman's special (why do the handyman's specials come my way?). Now being a guitar player, a FVS is far more than the studio owner needs. He just wants a polysynth with MIDI that he can use right away in his studio. I don't want to saddle him with a vintage piece that requires an overhaul and maintenance, so I was going to offer him a new P'08

Other than the discrete polyphonic scanner and programmer (boy those patents help to understand these things!), there isn't anything I haven't already seen in Memorymoogs and OBX/OBSX. The good news is that the super-rare custom EG ICs in the programmer and multiturn frequency pots on the SEMs are OK.

I'm gonna look forward to this restoration project (cue sinister snicker)

[MRNUTTY]

I'm a digital guy, so excuse the ignorance, but are those ball bearings in those pots? if so, wow!

Loved all those synths from the 70's and early 80's. Music shops had so many different types way back then. I remember helping the keyboard player in our band choose between the Juno-6, DX9, and Emulator. I wanted the DX9 sound, he wanted the Juno, and neither of us could afford the Emulator. LOL!

Good luck on your rebuild MC. Cool stuff

[MC]

They're not ball bearings. The pot has two shafts. The inner shaft - fine tune - couples to the actuator of the wiper through the balls, which acts as a multiturn. The outer shaft - coarse tune - is the wiper, and if you rotate the outer shaft the inner one will also rotate.

It was a neat idea to conserve panel space, but in the end Tom Oberheim admitted that he never liked them. His new SEM uses a standard pot with a second fine tune pot with extended shaft just protruding beyond the panel, much more practical and economical. Buchla did this back in the 70s with his 200 series VCOs.

The only other synth I have seen them is the Memorymoog.

Replacements are extremely hard to find and $$$. So are the concentric knobs.

I have seen at least one standalone SEM with this fancy pot on the VCF cutoff too.

[MRNUTTY]

I see it now. Pretty neat, but complicated. Must have been expensive/low volume even in it's hayday. Thanks for explaining that @MC.

[MC]

Well I heard back from the studio owner.

He wants a vintage piece. Any client is going to have their own keyboards so he wants something vintage they are not likely to own, thus nothing they can walk into GC and purchase. Can't argue with that, I have visited his studio. He also wants something that looks cool.

Last year I bought a 2nd Memorymoog from a friend who was urgent for money. It needs to be fixed. So I offered the studio owner a restored Memorymoog for the FVS package. That got his interest, we'll see what comes out of that.