Oberheim Xpander and modern analogue synths

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Dave Peck
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Re: Oberheim Xpander and modern analogue synths

Post by Dave Peck » Mon Jan 11, 2021 3:13 pm

I have an Xpander with one voice that's misbehaving. The voice seems to be 'stuck' in the wrong filter mode and won't respond to the panel encoder when trying to select another filter mode, and the filter doesn't respond to parameter changes from the panel encoders (i.e. won't change cutoff or resonance).

The oscs are messed up too - they are at a pitch that is way too high, and the panel encoders will affect the osc pitch but not correctly - the oscs jump several notes with each encoder click instead of the expected 1/2 step per click.

So it sounds like it may not be an issue with the analog voice circuitry, but rather some error in the way all of the parameter info is being sent to the voice...?

I suppose it could be something in the analog voice that is more systemic, like a power issue on the voice board, but aren't there actually two voices per physical circuit board? I don't see how a power issue could affect only one of two voices that are on the same PCB...?

Anybody else had this issue and know what it is? I have no problem repairing synths once I know what the problem is but this may be beyond my troubleshooting abilities.

Or a recommendation for Xpander service in the SF bay area?

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Re: Oberheim Xpander and modern analogue synths

Post by Sir Ruff » Mon Jan 11, 2021 3:17 pm

There is an active Xpander/M-12 FB group, i would join and post. Lots of potential tech input there.

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Re: Oberheim Xpander and modern analogue synths

Post by MisterJ » Mon Jan 11, 2021 6:15 pm

Xpander service manual, if you need one.

https://manuals.fdiskc.com/flat/Oberhei ... Manual.pdf

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Re: Oberheim Xpander and modern analogue synths

Post by Dave Peck » Mon Jan 11, 2021 6:50 pm

Thanks guys, this is a good start!

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Re: Oberheim Xpander and modern analogue synths

Post by BenJBX » Tue Jan 12, 2021 2:15 am

MisterJ wrote:
Mon Jan 11, 2021 6:15 pm
Xpander service manual, if you need one.

https://manuals.fdiskc.com/flat/Oberhei ... Manual.pdf
Thanks for this!

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Re: Oberheim Xpander and modern analogue synths

Post by Portabella » Tue Jan 12, 2021 4:26 am

Does someone have a better scan of the service manual?
the version floating around the WWW is so badly scanned one can't even read the schematics

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Re: Oberheim Xpander and modern analogue synths

Post by MisterJ » Tue Jan 12, 2021 3:32 pm

This copy is larger, it looks like the same scan but I can definitely read the schematics on it. I wasn't sure the forum would allow a 37mb attachment but it looks like it's going to post.
You do not have the required permissions to view the files attached to this post.

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Re: Oberheim Xpander and modern analogue synths

Post by maxwellravitz » Wed Jan 13, 2021 12:40 pm

Having just spent a year repairing an xpander with weird voice issues, there are many points at which things could go wrong. What happens when you running the tuning cals? Does the voice exhibiting issues fail the test?

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Re: Oberheim Xpander and modern analogue synths

Post by Dave Peck » Wed Jan 13, 2021 1:57 pm

maxwellravitz wrote:
Wed Jan 13, 2021 12:40 pm
Having just spent a year repairing an xpander with weird voice issues, there are many points at which things could go wrong. What happens when you running the tuning cals? Does the voice exhibiting issues fail the test?
Yes, that voice fails.

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Re: Oberheim Xpander and modern analogue synths

Post by Portabella » Wed Jan 13, 2021 3:40 pm

Sir Ruff wrote:
Sat Nov 28, 2020 1:17 am
Speaking of non-sucky patches, I have decided to pass along my collection of patches from 13 years of Xpander/M-12 programming: ~90/100 are mine, have held these close to my chest but now that the M-12 is leaving I'm happy to share. Names are (mostly) meaningless. Many are velocity sensitive (indicated often, but not always, with a "V" at end of name), a few use AT. Feel free to use as you wish of course, but if one really resonates with you in a track, I wouldn't mind hearing about it.
I realized this is a sysex file .. how do I upload this to my xpander?
I was only using patches in and out via the cassette interface up until now.
Is there a better way?

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Re: Oberheim Xpander and modern analogue synths

Post by Portabella » Wed Jan 13, 2021 3:56 pm

MisterJ wrote:
Tue Jan 12, 2021 3:32 pm
This copy is larger, it looks like the same scan but I can definitely read the schematics on it. I wasn't sure the forum would allow a 37mb attachment but it looks like it's going to post.
Awesome thanks

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Re: Oberheim Xpander and modern analogue synths

Post by Sir Ruff » Wed Jan 13, 2021 4:11 pm

Portabella wrote:
Wed Jan 13, 2021 3:40 pm
Sir Ruff wrote:
Sat Nov 28, 2020 1:17 am
Speaking of non-sucky patches, I have decided to pass along my collection of patches from 13 years of Xpander/M-12 programming: ~90/100 are mine, have held these close to my chest but now that the M-12 is leaving I'm happy to share. Names are (mostly) meaningless. Many are velocity sensitive (indicated often, but not always, with a "V" at end of name), a few use AT. Feel free to use as you wish of course, but if one really resonates with you in a track, I wouldn't mind hearing about it.
I realized this is a sysex file .. how do I upload this to my xpander?
I was only using patches in and out via the cassette interface up until now.
Is there a better way?
check instructions on previous page ;)

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Re: Oberheim Xpander and modern analogue synths

Post by Dave Peck » Wed Jan 13, 2021 4:44 pm

A bit more info.

The problem is happening on voice #2. I opened the synth and I see that all six voices are on one large PCB (I had remembered incorrectly, thinking it had 3 PCBs with 2 voices per board). So it's not possible to do any kind of hardware voice swapping to try to troubleshoot. Darn it.

The filter for voice #2 seems to be stuck in one of the bandpass filter modes, but the filter's fc and res DO respond to the panel encoders.

Both of the voice #2 oscs are responding to the panel encoders in a really weird way when you attempt to change the osc pitch:

- From encoder value zero to value 11, the pitch DROPS instead of increasing, and the steps are larger than the correct half-step.
- At encoder value 12, the pitch jumps up more than an octave and descends again until value 28, again with incorrect pitch change per step.
- At value 29, the pitch jumps up even higher and descends until value 44, again with incorrect pitch change per step.
- At value 45, the pitch jumps way up, maybe three octaves, and descends until value 54, incorrect pitch changes per step again.
- At value 55 though 63, the pitch goes UP by a half step per encoder step, which is correct. These last nine encoder steps are the only steps that result in correct oscillator behavior.

And the other encoder, for FINE TUNE, is working backward for both oscs. It goes flat when you increase the value.

Weird.

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Re: Oberheim Xpander and modern analogue synths

Post by maxwellravitz » Wed Jan 13, 2021 6:05 pm

Actually most or all of the chips are socketed, so it's quite easy to do some swapping to help narrow down the issue. The oscillators are the 3374 chips, and the filters are the 3372 chips, so you can begin by swapping those chips with another voice to see if those chips are causing your problem. My guess is this has more to do with something relating to the DAC, and I'd bet you have a couple 4051 chips that have gone bad. I would start by installing new 4051's in the DAC section (or honestly replacing all the 4051's isn't a bad idea, but there are quite a few in the Xpander), and also replacing the two polystyrene caps by the DAC (C805 and C806) because those can cause issues. It's also worth replacing the LM311 at U108. I'd say that's a pretty easy place to start, and can point to other places to look if that doesn't solve things.

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Re: Oberheim Xpander and modern analogue synths

Post by BenJBX » Wed Jan 13, 2021 6:24 pm

Dave Peck wrote:
Wed Jan 13, 2021 4:44 pm
A bit more info.

The problem is happening on voice #2. I opened the synth and I see that all six voices are on one large PCB (I had remembered incorrectly, thinking it had 3 PCBs with 2 voices per board). So it's not possible to do any kind of hardware voice swapping to try to troubleshoot. Darn it.

The filter for voice #2 seems to be stuck in one of the bandpass filter modes, but the filter's fc and res DO respond to the panel encoders.

Both of the voice #2 oscs are responding to the panel encoders in a really weird way when you attempt to change the osc pitch:

- From encoder value zero to value 11, the pitch DROPS instead of increasing, and the steps are larger than the correct half-step.
- At encoder value 12, the pitch jumps up more than an octave and descends again until value 28, again with incorrect pitch change per step.
- At value 29, the pitch jumps up even higher and descends until value 44, again with incorrect pitch change per step.
- At value 45, the pitch jumps way up, maybe three octaves, and descends until value 54, incorrect pitch changes per step again.
- At value 55 though 63, the pitch goes UP by a half step per encoder step, which is correct. These last nine encoder steps are the only steps that result in correct oscillator behavior.

And the other encoder, for FINE TUNE, is working backward for both oscs. It goes flat when you increase the value.

Weird.
You may have already covered this but when you have chips doing straight up wierd stuff it's worth checking the ripple on the supply rails and replacing any electrolytics in the PSU if needed.

Certain digital circuits do not like ripple and harmonics at all, it interacts with their clocking frequencies and causes all manner of funny business.

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Re: Oberheim Xpander and modern analogue synths

Post by Dave Peck » Wed Jan 13, 2021 6:57 pm

Yup I'll be doing that as part of the repair. But due to the way it affects multiple aspects of voice #2, but ONLY voice #2, it's looking like this is something more than just a PS issue.

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Re: Oberheim Xpander and modern analogue synths

Post by MisterJ » Wed Jan 13, 2021 9:22 pm

I've never had that sort of problem with my Xpander but from the description it could be a failure of the sample and hold for that voice. I just took a quick look at the service manual and the control voltages are put on the VCOs and VCFs with a sample and hold circuit which is the way polyphonic synths from that era generally work. An unstable voltage on the sample and hold could cause the problems described. If the sample and hold isn't holding properly, what the encoders put out to the DAC isn't going to be what the CV inputs see. That could be a failure of the capacitor in the S&H for that voice as well. It would be pretty easy to put a volt meter across the CV inputs for the filter and oscillator and compare CVs on the bad voice with a good one. An oscilloscope would be better because you would see any AC ripple on the CVs but voltmeter would be sufficient to check them for stability.

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Re: Oberheim Xpander and modern analogue synths

Post by maxwellravitz » Wed Jan 13, 2021 9:51 pm

As I mentioned before, I would really start by replacing the two .22uf poly caps and the 4051 multiplexers in the DAC section of the voice board. The caps will require soldering, and might be a bit tricky to track down replacements, but the 4051s are all socketed and easy to replace.

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Re: Oberheim Xpander and modern analogue synths

Post by MisterJ » Thu Jan 14, 2021 7:05 pm

I would study the service manual thoroughly before I opened it up and desoldered any caps. The more you understand how something works, the better chance you have to diagnose and fix it. With something as rare and valuable as the Xpander, a kid glove approach is recommended. When you do open it up, do a thorough visual inspection of the voice board. Clean it of any dust or cat hair or anything else that managed to get inside. I've found dead bugs and spider webs in gear I've serviced that were the cause of the problem. To do testing, use the basic patch with the VCA opened up so you have a triangle wave going through all 6 voices at a constant amplitude with no modulation. If you don't have good soldering skills and the right equipment, I wouldn't try doing any repairs that require soldering. I'd also run the internal diagnostics and tuning routines just to see what happens.

That being said, I took a better look at the schematics and I agree that the 4051s are likely suspects, much more than the caps because they are being worked hard and switched constantly at 10ms. There are also 4 op amp buffers between the 4051s and the CEMs on one TL084 chip that could be the problem.

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Re: Oberheim Xpander and modern analogue synths

Post by Dave Peck » Thu Jan 14, 2021 9:01 pm

Internal diagnostics confirm what the actual behavior indicates - oscs & filter on voice #2 fail, everything else is checking OK. I don't have a problem desoldering & repairing through hole PCBAs once I figure out what needs repairing. And all of the ICs are DIP package and socketed.

The service manual doesn't provide much useful troubleshooting info but the schematics are helpful. And the board layout is really clear and easily serviceable. I've been looking up sources for the various parts that could be the culprit(s) and they are all readily available and cheap so I'll just place a 'shotgun effect' Mouser order for anything in the voice #2 section that could be responsible (it will cost maybe two dollars more than narrowing it down to the exact part first), plus the two big power supply caps. I'll probably have to substitute a radial package for those, laying flat and with a long insulated buss wire to connect one of the leads, since the big old axial package caps are not easy to find. Wish me luck!

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Re: Oberheim Xpander and modern analogue synths

Post by MisterJ » Fri Jan 15, 2021 9:01 am

It's always good to order spare parts and have them on hand. Mouser is a good source along with Digi-key. I always order more than I'm actually going to use. I have two badly distorted voices on my Studio 440 and several other issues. I've already figured out that it isn't the rare and expensive 3389 CEM and it looks like it's either the DACs or one of the logic chips that feed data to it. DACs don't usually fail but the 12 bit DACs were really hard to find and I found only one source, an eBay dealer in Hong Kong. When he lowered the price from $7.99 to $2.99, I just ordered 4 of them. Even if they aren't the problem, I want to have them on hand and it's a good idea to get them anyway while someone still sells them. I'm definitely following my own advice on this and have just rerouted the sounds to the good voices until I do a better diagnosis with an oscilloscope and get a few more parts for other issues. I won't get around to it for a few months and right now I have a Maxi-Korg DV-800 on my bench that I'm restoring and modding. Vintage gear has its challenges when it comes to sourcing parts but the upside is that the old circuit boards are much easier to work on than modern surface mount boards.

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