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Oberheim OB-Xa Servicing?
MUFF WIGGLER Forum Index -> Music Tech DIY Goto page 1, 2, 3, 4  Next [all]
Author Oberheim OB-Xa Servicing?
mOBiTh
Hey guys

Anyone have any experience of owning and/or servicing one of these beasts?

Just about to pick one up that looks like it may need some attention (although I'm told the voices are all working)

There are some iffy keys so I'm guessing re-bushing the entire keyboard would be a good idea. Do these keyboards have brass electrical contacts?

Also is there anything I should look out for in terms of the voice cards, and making sure they're all fully functional? Not sure how long I'll have to check it over so running through the whole service manual seems unlikely!

PSU should be a fairly straightforward check so not too worried about that.

Cheers

M 8_)
The Real MC
Key contacts are gold plated and re-bushing is always a good idea with a board this old.

Checking out voicecards - they all have gate LEDs, usually two layers of four voicecards each. As you play a note you will see the LED light up, and you should hear audio.

I am restoring an OB-X which isn't too far from the Xa, check my webpage for restoration tips on Oberheim boards. Lot of things to go wrong for a board this old.
mOBiTh
cheers mate - your website looks super useful (and fascinating!) 8_)

I'll document my progress with it on here if you're interested
mOBiTh
Oh my god...................

screaming goo yo screaming goo yo w00t w00t hihi hihi 8_) 8_) It's motherfucking bacon yo It's motherfucking bacon yo


MY ASS IS BLEEDING MY ASS IS BLEEDING MY ASS IS BLEEDING MY ASS IS BLEEDING MY ASS IS BLEEDING
The Real MC
Did someone get a new toy...?
mOBiTh
The Real MC wrote:
Did someone get a new toy...?


I'm not sure if I should call it a toy incase it gets angry eek! hihi hihi

It's fantastic.

Your website has proved very helpful so far, so thanks again! The DIP switch got swapped out last night so all 8 voices are triggering perfectly now.

The autotune generally seems to work but a couple of times last night it cancelled a couple of voices (different ones each time!) after tuning. I've disabled it for now with Test 1 switch inside.

I reloaded all the factory presets off the midi board and everything seems in order, apart from the first time I tried to use the Double and Split presets - the synth kicked out a weird squiggle noise each time I changed preset, but didn't actually change preset. Hmn. It was working fine later on though.

I notice there were some bug fixes for the midi firmware in few years ago so I'll try installing the new firmware (can't tell if it's already been installed) and see if that cures it.

My unit seems to be pre- the quantised glide/portamento option, which is a bit of a shame as I'm sure that would have been fun.

I wonder if this can be upgraded-to, any ideas?

PSU seems to be in good shape but I'll probably whip it out and replace the caps and bridge rectifiers as you suggest.

The most obvious sonic 'idiosyncrasy' appears to be that when using a long glide, some voices glide significantly quicker than others. Not sure how intentional this was as the later units had two modes for glide it seems (tight and loose). It sounds totally immense though so I might be tempted to leave it alone unless it gets annoying.

Modulation control panel is a bit of a state and needs calibrating and generally tidying up. Then it's just cleaning and replacing the keyboard bushes I hope!
The Real MC
The OB-8 had quantized glide, but not OB-Xa.

Early OB-Xa had analog glide processors, later OB-Xa had digital glide (my webpage needs an update, I recently discovered this in an ECO). It sounds like you have the analog glide processors because the gilde rates differ between voices. There is no way to get matched glide rates because of tolerances between the OTAs used in the glide circuit. Oberheim had a matching procedure but it only did so much.

Ultimately Oberheim went to digital glide to match up the glide rates. But this requires upgrading the EPROMs, no idea where to get that done. FWIW, my Memorymoog has analog glide processors that doesn't suffer from this problem, haven't looked into an analysis yet.

Heck, my OB-X is a REAL early one and the OTAs in the glide circuits aren't matched at all - and it's pretty obvious with the different glide rates.
mOBiTh
Ah yes! This rings a bell, I read about the switch to digital glide somewhere, but can't remember where.

Cool - I'm really glad I have the analogue glide, it's something special 8_)

One thing I did think of which might be cool to try (but I'm not sure if it's possible) is there's a mod (ECO?) to adjust the noise source level by changing a couple of resistors on the control board.

So I wondered whether it might be possible to break-out this mod to a pot on the rear of the synth (or maybe recessed into the end cheek) so you have more control over the noise level. A foot pedal control would be even better!

Can you think of any obvious reasons why this might be a bad idea?!
The Real MC
There is no ECO that I know of, but that would be an easy mod.
mOBiTh
Cool, I like easy hihi

I wonder whether the same would be possible with oscillator two level. I'm not really worries about this as much though

Hopefully get time to study the docs a bit later

God knows how I've managed to land this beauty, what a stroke of luck!
Nordcore
Chganging the noise level is simple:



Change the 100k over the OP-Amp (A9) to change overall noise level. (Higher=louder, proportional, so 47k will give 6dB less noise level.)
Change the 100k on the left of the switch (A10), to adjust only the "half" level. Larger value will make it quieter. Amplification is proportional to this resistor + 64k (47k+47k||27k). Thus, with 100k original the "half" is 100/164=8dB quieter than "full". )

EDIT: sorry, this is for the OBX. Dunno how different the obx-a is here...
mOBiTh






8_)
The Real MC
SWEEEEEET....! Eight voices AND the Encore MIDI retrofit! That's a pile of CEMage in there!

That LH panel has been modified - what's that toggle switch?
The Real MC
Nordcore wrote:
EDIT: sorry, this is for the OBX. Dunno how different the obx-a is here...


The OB-X/Xa share a lot of circuitry. The noise circuit may even be the same.
mOBiTh
Nordcore wrote:
Chganging the noise level is simple:



Change the 100k over the OP-Amp (A9) to change overall noise level. (Higher=louder, proportional, so 47k will give 6dB less noise level.)
Change the 100k on the left of the switch (A10), to adjust only the "half" level. Larger value will make it quieter. Amplification is proportional to this resistor + 64k (47k+47k||27k). Thus, with 100k original the "half" is 100/164=8dB quieter than "full". )


Great! Cheers 8_)
mOBiTh
The Real MC wrote:
SWEEEEEET....! Eight voices AND the Encore MIDI retrofit! That's a pile of CEMage in there!

That LH panel has been modified - what's that toggle switch?


screaming goo yo

Super chuffed so far.

the PSU also seems to measure up fine despite the apparent age.

That actually isn't a toggle switch - it's an upside down M3 bolt that the previous owner put in upside down. For some reason that control section had been taken apart and the leads to the pitch bend resoldered (back to front!)

One thing that remains a bit of an unknown for me is that one of the white keys is not working at all. I'll order the new bushes tomorrow but did you experience any non-working keys on your boards? They seem so heavily engineered (and I can't see any damage) so curious to know what the possible modes of failure are for these keybeds... hmmm.....
The Real MC
Those are old Pratt-Read keyboards with j-wires for contacts. J-wires often get bent and that makes contacts intermittent or they won't work at all. Sometime they break too. I have a few intermittent keys on my unit, that's next on the list.

Just checked the polyphonic analog glide on the memorymoog schematics... they use CEM3360 instead of CA3080s, and there are trimpots to match glide rates.
mOBiTh
Cool. I guess the CA3080 option was cheaper for oberheim?

I've taken the keybed off - not a massively enjoyable task as there's not much to grab hold off that isn't fairly, or extremely, delicate! Also you have to unscrew the keyboard from the bottom which is fairly tricky when working on your living room floor meh

The broken J-wire was easy to spot - removed for the photo:

The Real MC
mOBiTh wrote:
Cool. I guess the CA3080 option was cheaper for oberheim?


Nah - the CEM3360 didn't exist when the OB-X was designed.
mOBiTh
The Real MC wrote:
mOBiTh wrote:
Cool. I guess the CA3080 option was cheaper for oberheim?


Nah - the CEM3360 didn't exist when the OB-X was designed.


Aha!

Two new problems have become apparent now that I've had chance to do some more testing.

1) Wiggling the Freq Cutoff knob quickly near minimum causes some popping/stepping sounds. This seems to happen on all sounds so would appear to be either a bad pot (these seem very heavy duty though) or perhaps an ADC problem? Actually I have no idea yet how these are encoded. Seems to be only this pot that has a problem.


2) I attempted to upgrade the firmware via midi (I wasn't running the latest version because the midi channel setting was non-persistent - a good indicator!) The upgrade process appeared to be going well with the bank lights incrementing in a binary fashion but when it reached the end of the file the keyboard hung and now is completely inoperable. Fuck. sad banana

Anyone know if encore actually exist still?! Haven't had any response to my previous email...
mOBiTh
I'm guessing this has wiped/corrupted the eeprom on the encore board, which is pretty annoying.

Shame there isn't more information on their website if the upgrade procedure is this risky!
Dubka
Ah, I'm glad someone brought this up.. I've got a fairly badly behaved (but ultimately awesome) OB-Xa.

The bottom 4 voices all have slow attack in comparison to the top 4 voices.
I'm guessing this is to do with the control/motherboard as it's happened to all 4 voices overnight. Top 4 all respond properly to the attack knob on the front panel btw.

Any ideas where I should be looking?
mOBiTh
mOBiTh wrote:
I'm guessing this has wiped/corrupted the eeprom on the encore board, which is pretty annoying.

Shame there isn't more information on their website if the upgrade procedure is this risky!


Got a fantastically quick response from Encore, but as expected it's going to need to be shipped back to the states for reprogramming.

In the mean time I'll get the keyboard rebushed and new end cheeks made to incorporate midi sockets and potential mod controls.
shaft9000
OB-Xa's are finicky boards

mine has had a variety of issues - one day it would NOT play in tune except in unison, got progressively worse, and then suddenly when using the pitchbend paddle would inextricably cause all the voices to go out of tune; haywire-like
mOBiTh
So I finally got the reprogrammed EEPROM back from Encore Electronics this morning. Excitement!

Unfortunately it doesn't work. Utter dismay...

I struggle to remain patient in situations like these. If only there was a convenient way of programming the chip in situ instead of messing around with three weeks of international postage...

In the mean time I've had some beautiful end cheeks made up for the synth in dark wenge with cut outs for mods and midi connectors. More photos to follow soon.

I've also rebushed the entire keyboard with fresh bushing so it feels great again.

The manufacturing tolerance of the bushing I received were surprisingly varied so I'd hesitate to recommend them really. Has taken a fair amount of pressing and pulling on the keys, to bend the metal bushing hangers, in order to get them vaguely level - something I'd have hoped not to do.

Back to the waiting game...
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