Oberheim Sem Schematic

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1. Oberheim Sem Module
2. Oberheim Sem Schematic
3. Oberheim Sem Schematic
4. Oberheim Matrix 12

OBERHEIM SEM-01A SYNTHESIZER EXPANDER MODULE

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Last Update 10-11-2014 #1#2#3

Inherent Design Errors

How to restore an OB-X

New Voicecards

'Tom Sawyer' rezz

Features

OB-X Revs

OB Model comparisions

Back in2000, Cary Roberts brought his eight voice OB-X to AHMW (the midwestgathering of Analogue Heaven members). I was really impressed bythe sound of the OB-X -thick, organic, edgy, different from my Memorymoog. I used toown a Matrix-6R but it didn't sound near as good as the OB-X. TheOberheim OB-X is a unique sounding classic analog polyphonic, butwith many of them in disrepair they are not easy nor cheap torestore. This is not a restore job for the novice DIYer. Bythe time I was in a position moneywise, fully loaded eight voice OB-Xwere not to be found anywhere. After a few years, I did spot twofullyloaded eight voice OB-X on the 'bay. One sold quickly to a BINbutneeded new endcaps and a restoration. Another was a dead one thathad been stored in a shed and had been a home to rodents (the telltalesign of rodent urine on the computer PC board was a deal-killer, thatstuff is a serious toxin). Spare voice cards were extremelyrare and expensive so I passed on the four and six voice varities thatappeared. I wanted a fully loaded eight voice.
For lack of finding a suitable OB-X, I snagged this basketcase OberheimOB-SX from the 'bay back in '08. Some researchonAH revealed that it had its roots in the OB-X so I opted for the SXuntil I found the OB-X I really wanted. The OB-SXneeded a restoration and I had made progress, but then I got very busyat work and it sat on the back burner. After a while I decided Ireally wanted an honest-to-goodness OB-X.
Any OB-X seems to be a rare bird on the 'bay, fully functional onesrarer indeed. I had read that only 600 of these were made. So I decided I was going to have to compromise and attempt to fix ahandyman's special if I wanted one at all, and I could land afixer-upper for a lot less $$$. So when a dead four voiceappeared, so did extra voicecards and a motherboard for the extravoices so it was perfect timing. Now I had a system for fullexpansion to eight voices. It wasn't a mint specimen but thatwasn't a concern. The Oberheim sticker on the top right is notoriginal but I decided to leave it on.
Boy can I pick 'em! This was a real basketcase as there were manyblown components, likely due to a very malfunctioning powersupply. It powered on, but the front panel would notrespond. OB-Xs do not have a good history as their componentstend to go bad,and there is a reason why many of them that turn up for salemalfunctioning as I later discovered.Unless they have been completely restored, pretty much ALL oldOberheims are malfunctioning.

All OB-X, OB-SX, and OB-Xasynthesizershave a design error

Curiousity stirred me and I wanted to know why these synthesizers aresoprone to breaking down. After studying the schematics, Iuncovered a few design errors that combined to cause the problem ofdead OB-Xs:

1. A design violation in the CMOS 4051s on the CV S&Hs. The voltage on the input pin of the 4051 must be within VEE-0.5< Vin < VDD+0.5. I found thatthe DAC VREF and VDDare the same value (5.6VDC), and sitting between them was an opampbuffer with +/-15VDC rails. On power transitions, the outputstate of any opamp is indeterminant and can swing anywhere between+/-15VDCrails as they are ramping up, way outside the VEE-0.5< Vin< VDD+0.5 rule. Even though thiscould occur in a split second, this will damage the CMOS gate andultimately blow the 4051s over time.
2. The 5.6VDC source is a mere diode piggybacked to the 5VDCrail. This is not a voltageregulator! The CMOS ICs on the 5.6VDC VDDrail will subject that source to high current transients when the CMOSswitches between states. Under this condition, the 5.6VDC diodecircuit cannot maintain its steady voltage, thus the rail voltage willdrop during transients. If this happens while any analog input onthose 4051s is greater than VDD, the VEE-0.5< Vin< VDD+0.5 rule is violated and willblow the 4051. And on the front panel circuit with the pots, the5.6VDC rail is supplied by a proper 7805 regulator biased at the commonterminal with a diode, which is the correct design (insert facepalmhere).
3. The original CMOS ICs were RCA components. I have the RCAdatabook for that era, and discovered that the first generation CMOS made by RCA didnot have input overvoltage protection diodes now standard on modernCMOS! Also these ICs were made on early 6 micronfabrication lines and are prone to outright dying due to chemicalinteractions which causes a short to ground. Today's modern CMOSare made on 0.05 micron lines and the chemical interactions werecorrected, making them far more robust. Coupled with the twoaforementioned design errors, it is no small wonder that so many OB-Xsturn up dead or seriously malfunctioning.
   

These design problems also existed in the OB-SX and OB-Xa. Thiswas finallycorrected in the OB-8.
The corrections to these problems are:

1. Replace the pseudo-regulator diode circuit with a properregulator such as a 317-based circuit. It does not draw muchcurrent and probably won't even need a heat sink. Make sure thereis a 10uf cap between the adjust terminal and ground for betterregulation under the CMOS switching transients. Setting the railvoltage a little higher than 5.6VDC - higher than the rail on the frontpanel pots - is the safest approach, so a trimpot would be wise. This is the new VDD for the CMOS 4051s onthe S&H CVs. If you set VDD toohigh above the 5VDC rail on the TTL decoders and demux's then the 4051swill not switch properly. You'll need to build a sub-board with aplace to mount it (I used a tall standoff mounted just to the left ofthe connectors leading to the voicecards, near the top of the controlboard).
   
2. Install two clipper diodes on the output of the opamp followingthe DAC, one should clip to ground the other to the new 5.6VDCrail. During power cycles, even if the 5.6VDC doesn't fully rampup this guarantees that the opamp will never swing outside the CMOS VEE-0.5< Vin< VDD+0.5 input rule.
3. Replace all RCA CMOSICs with new modern equivalents.

If you do not understand circuit design for these corrections, pleaseconsult a competent technician or engineer.
I do not offer repair or restoration services so please do not emailme because I am very busy and don't have time to address individualqueries. I often get emails from folks who want to know what myreplacement parts were, so here ya go:

• Multiturn sealed trimpots - Bourns 3296W
• Single turn sealed trimpots - Bourns 3386F
• TTL - the 74LS families will work fine I use parts from TexasInstruments
  
• CMOS - Texas Instruments or National Semiconductor
• Opamps - Texas Instruments or National Semiconductor
• Electrolytic caps - Panasonic HFS or Nichicon PW, 105Ctemperature rating. This unit uses a mix of axial and radialformats,pay attention when you order. Axial caps are getting harder tofind.
• Buy from digikey, mouser, allied, or other high volumesupplier. Hobbyist suppliers often sell substandard (even counterfeit!) parts andyou can't specify makers such as Texas Instruments. top
  

Restoringa dead OB-X

This section is applicable to other old Oberheims like OB-SX, OB-Xa,andOB-8 not just OB-X. It is also somewhat applicable to otheranalog polyphonics like the Memorymoog, Prophet-5, Roland polysynths,etc.
How prepared are you for a job of this magnitude? Take a look atthese imagesbelow. The one on the left shows my Oberheim OB-SX in the middleofrestoration. See that box to my right? That's the box ofNEW components. My OB-X needed MORE replacement components thanthe OB-SX in the picture. See the image onthe right? Those are 3/4 of the components that were discardeddue to age or malfunction (notshown are large electrolytic capacitors, disintegrated keyboardbushings, and manyother ICs). Not kidding,folks. Restoring an oldOberheim polyphonicsynthesizer is a labor intensive effort (plan extra $$$ for a competenttech) requiring about $300US in brand new parts. It's alot of work, but in the end you will have a reliable synthesizer thatstays in tune and autotunes when you aredone. Do not expect shortcuts here.

Thepatient on the operatingtable (no, the KEYBOARD silly!)	Thepile of replaced components (!!!)

This unit is serial # 7943xx, indicating it was built in the 43rd weekof 1979. The OB-X was introduced in June 1979 thus this is one ofthe early units. This one lacks a lot of field upgrades (aka ECOsor Engineering Change Orders) that have been developed over theyears. It had obviously been gigged with its share of scratcheson the panel and the wood endcaps were very soiled, which was a goodindicator of the abuse this unit had endured.
I don't often buy dead polysynths because of the time/effort inrestoration and part availability can be a problem. The OB-X wasa rare exception because its great sound justified the time/effort andwith very few exceptions (80C98s, RAM/ROM ICs, CEM3310s, CA3080s) youcan STILL get parts for these beasts. They used a lot of standardopamps, TTL, and CMOS ICs. CA3080s can fail but can still befound on the net or scavenged from other synths (the oft-scoffedpreset-only OB-SX is a popular target for scavenging valuableparts). Although the CEM3310 is the only custom IC (two on eachvoicecard) and are very expensive on the market, they are robust and donot fail often. Voicecards offered for trade are often strippedof valuable ICs like CEM3310s and 3080s, so if you find any be sure toconfirm that they still have them!
The very first thing you should check with any synthesizer is the powersupply. Nothing will work unless it is 100% functional. You should NEVER swap out ICs until youconfirm full functionality of the power supply because bad rails willblow brand new ICs. A multimeter is not enough - you need an oscilloscope which will revealfaults that will not appear on a meter. Inspection on my unitrevealed that there were several bad rails and multipledefective components on this power supply. That can blowcomponents downstream under the wrong conditions. The restorationwork on the power supply consisted of:

• Replacing bad regulators.
• Replacing bad trimpots.
  
• Replacing all electrolytic capacitors with fresh new ones ratedat 105 deg C.
• Replacing all tantalum capacitors on the power rails withelectrolytics. The maximum voltage rating on tantalum caps tends to degrade with age,which is why they have a history of prematurely failingand shorting out the power rails with them.
  
• Replacing the diode bridges with heftier ones capable of highercurrents (the originals were underspec'd and will burn out).
• Replacing the battery that retained patches in RAM memory. These batteries have a lifespan of 25 years and are long past theirexpiration date. They are used to backup the RAM where yourpatches are stored, and when the battery wears out you will lose yourpatches.
• Installing an IEC power inlet with integrated AC line filter,which was installed in later units and helped to shunt RF interferenceand other power foibles that can trip the embedded system which issensitive to power fluctuations.
  
• Implementing ECOs known to improve power supply operation. This thing has had zero upgrades since it left the factory.
  

Electrolytics are polarized soattention is required so they are not installed backwards or you willhave fireworks when you apply power! I left the tantalum caps intheaudio path intact (usually at the end of the audio path on thevoicecard), they are not subject to the large voltages like power railsand I wanted to retain that 'dirty' sound that Oberheims are renownedfor (tantalum caps are not renowned for their high fidelity, and thisis one beast where imperfections can be a good thing).
With the power supply now verified, it was time to move on totheembedded system. The front panel would light up but nothing wouldwork. The OB-X has no bootup fault reporting system. Something was locking up the embedded system. Oneproblem with troubleshooting this kind of fault is that the schematicsare so poorly drawn that I had to re-draw them to get a logicaloverview of the embedded system. The embedded system is lockingup because one or more rogue components is tying down either the databuss or the address buss, and the troubleshooting process to isolatethe rogue components is what separates the men from the boys. From that I was able to quicklyisolate the problem which turned out to be a bad 80C98 shorting out thedatabuss. This re-drawn diagram is very large and not capable ofbeing scanned, so a friend is currently putting that diagram into a CADsystem so an electronic copy will be available in the future.
With the embedded system now living and breathing, it was obvious thiswas going to need a majoroverhaul. There were a lot of defective circuits - most of theCVs from the demultiplexed system were not present, and several logiccontrol lines to the voicecards were defective. If I turned upthe portamento control, the pitch would shift and throw all the voicesout of tune. I got it to makeaudio noise, but with many of the control signals defective it wasn'tgoing to make music. I decided that I was going to do the fullmontyand replace all the front panel switches, all the trimpots, CMOS ICs,TTL ICs, opamps, tantalum caps,and electrolytic caps. With the condition that the power supplywas in, it's a safe bet that many ICs were either damaged or stressedalmost to the point of total failure. So I ordered a few hundreddollars of replacement components and a new set of keyboard bushings toreplace the dried out ones that go 'clack clack clack' when you depresskeys.
I already had a stash of NOS front panel switches with caps of varyingcolors so I elected to add a personal touch with black, grey, blue, andred button caps. I have white button caps but they clashed withthe panel so I did not use them. As I removed the originalswitches, they revealed an oily substance under the switch casing thatreeked of nicotine - cigarette smoke. Before installing newswitches, I cleaned off this substance to prevent future low impedanceshorts (and remove a known carcinogen). I arranged the button capcolor coding into logical groups. I reserved the red button capsfor functions you REALLY shouldn't activate accidentally (such asWRITE) or functions you may want in a hurry (such as AUTOTUNE). EDIT and MANUAL got the blue button caps; the HOLD, RESET, and programbuttons got the grey button caps; and the programmable synthesizerswitches got black button caps as originally installed. You cansee this arrangement in the picture at the top of this page - theydon't look out of place.
Unfortunately this project had to be suspended due to work demands andserious health issues. It was a few years before I resumedrestoration. The annual AHMW2013 event was coming up and Idecided toget this unit in working order. I didn't have much time so itwouldn't be a complete restore, just enough to get it functioningmusically. And I was pretty anxious to hear what this thing coulddo..
When you are performing a mass component replacement, you MUST perform them in logical orderper strategic section and verify functionality of each section. This was the order I performed:

1. Replace trimpots on the control board and recalibrate. Thisisimportant for lining up the DAC output range, the LFO frequency range,and the octave and pitch bend ranges. If this isn't done firstthenvoicecard calibration will be incorrect.
2. Correct the 5.6VDC design error.
3. Replace all CMOS 405x switches in the CV demultiplexer, the LFOwaveform switching, and modulation source switching.
4. Replace all quad buffer opamps on the outputs of the 4051s in theCV demultiplexer circuit.
5. Replace all the 7442 decoders and 74174 latches that generatecontrol signals to demultiplexer circuits, to ROM/RAM demux, and tovoicecards.
6. Replace the CA3240s in the polyphonic glide circuits with LF353s(ECO recommends TL082s but the LF353 is compatible)
   
7. When I verified the CVs and logic controls to the voicecards, Iidentified a bad pitch CV to voicecard #4 and traced it to a bad 3080OTA in the polyphonic glide circuit.

For a system this complex, it isbest to divide up into separatesessions and check that the computer still works. After you finish, apply power andverify that the computer still works. Then replace trimpots andelectrolytics on the voicecards.
The best plan of ICreplacement is to fan out from the DAC. Replace the 4051s, then power up and verify. Then the quadopamps, power up and verify. Then the 7442s, 74174s. Rinse,repeat, and recycle. Divide into small sessions and you will keepyour head from spinning. Then probe CVs and logic signals at thevoicecard pins, confirming that they change when you manipulate thefront panel controls. Then replace the TTLs in the computercircuits, theopamps in the CV and PWM summers, the LFO, etc. Divide andconquer, divide and conquer.

This iswhy a mass component replacement must be done strategically - thevoicecards are dependent on everything upstream so you must verify 100% operation of thecontrol board which is generating the control signals.
If your OB-X doesn't autotune,DON'T attempt to troubleshoot it yet. The autotune circuit is acomplex closed-loop system that is dependent on proper CV generationandcalibration of the voicecards. In other words, it may work justfine when you finish in the end, which was the case with me.

Other known failures:

1. Bad miniDIPvoice select switch. This allows a tech todisable other voices so that one can be isolated fortroubleshooting. It is an IC-shaped assembly with eight littlerocker switches, located under the front panel to the left. Ifyou have voices that will not trigger then this should bereplaced. Very common failure point. My OB-X was earlyenough that it had three largeslideswitches instead of the miniDIPswitch to select voices.
2. Cracked ceramiccaps. These tend to disintegrate withage. They are many in the OB-X. The leads can completelyseparate from the cap body rendering it completely broken.
3. LeakingS&H caps on the processor board. If you see thetelltale sign of residue around the leads, they're leaking. Evenif they aren't, they're old enough to be replaced with WIMA caps.
4. Crackedsolder joints on the voicecard motherboard. You have toremove the voicecards and remove the motherboard to reflow thesejoints. Reflow all of them because I uncovered the cause of these cracked solderjoints and you can eliminate a lot of malfunctions with this correction.
5. Inspect for excess flux. You can use a small wire brush toscrape it free, just be careful not to damage any board traces. Aged excess flux can cause malfunctions via low impedance shorts.
6. Blown LM311 can render pots inoperative.
7. Erratic intermittent operation could be due to bad 6508 staticRAMs. I found one in my OB-SX. I had recently read thatstatic RAMs can be restored by shorting all the pins through a fewlayers of conductive aluminum foil. Haven't tried it myself, butit does sound possible.
8. Failing pan trimpots on the voicecard motherboards. If youhear audio crackling when you rotate them, you're losing signal. Unfortunately the original Piher trimpots are no longer made and thereisn't a trimpot that fits the same footprint, so you need to useminiature pots mounted somewhere.
9. A thorough inspection revealed a broken solderfillet on one ofthe polyphonic glide circuits that rendered that pitch CV inoperative.
10. Check for the presence of insulating fish paper underneath thecomputer board, right above the big polystyrene caps in the polyphonicglide circuits. There is close clearance here and the fish paperprevents the leads of the caps from shorting to the traces on thecomputer board. The tape on the fish paper can dry out and thepaper may be loose or missing. Replace with fresh tape or use RTVto secure the paper.
    

It's a REAL GOOD IDEA to do these steps BEFORE probingcircuit malfunctions. You don't want to waste time chasing down aghost. I wasn't kidding when I warned that a project like thistakes time!

By this point I had verified that manipulating front panelswitches and pots correlated to associated control lines to thevoicecards - now it was time to restore the voicecards. This iswhy a mass component replacement must be done strategically - thevoicecards are dependent on everything upstream so you must verify 100% operation of thecontrol board which is generating the control signals.
Remember the design errors? Sitting between the DAC and the CMOS4051s is an opampbuffer with +/-15VDC rails. On power transitions, the outputstate of any opamp is indeterminant and this innocent little puppy canswing anywhere between+/-15VDCrails as they are ramping up, violating the VEE-0.5< Vin< VDD+0.5 input rule of any CMOSinputs downstream. Even though thiscould occur in a split second, this will damage the CMOS gate andultimately blow the 4051s over time (poof!). This is anotherreason why old Oberheim often malfunction with age.
One other correction is due - install two clipper diodes on the outputpin of the opamp followingthe DAC; I used a 1N34 to ground (anode to ground, cathode to outputpin) and a 1N4148 to the new (+)5.6VDCrail (anode to output pin, cathode to (+)5.6VDC). This forms ahardware limiter that prevents the opamp outputs from exceedingvoltages that can damage the CMOS ICs downstream. I chose the1N34 because it is a germanium diode which will limit the low voltageto (-)0.3VDC, adding a safety margin to the VEE-0.5term of the CMOS input rule. During power cycles, even if the5.6VDC doesn't fully rampup the clipper diodes guarantee that the output of the opamp will never swing outside the CMOS VEE-0.5< Vin< VDD+0.5 input rule. Eventhough you will be installing new CMOS ICs with input protectiondiodes, this is another shield against damage. I included a picon how I tack-soldered the diodes on the solder side of the processorboard, and don't forget the tubing to insulate the leads of the diodesto prevent shorts.
It's a good idea to confirm that theCVs can swing between zero and5VDC after you install the diode clippers.
So I closed the hood and tried to play the partially restoredOB-X. I lost an hour playing, it sounded so good. 3 of 4voicecards were working, the bad one was isolated and I located onefrom the surplus that would function so that brought it to arespectable four voice system. Naturally the tuning was way offon the voicecards because there was work to be done on them. Before I could calibrate any voicecards I had to:

1. Replace the V/OCT trimpots on each voicecard
   
2. Replace the RANGE trimpots
3. Remove the tantalum capacitors on the power rails, which have atendency to fail and short out. I left the tantalums in the audiopath intact, as I wanted to preserve that classic Oberheim edge anddirt and the less-than-perfect tantalums are a factor in that sound andare not subject to the high voltage as those on power rails.
   

This was enough to properly calibrate tuning on each voicecard - thepulse width and high tracking trimpots would be replaced later.
I was making good time so I proceeded to install the Encore ElectronicsMIDI retrofit, a comprehensive improvement to the OB-X. Installation went smoothly with theexceptionof routing the RESET line - my unit was old enough that theinstallation instructions did not match my unit and I had to trace thecorrect PC board trace manually. Upon firing up, the retrofitworkedflawlessly. The MIDI output would come in handy because I coulduse a Kurzweil 1000SX as a rock steady digital tuning reference forcalibrating the tuning on the voicecards.
Encore's products have been emptying out and they don't appear to bemaking any more of them. They still have OB-X retrofitsavailable, butthere may come a day when they are gone.
I removed the tape interface jacks - they're long obsolete and I nolonger needed them with the Encore retrofit. I also removed theCV/trigger input/output system due to obsolescence, and I did not wantthose jacks to wreak any havoc on the critical CVs going to thevoicecard.
Calibration went pretty painless and the OB-X now played in tune! But the big acid test remained.. autotune. If it doesn't work,the system will defeat any voices it cannot autotune. A press ofthe button, deep breath, play the keyboard to cycle through the voices- and all four are playing in tune. Autotune is working! If none of the voices will autotune, theaudio summer opampon the autotune buss on the voicecard motherboard has been known to gobad. The 2nd version of the OB-X service manualincludes a technical description of the autotune circuit fortroubleshooting. I drew up a clearer schematic and descriptionfor reference:

And then another problem surfaces: some of the connector pins onthevoicecard motherboard wereintermittent. I had read this was a common issue. But timewas short and this would have to wait later. Luckily that problemdidn't surface while at AHMW2013 (knock-knock-knocking on wood). I alsocouldn't get the filter cutoff frequencies trimmed between voices whichis a bias issue to be addressed later. One of the voicecards hadan EG that isn't the same timing as the others, likely caused by thetiming cap that is thirty plus years old. The restoration isn'tcomplete and more work will follow the AHMW2013 event.
After having the instrument in this condition for a few days and atravel 1/3 across the US and back, I cannot stress enough theimportance of new trimpots. The OB-X fired up in tune - everytime - and stayed in tune much better than the Memorymoog at theAHMW2013event. I didn't even have to autotune when it was powered upcold. The original trimpots were CTS open frame and cheapquality, which often failed completely with age. The Oberheimaccountants cut a few too many corners in sourcing cheapercomponents. The trimpot settings were 'locktited' into place whenthey left the factory but this made no difference after thirtyyears. This is another reason why old Oberheim show up for salemalfunctioning. New replacements are the only solution. Thelayout on the circuit board hint that they were originally intended toreceive Piher trimpots but I recommend sealed trimpots fromBourns. I have had good luck with the Bourns trimpots over theyears in other instruments.
I now had a 100% functional four voice OB-X ready for the AHMW2013event. All I had to do was dial up 120 patches in two days(right!). I settled for the factory patches that can be recalledfrom the Encore retrofit with a power-up reset sequence, and made myown bank of some classic Oberheim sounds such as strings, brass, pads,filter sweeps, fat unison leads and basses, special effects.. here aresome of the sounds it was making.
A lot of AHMW attendees were looking forward to trying the OB-X. It got a great reaction although it had to compete with a cool soundingRolands100M modular that one of the folks brought. Everybody liked bothbut the favorites were split between the Roland and the OB-X. Doesn't break my heart. But I must admit after being a Moogfanboy for years that I like the unique sounds coming from the OB-Xthat none of my Moogs could dream of. They both have theirapplications!

The night before the AHMW2013show Iwas playing the OB-X in my hotel roomto dial up some more sounds. I placed the OB-X on a footstool andgot comfortable on a chair. As I was working, my freshly restoredOB-X startedmalfunctioning. I noticed if I placed pressure on the case Icould make it malfunction. It dawned on me that the base of thechassis was thin enough that it could flex. With the footstoolsupporting the base in the middle, that was exactly what washappening. This flexing can transmit to the PC boards inside,which will cause malfunctions. This is a root cause why theconnector pins on the voicecard motherboards often have cracked solderjoints that have to be reflowed. I have added angled extrudedaluminum to the base of my unit at strategic points to correct thisproblem. It is stronglyadvised that your keyboard stand use arms that support your OB-X fullyfrom front to back (like Standtastic) or as wide as possible and avoidany support point in the middle of the base. If you useyour OB-X on a tabletop with the endcheeks to support it, this is safe.
I set out to expand the voicecards in my unit. Mine was a fourvoice, and in order to expand beyond four voices you need some veryrare parts. At the time Iacquired my OB-X I was fortunate to find:

1. Five more voicecards, enough for a full eightvoice plus a spare.
2. Extra voicecard motherboard to accommodate four morevoicecards. These are very hard to find. Voices 1-4 sit inthe lower motherboard while 5-8 are suspended directly above it.
   
3. Metal mounting tray for the motherboard, which iseven harder to find. This provides a rigid mounting frame toswing the assembly out of the way for calibration. Without it,the motherboard will flex and crack with handling.
   

Fortunately, my good friend Scott 'Old Crow' Riderwas inspired to replicate the OB-X voicecard using modern componentsafter hearing my unit at the AHMW2013 show. He is also buildingthemotherboard (but not the mounting tray) for OB-X owners wishing toexpand their units to full eight voice. I have heard the new cardin person and can confirm that he nailed the vintage OB-Xsonority. Bare boards are available for DIY assembly, and he isnear completion on an SMT version for assembled cards for those whodon't have DIY skills. Full details here.
The components already exist on the processor board for up to eightvoices. You will need to build connectors between the processorboard and extra motherboard, and for connector J between upper andlower motherboards. Connector G is not used on the uppermotherboard. Motherboards were configured for either voices 1-4or 5-8, there should be a label to the right of the pan trimpots. The motherboard for voices 5-8 are the same circuit boardwith unneeded components unpopulated. If you acquire themotherboard for voices 1-4 with intention for voices 5-8, you mustremove the active components in the voice summing circuit and the 7x15regulators for it to work with voices 5-8. If you do not know howto interpret schematics then you should refer this task to a competenttech.
It required some effort to getting the extra voicecards restored, theyall had faults that had to be corrected. I found bad 7x15s,opamps, FETs, 3080s. The CA3086s have been known to fail, minewere fine. All the trimpots, electrolytic caps, and theaforementioned caps vulnerable to cracking were replaced. Onevoicecard even had a broken trace, that took some detective work. DO NOT HOT SWAP VOICECARDS WITHTHE POWERON! I knew this precaution but on one occasion I forgot toturn off the power when I installed one of the freshly repairedvoicecards. That fried a CMOS IC on the board POOF!
One complaint I have read often was that inOB-X consisting of greater than four voicecards, VCO calibration wasdifficult in the lower voicecards. Each motherboard fitted fourvoicecards each, voices 1-4 are on the lower motherboard with voices5-8 directly above it.
Everyone knows that heat rises and that accumulated heat throws analogVCOs out of tune. Studying the OB-X framework, there is a largealuminum wall separating thekeyboard and voicecards that also supports the front edge of the panelwhen it is closed. That wall combined with the standoffs for theupper motherboard combined with a solid metal tray that the boardmountsonto (Oberheim failed to put vent holes in the metal tray!) created aclosed box that restricts airflow and traps heat in the lowervoices. There is little room at the rear of the OB-X for heat toescape. This is another reason why OB-X have a reputation fortuning drift. This is why calibration is difficult on the lowervoices - when you raise the upper voices out of the way to calibratethe lower ones, the heat has now escaped. Close the upper voicesback down, and the heat is again trapped and throws off yourcalibration! To make matters worse, the VCF exponential converterhas zero temperature compensation thus raising/lowering the upper traymakes VCF calibration an empirical job with the heat changes!!!
The easiest solution is raising the standoffs for the upper motherboardby1/2',providing better ventilation for the lower motherboard beneathit. There are six standoffs, three at each end. One set ofthree have hinges to swing the upper motherboard like a hood, the otherset are fixed height. The hinged set is a two piece assemblyconsisting of a fixed 1' standoff coupled to a hinge that is another1/2', total 1-1/2' height. The other set is fixed height of1-1/2'. Since we want to raise them 1/2', we can replace the 1'fixed standoffs with the other 1-1/2' ones, when combined with the 1/2'hinges results in a new 2'height. All you need are three new 2' fixed heightstandoffs. I confirmedthere is ample clearance for all components when the hood wasclosed. After using this modification for a year I can reportthat the lower voicecards are very stable.
The later OB-Xa and OB-8 replaced the wall between the keyboard andvoicecards with a pair of standoffs. This allowed air tocirculate and they are easier to calibrate and stay in tune better thanthe OB-X. top

That coolopening sound in 'Tom Sawyer'

Rush's 'Tom Sawyer' opens with thisreally cool resonance sweep (aka 'rezz') that has evaded emulation for YEARS on other synthesizers. I tried to emulate it myself and never quite got it. At the timeof the making of Moving Pictures,Geddy Lee was using both his old eight-voice SEMpolysynth and an OB-X, which can be seen in his picture in the inlet ofthe album. Rush has a DVD on the making of the MovingPictures album where Geddy Lee confirmed that they created thatsound on 'the oberheim' but he doesn't say which one! Nonetheless, both can get that sound. These are the secrets to'that' sound.
SEM Multimode filter. The OB-X uses the same multimode filter asthe older SEM-based polyphonic systems. It is a 12dB/oct statevariable filter that can generate simultaneous lowpass, bandpass, andhighpass outputs. No other filter, not eventhe Moog ladder filter in 12dB/oct mode, will get this sound. Itried. The SEM filter has that unique grind at high resonanceand growl at low frequency cutoff that contributes to the Tom Sawyerrezz. Later OB models abandoned the SEM filter. TheCEM3320-based OB-SXand OB-Xa were almost as good, OB-8 and later OBs lost it. TheSEM filter is THE tool for this sound.
Knowing this, I fiddled and fiddled to get THAT sound when I first gotmy OB-X. Only when I pressed the UNISON switch was that momentwhen I said 'WOOOOAH, so THAT'S how they did it'. That soundrequiresmultiple voices firing in unison. A single voice (such as amonophonic) willnot work, you need a polyphonic with unison mode firing multiple voicesat once. There's another slight clue in the song that confirmsthis. If you're not in unison mode, the OB-X operates in multipletrigger mode which means the EG re-fires with every key. There isone place in the song where the EG does not re-fire between A and Gkey. If you're in unison mode on the OB-X, it is operating insingle trigger mode where the EG does not re-fire until you release allthe keys. That one place in the song is only possible with unisonmode.
But I had done the same trick on my Memorymoog and Andromeda and theydidn't sound like the OB-X. I'm no slouch with analogprogramming, so something wasmissing.
That 'something' turned out to be inherent filter tracking error andmismatched EG decay rates between voices. On newer polyphonicslike Memorymoog, Andromeda, et al they all perfected the matching offilter tracking and EG rates between voices. The OB-X is one ofthose cases where imperfections canbe a good thing. The SEM multimode filter in the OB-X hasits faults (or features depending on your perspective). Thefilter tracking of SEM filters is only within a paltry three octaverange, outside of that they are not guaranteed to track. Thecheap expo converter in the CV summing circuit for cutoff controlcontributes to this 'fault'. Once outside that three octaverange, the error between voicecards will not be the same. Translated, the cutoff frequency between voices will vary despite bestefforts to calibrate them. Indeed,filter calibration has been a common complaint because you will never get all the voicecards to track thesame. In unison mode with multiple voices firing at onces,that tracking error causes a flanging effect- due to multiple voices with mismatched filter cutoffs - with the 'TomSawyer' rezz as the cutoff sweeps down. Also listenclosely to the original song - when the filter closes, you're hearingmismatched cutoffs. They're panning the voicecards across thestereo field. It's a cool stereo effect that you can't get with amodulated delay like chorus or flanging.
Adding to the filter tracking error is the tolerances of the EG timingbetween voicecards. The original timing capacitors used for theEGs on the voicecards were wide enough tolerance that the decay ratevaried slightly between voicecards. That exaggerated the filtersweep flanging effect between voicecards further, especially as thefilter sweep fallsfrom high cutoff. This was further confirmed when I went just alittle too far in restoration and replaced those 'faulty' timing capswith new WIMAs and 'lost' that fault, but I have another modificationin the works to get it back. Another confirmation was playingwith an Oberheim Two-Voice as I tried that rezz sound. I usedboth SEMs for the sound. With multiple SEMs, there is no way to get the EG decayperfectly matched. Indeed, as I varied the decay of one of theSEMs while the other was untouched, the unique 'growl' of the TomSawyer rezz began to emerge.

This iswhy a mass component replacement must be done strategically - thevoicecards are dependent on everything upstream so you must verify 100% operation of thecontrol board which is generating the control signals.
If your OB-X doesn't autotune,DON'T attempt to troubleshoot it yet. The autotune circuit is acomplex closed-loop system that is dependent on proper CV generationandcalibration of the voicecards. In other words, it may work justfine when you finish in the end, which was the case with me.

Other known failures:

1. Bad miniDIPvoice select switch. This allows a tech todisable other voices so that one can be isolated fortroubleshooting. It is an IC-shaped assembly with eight littlerocker switches, located under the front panel to the left. Ifyou have voices that will not trigger then this should bereplaced. Very common failure point. My OB-X was earlyenough that it had three largeslideswitches instead of the miniDIPswitch to select voices.
2. Cracked ceramiccaps. These tend to disintegrate withage. They are many in the OB-X. The leads can completelyseparate from the cap body rendering it completely broken.
3. LeakingS&H caps on the processor board. If you see thetelltale sign of residue around the leads, they're leaking. Evenif they aren't, they're old enough to be replaced with WIMA caps.
4. Crackedsolder joints on the voicecard motherboard. You have toremove the voicecards and remove the motherboard to reflow thesejoints. Reflow all of them because I uncovered the cause of these cracked solderjoints and you can eliminate a lot of malfunctions with this correction.
5. Inspect for excess flux. You can use a small wire brush toscrape it free, just be careful not to damage any board traces. Aged excess flux can cause malfunctions via low impedance shorts.
6. Blown LM311 can render pots inoperative.
7. Erratic intermittent operation could be due to bad 6508 staticRAMs. I found one in my OB-SX. I had recently read thatstatic RAMs can be restored by shorting all the pins through a fewlayers of conductive aluminum foil. Haven't tried it myself, butit does sound possible.
8. Failing pan trimpots on the voicecard motherboards. If youhear audio crackling when you rotate them, you're losing signal. Unfortunately the original Piher trimpots are no longer made and thereisn't a trimpot that fits the same footprint, so you need to useminiature pots mounted somewhere.
9. A thorough inspection revealed a broken solderfillet on one ofthe polyphonic glide circuits that rendered that pitch CV inoperative.
10. Check for the presence of insulating fish paper underneath thecomputer board, right above the big polystyrene caps in the polyphonicglide circuits. There is close clearance here and the fish paperprevents the leads of the caps from shorting to the traces on thecomputer board. The tape on the fish paper can dry out and thepaper may be loose or missing. Replace with fresh tape or use RTVto secure the paper.
    

It's a REAL GOOD IDEA to do these steps BEFORE probingcircuit malfunctions. You don't want to waste time chasing down aghost. I wasn't kidding when I warned that a project like thistakes time!

By this point I had verified that manipulating front panelswitches and pots correlated to associated control lines to thevoicecards - now it was time to restore the voicecards. This iswhy a mass component replacement must be done strategically - thevoicecards are dependent on everything upstream so you must verify 100% operation of thecontrol board which is generating the control signals.
Remember the design errors? Sitting between the DAC and the CMOS4051s is an opampbuffer with +/-15VDC rails. On power transitions, the outputstate of any opamp is indeterminant and this innocent little puppy canswing anywhere between+/-15VDCrails as they are ramping up, violating the VEE-0.5< Vin< VDD+0.5 input rule of any CMOSinputs downstream. Even though thiscould occur in a split second, this will damage the CMOS gate andultimately blow the 4051s over time (poof!). This is anotherreason why old Oberheim often malfunction with age.
One other correction is due - install two clipper diodes on the outputpin of the opamp followingthe DAC; I used a 1N34 to ground (anode to ground, cathode to outputpin) and a 1N4148 to the new (+)5.6VDCrail (anode to output pin, cathode to (+)5.6VDC). This forms ahardware limiter that prevents the opamp outputs from exceedingvoltages that can damage the CMOS ICs downstream. I chose the1N34 because it is a germanium diode which will limit the low voltageto (-)0.3VDC, adding a safety margin to the VEE-0.5term of the CMOS input rule. During power cycles, even if the5.6VDC doesn't fully rampup the clipper diodes guarantee that the output of the opamp will never swing outside the CMOS VEE-0.5< Vin< VDD+0.5 input rule. Eventhough you will be installing new CMOS ICs with input protectiondiodes, this is another shield against damage. I included a picon how I tack-soldered the diodes on the solder side of the processorboard, and don't forget the tubing to insulate the leads of the diodesto prevent shorts.
It's a good idea to confirm that theCVs can swing between zero and5VDC after you install the diode clippers.
So I closed the hood and tried to play the partially restoredOB-X. I lost an hour playing, it sounded so good. 3 of 4voicecards were working, the bad one was isolated and I located onefrom the surplus that would function so that brought it to arespectable four voice system. Naturally the tuning was way offon the voicecards because there was work to be done on them. Before I could calibrate any voicecards I had to:

1. Replace the V/OCT trimpots on each voicecard
   
2. Replace the RANGE trimpots
3. Remove the tantalum capacitors on the power rails, which have atendency to fail and short out. I left the tantalums in the audiopath intact, as I wanted to preserve that classic Oberheim edge anddirt and the less-than-perfect tantalums are a factor in that sound andare not subject to the high voltage as those on power rails.
   

This was enough to properly calibrate tuning on each voicecard - thepulse width and high tracking trimpots would be replaced later.
I was making good time so I proceeded to install the Encore ElectronicsMIDI retrofit, a comprehensive improvement to the OB-X. Installation went smoothly with theexceptionof routing the RESET line - my unit was old enough that theinstallation instructions did not match my unit and I had to trace thecorrect PC board trace manually. Upon firing up, the retrofitworkedflawlessly. The MIDI output would come in handy because I coulduse a Kurzweil 1000SX as a rock steady digital tuning reference forcalibrating the tuning on the voicecards.
Encore's products have been emptying out and they don't appear to bemaking any more of them. They still have OB-X retrofitsavailable, butthere may come a day when they are gone.
I removed the tape interface jacks - they're long obsolete and I nolonger needed them with the Encore retrofit. I also removed theCV/trigger input/output system due to obsolescence, and I did not wantthose jacks to wreak any havoc on the critical CVs going to thevoicecard.
Calibration went pretty painless and the OB-X now played in tune! But the big acid test remained.. autotune. If it doesn't work,the system will defeat any voices it cannot autotune. A press ofthe button, deep breath, play the keyboard to cycle through the voices- and all four are playing in tune. Autotune is working! If none of the voices will autotune, theaudio summer opampon the autotune buss on the voicecard motherboard has been known to gobad. The 2nd version of the OB-X service manualincludes a technical description of the autotune circuit fortroubleshooting. I drew up a clearer schematic and descriptionfor reference:

And then another problem surfaces: some of the connector pins onthevoicecard motherboard wereintermittent. I had read this was a common issue. But timewas short and this would have to wait later. Luckily that problemdidn't surface while at AHMW2013 (knock-knock-knocking on wood). I alsocouldn't get the filter cutoff frequencies trimmed between voices whichis a bias issue to be addressed later. One of the voicecards hadan EG that isn't the same timing as the others, likely caused by thetiming cap that is thirty plus years old. The restoration isn'tcomplete and more work will follow the AHMW2013 event.
After having the instrument in this condition for a few days and atravel 1/3 across the US and back, I cannot stress enough theimportance of new trimpots. The OB-X fired up in tune - everytime - and stayed in tune much better than the Memorymoog at theAHMW2013event. I didn't even have to autotune when it was powered upcold. The original trimpots were CTS open frame and cheapquality, which often failed completely with age. The Oberheimaccountants cut a few too many corners in sourcing cheapercomponents. The trimpot settings were 'locktited' into place whenthey left the factory but this made no difference after thirtyyears. This is another reason why old Oberheim show up for salemalfunctioning. New replacements are the only solution. Thelayout on the circuit board hint that they were originally intended toreceive Piher trimpots but I recommend sealed trimpots fromBourns. I have had good luck with the Bourns trimpots over theyears in other instruments.
I now had a 100% functional four voice OB-X ready for the AHMW2013event. All I had to do was dial up 120 patches in two days(right!). I settled for the factory patches that can be recalledfrom the Encore retrofit with a power-up reset sequence, and made myown bank of some classic Oberheim sounds such as strings, brass, pads,filter sweeps, fat unison leads and basses, special effects.. here aresome of the sounds it was making.
A lot of AHMW attendees were looking forward to trying the OB-X. It got a great reaction although it had to compete with a cool soundingRolands100M modular that one of the folks brought. Everybody liked bothbut the favorites were split between the Roland and the OB-X. Doesn't break my heart. But I must admit after being a Moogfanboy for years that I like the unique sounds coming from the OB-Xthat none of my Moogs could dream of. They both have theirapplications!
The night before the AHMW2013show Iwas playing the OB-X in my hotel roomto dial up some more sounds. I placed the OB-X on a footstool andgot comfortable on a chair. As I was working, my freshly restoredOB-X startedmalfunctioning. I noticed if I placed pressure on the case Icould make it malfunction. It dawned on me that the base of thechassis was thin enough that it could flex. With the footstoolsupporting the base in the middle, that was exactly what washappening. This flexing can transmit to the PC boards inside,which will cause malfunctions. This is a root cause why theconnector pins on the voicecard motherboards often have cracked solderjoints that have to be reflowed. I have added angled extrudedaluminum to the base of my unit at strategic points to correct thisproblem. It is stronglyadvised that your keyboard stand use arms that support your OB-X fullyfrom front to back (like Standtastic) or as wide as possible and avoidany support point in the middle of the base. If you useyour OB-X on a tabletop with the endcheeks to support it, this is safe.
I set out to expand the voicecards in my unit. Mine was a fourvoice, and in order to expand beyond four voices you need some veryrare parts. At the time Iacquired my OB-X I was fortunate to find:

1. Five more voicecards, enough for a full eightvoice plus a spare.
2. Extra voicecard motherboard to accommodate four morevoicecards. These are very hard to find. Voices 1-4 sit inthe lower motherboard while 5-8 are suspended directly above it.
   
3. Metal mounting tray for the motherboard, which iseven harder to find. This provides a rigid mounting frame toswing the assembly out of the way for calibration. Without it,the motherboard will flex and crack with handling.
   

Fortunately, my good friend Scott 'Old Crow' Riderwas inspired to replicate the OB-X voicecard using modern componentsafter hearing my unit at the AHMW2013 show. He is also buildingthemotherboard (but not the mounting tray) for OB-X owners wishing toexpand their units to full eight voice. I have heard the new cardin person and can confirm that he nailed the vintage OB-Xsonority. Bare boards are available for DIY assembly, and he isnear completion on an SMT version for assembled cards for those whodon't have DIY skills. Full details here.
The components already exist on the processor board for up to eightvoices. You will need to build connectors between the processorboard and extra motherboard, and for connector J between upper andlower motherboards. Connector G is not used on the uppermotherboard. Motherboards were configured for either voices 1-4or 5-8, there should be a label to the right of the pan trimpots. The motherboard for voices 5-8 are the same circuit boardwith unneeded components unpopulated. If you acquire themotherboard for voices 1-4 with intention for voices 5-8, you mustremove the active components in the voice summing circuit and the 7x15regulators for it to work with voices 5-8. If you do not know howto interpret schematics then you should refer this task to a competenttech.
It required some effort to getting the extra voicecards restored, theyall had faults that had to be corrected. I found bad 7x15s,opamps, FETs, 3080s. The CA3086s have been known to fail, minewere fine. All the trimpots, electrolytic caps, and theaforementioned caps vulnerable to cracking were replaced. Onevoicecard even had a broken trace, that took some detective work. DO NOT HOT SWAP VOICECARDS WITHTHE POWERON! I knew this precaution but on one occasion I forgot toturn off the power when I installed one of the freshly repairedvoicecards. That fried a CMOS IC on the board POOF!
One complaint I have read often was that inOB-X consisting of greater than four voicecards, VCO calibration wasdifficult in the lower voicecards. Each motherboard fitted fourvoicecards each, voices 1-4 are on the lower motherboard with voices5-8 directly above it.
Everyone knows that heat rises and that accumulated heat throws analogVCOs out of tune. Studying the OB-X framework, there is a largealuminum wall separating thekeyboard and voicecards that also supports the front edge of the panelwhen it is closed. That wall combined with the standoffs for theupper motherboard combined with a solid metal tray that the boardmountsonto (Oberheim failed to put vent holes in the metal tray!) created aclosed box that restricts airflow and traps heat in the lowervoices. There is little room at the rear of the OB-X for heat toescape. This is another reason why OB-X have a reputation fortuning drift. This is why calibration is difficult on the lowervoices - when you raise the upper voices out of the way to calibratethe lower ones, the heat has now escaped. Close the upper voicesback down, and the heat is again trapped and throws off yourcalibration! To make matters worse, the VCF exponential converterhas zero temperature compensation thus raising/lowering the upper traymakes VCF calibration an empirical job with the heat changes!!!
The easiest solution is raising the standoffs for the upper motherboardby1/2',providing better ventilation for the lower motherboard beneathit. There are six standoffs, three at each end. One set ofthree have hinges to swing the upper motherboard like a hood, the otherset are fixed height. The hinged set is a two piece assemblyconsisting of a fixed 1' standoff coupled to a hinge that is another1/2', total 1-1/2' height. The other set is fixed height of1-1/2'. Since we want to raise them 1/2', we can replace the 1'fixed standoffs with the other 1-1/2' ones, when combined with the 1/2'hinges results in a new 2'height. All you need are three new 2' fixed heightstandoffs. I confirmedthere is ample clearance for all components when the hood wasclosed. After using this modification for a year I can reportthat the lower voicecards are very stable.
The later OB-Xa and OB-8 replaced the wall between the keyboard andvoicecards with a pair of standoffs. This allowed air tocirculate and they are easier to calibrate and stay in tune better thanthe OB-X. top

That coolopening sound in 'Tom Sawyer'

Rush's 'Tom Sawyer' opens with thisreally cool resonance sweep (aka 'rezz') that has evaded emulation for YEARS on other synthesizers. I tried to emulate it myself and never quite got it. At the timeof the making of Moving Pictures,Geddy Lee was using both his old eight-voice SEMpolysynth and an OB-X, which can be seen in his picture in the inlet ofthe album. Rush has a DVD on the making of the MovingPictures album where Geddy Lee confirmed that they created thatsound on 'the oberheim' but he doesn't say which one! Nonetheless, both can get that sound. These are the secrets to'that' sound.
SEM Multimode filter. The OB-X uses the same multimode filter asthe older SEM-based polyphonic systems. It is a 12dB/oct statevariable filter that can generate simultaneous lowpass, bandpass, andhighpass outputs. No other filter, not eventhe Moog ladder filter in 12dB/oct mode, will get this sound. Itried. The SEM filter has that unique grind at high resonanceand growl at low frequency cutoff that contributes to the Tom Sawyerrezz. Later OB models abandoned the SEM filter. TheCEM3320-based OB-SXand OB-Xa were almost as good, OB-8 and later OBs lost it. TheSEM filter is THE tool for this sound.
Knowing this, I fiddled and fiddled to get THAT sound when I first gotmy OB-X. Only when I pressed the UNISON switch was that momentwhen I said 'WOOOOAH, so THAT'S how they did it'. That soundrequiresmultiple voices firing in unison. A single voice (such as amonophonic) willnot work, you need a polyphonic with unison mode firing multiple voicesat once. There's another slight clue in the song that confirmsthis. If you're not in unison mode, the OB-X operates in multipletrigger mode which means the EG re-fires with every key. There isone place in the song where the EG does not re-fire between A and Gkey. If you're in unison mode on the OB-X, it is operating insingle trigger mode where the EG does not re-fire until you release allthe keys. That one place in the song is only possible with unisonmode.
But I had done the same trick on my Memorymoog and Andromeda and theydidn't sound like the OB-X. I'm no slouch with analogprogramming, so something wasmissing.
That 'something' turned out to be inherent filter tracking error andmismatched EG decay rates between voices. On newer polyphonicslike Memorymoog, Andromeda, et al they all perfected the matching offilter tracking and EG rates between voices. The OB-X is one ofthose cases where imperfections canbe a good thing. The SEM multimode filter in the OB-X hasits faults (or features depending on your perspective). Thefilter tracking of SEM filters is only within a paltry three octaverange, outside of that they are not guaranteed to track. Thecheap expo converter in the CV summing circuit for cutoff controlcontributes to this 'fault'. Once outside that three octaverange, the error between voicecards will not be the same. Translated, the cutoff frequency between voices will vary despite bestefforts to calibrate them. Indeed,filter calibration has been a common complaint because you will never get all the voicecards to track thesame. In unison mode with multiple voices firing at onces,that tracking error causes a flanging effect- due to multiple voices with mismatched filter cutoffs - with the 'TomSawyer' rezz as the cutoff sweeps down. Also listenclosely to the original song - when the filter closes, you're hearingmismatched cutoffs. They're panning the voicecards across thestereo field. It's a cool stereo effect that you can't get with amodulated delay like chorus or flanging.
Adding to the filter tracking error is the tolerances of the EG timingbetween voicecards. The original timing capacitors used for theEGs on the voicecards were wide enough tolerance that the decay ratevaried slightly between voicecards. That exaggerated the filtersweep flanging effect between voicecards further, especially as thefilter sweep fallsfrom high cutoff. This was further confirmed when I went just alittle too far in restoration and replaced those 'faulty' timing capswith new WIMAs and 'lost' that fault, but I have another modificationin the works to get it back. Another confirmation was playingwith an Oberheim Two-Voice as I tried that rezz sound. I usedboth SEMs for the sound. With multiple SEMs, there is no way to get the EG decayperfectly matched. Indeed, as I varied the decay of one of theSEMs while the other was untouched, the unique 'growl' of the TomSawyer rezz began to emerge.
So the secrets of the 'Tom Sawyer' rezz are the SEM multimode filterandimperfection. All succeeding polyphonics perfected the 'faults'of the OB-X, thus they are not capable of replicating that sound. top

Features

Oberheim's previous polyphonic synthesizers was a bunch of SEM modules(SEM was a compact synthesizer module) ganged together and coupled to apolyphonic keyboard scanning system. The popular four voice wasmodel FVS (Four Voice System), while the bulky (and ultimatelyunwieldly) eight voice was model EVS. Oberheim retrofitted userpatch storage to both polyphonics but this programmer did not memorizeall the settings on the SEM modules.
In 1978 the Prophet-5 polyphonic synthesizer was released, and its mainfeature of memorization of allsynthesizer parameters attracted legions of customers. The OB-Xwas introduced in June 1979 to compete with the Prophet-5. TheOB-X voicecards were near identical to the SEM, only now everyparameter was programmable. They were offered in four, six, andeight voice configurations with list prices of US$4595, $5395, and$5995 respectively. These were very expensive machines for1979. Owners could add more voicecards later, up to eighttotal. The OB-X has no split or layer capability later to beintroduced on the OB-Xa. Unofficially they are categorized intofour revs with the latest ones sporting the pre-MIDI 'OberheimInterface' in theleft hand endcap to communicate with their DSX sequencers and DMX drummachines (which wouldn't be released until AFTER the OB-X wasdiscontinued..!). None of them ever had a MIDI upgrade from thefactoryas the OB-X was discontinued in early 1981, two years before the birthof MIDI.
There are 19 controlknobs and 21 buttons on the front panel for tweaking the synthesizerpatch, all of which can bestored for instant recall. You have to press the EDIT button totweak the patch using the knobs and buttons, and the MANUAL buttonmakes a brand new patch using the current position of the pots. Memory provided storage for 32 user patches (the Encore Electronicretrofit expanded this to 120 patches). There is a bigslideswitch under the hood to disable write capability and protect yourpatches (not useable with the Encore retrofit). Left, right, andmonophonic outputs with pan trimpots under the hood route the audio torespective jacks. Volume is global tothe instrument, there is no programmable volume per preset.
Usually the serial badge on the rear will specify how many voices arepresent (unless a unit has been expanded to more voicesin thefield). Thevoicecard architecture is identical to the classic SEM discretecircuit, you could follow the schematics from left to right and it is90% identical with exceptions of circuit differences in the filter,lowpass only in the OB-X, and omission of PWM/FM via VCF EG. Eachvoice featured two VCOs,each capable of providing ramp or variable pulse width waveforms. VCO2 can be hard synced to VCO1 and you can configure the left handpitch bend lever to modulateonly VCO2 for screaming hard sync effects. The OB-X was one ofthe meanest sounding hard sync sounds. One pulse width control isshared between both VCOs. There is an option for oscillator crossmodulation (XMOD) using VCO2 ramp to VCO1 frequency, but the depth isfixed and VCO2 cannot be put in LFO mode. There is a pinknoise source on the OB-X. Crude mixer level controls ofeach oscillator islimited to full/off for VCO1, full/half/off for VCO2, and full/half/offfor noise. Youcannot combine waveforms - ramp or pulse, not both.
The OB-X features polyphonic glide ('portamento' in Oberheim speak),which the Memorymoog had but theProphet-5 did not (P5 glide only worked in unison). Thepolyphonic glide is totally analog. It does also have a fault - or feature depending on how youappreciate it. Because each voice has its own glide circuit, thekey component - the CA3080 OTA - isn't exactly matched betweenvoices. This means the glide rate is different betweenvoices. When used with unison with all voices firing, the unequalgliderate results in a unique fat detuned sound as you transition betweennotes. Using the stereo outputs with voices panned across thiswas a dramatic effect. Later OB-Xs used matched OTAs - markedwith a red dot - in the glide circuit for better but not perfectmatching betweenvoicing. The mismatch was prettypronounced on my unit, enough that I swapped out the unmatched OTAs forbetter ones. Digital glide was first introduced on the 56-preset OB-SX(which emulated the unequal glide rates in the OB-X!!!) while the OB-Xa(produced at the same time of the 56-preset OB-SX) still had analogglide. Oberheim finally progressed todigital glide in the OB-8 that was generated by software and guaranteedmatching glide rates between voices. That eliminated the cost ofthehigh partscount and the labor intensive matching for the OTAs that were thorns ofanalog polyphonic glide.
Modulation is pretty basic on the OB-X. There is no voicemodulation like that offered on the SEM module, Prophet-5, orMemorymoog. There is no provision to route filter EG to VCO frequency (later addedto OB-Xa) or pulse width (later added to OB-8), nor can you impartaudio FM on the filter cutoff with a VCO (later added to OB-8). There is a single LFO with triangle (panel is incorrectly labeled'sine'), square, and S&H waveforms with two independent depthcontrols for frequency or pulse width modulation. The mod wheel(OK I know they're pinball flippers notwheels, but I'm retaining the convention used in the rest of the world,especially MIDI) always routes the triangle LFO waveform to bothVCOs. A control pedal via the rear paneljack can also be used to blend in vibrato like the mod wheel.

Oberheim Sem Module

The VCF is the same 12dB/oct filter found in the SEM, but only withlowpass filtering. A switch turns on keyboard tracking althoughthe service manual says that tracking is accurate only over a threeoctave range. The filter will not self-oscillate, but it has a wonderful resonance that iscolorful and unique. The only modulation option is from theLFO or the EG. There is a rear pedal jack for filter controlwhich affects all voicecards simultaneously.
The VCA is the same discrete circuit found on the SEM and is largelyresponsible for the dirty edgy sound to the OB-X. The VCA iscomprised of a CA3080 OTAfollowed by a TL081 to buffer the weak current output of the OTA. The3080 OTA is not exactly a high fidelity component and somedistortion is introduced here, which lends the OB-X its dirty edgysound. Definitely not clean but there is a pleasant mildcreamy overdrive here that is pleasing to the ears. At the timethe cheap-and-dirty CA3080 was the most cost effective that technologyhad to offer. The better ones weren't better enough to justifythe higher cost. This same circuit isalso used in the voice summing circuit, further distorting the signal(especially in unison). Interesting note: the SEM polysynths hadthe OTA VCA, but no OTA in the voice summing circuit! In fact youcan correlate the loss ofthis dirty edgey sound with the progression fromOB-X->OB-Xa->OB-8 as changes were made to cleaner VCAa and voicesummingcircuits (see the comparisiontable). No doubt when Tom Oberheim lamented that the OB-8 was'too clean' and that something was lost along the way..
On the left hand panel, a switchselects the range of pitch bend to either a full octave or twosemitones. Frankly the pitch bend is a little abrupt using the'pinball flipper'. Another switch routes pitch bendto VCO2 only, handy for hard sync sweeps. A third switch changesthe range of thekeyboard up or down one octave.

Oberheim Sem Schematic

CV/Trigger inputs on the rear panel only work on voicecard one and thejacks are wiredsuch that when used the computer will skip voicecard one. Thislets you use a sequencer on one voicecard with the remaining voicecardsare controlled by the keyboard, but because the OB-X does not featuremultitimbrality you can't get an individual patch on that sequencedvoicecard. There are CV/Trigger outputs provided but they are thesignals from voicecard one only. Because it preceded MIDI, theOB-X had a cassette interface system for archiving and restoringpatches. Since I was going to use the Encore retrofit whichfeature MIDI control and sysex patch I/O, the cassette interface andrear panel CV/trigger jacks were no longer needed and I removed them.
One of the coolest features is UNISON. This turns the OB-X into amonophonic instrument with all voices firing together. Becausethe calibration between voicecards isn't precisely matched, unisonyields some very thick sounds. A large part of this is due tofree-running VCOs in which none of them are oscillating insynchrony. Very few analog polysynths have this feature. Combined with polyphonic glide and the HOLD/RESET buttons, you canarrange one key to play a chord and get end-of-the-world effects.
Of the OB line, the OB-X has the least features but the most organicsound. It has that colorful alien resonance in the filters(especially in filter sweeps), that discrete girth and bite (especiallyin unison), that subtle pleasant distortion in the VCA and voicesummers which is a classic Oberheim trait.
The OB-Xa and OB-SX voicecard is neitherinterchangeable or compatible with OB-X. They used differentsignals and the connector pins do not match. top

Revs

There are unofficial 'revs' of the OB-X scattered about the net thathelp define how old an OB-X is. Some are obvious from theoutside, some aren't. Through significant circuit changes fromreading ECOs (Oberheim 'Engineering Change Orders') and cosmeticchanges in the panel, I narrowed down this list of unofficial 'revs'.

Rev #	Description
0	'OB-X' block typeface on topmiddle of panel, no Oberheim name on front panel, voicecard enablethree large slideswitches (like TEST1 and TEST2), only twoslideswitches on LH panel (OSC 2 ONLY and TRANSPOSE), rev A voicecards,no solder mask on any boards, mounting screws visible on LH panelslideswitches, no bezzle in hole for mod/pitch levers. Polyphonicglide circuit used CA3240 which fail with age.
1	'OB-X' block typeface andOberheim logo above keyboard, three slideswitches on LH panel (OSC 2ONLY, BROAD/NARROW, OCTAVE)
2a	'OB-X' fancy typeface top leftof panel, 'Oberheim' brand above keyboard on right, 'HOLD' 'RESET'labels right-aligned, voicecards have solder mask
2b	'HOLD' 'RESET' labelscenter-aligned, voicecard enable mini-DIP switch (common failurepoint). Noise filter added to AC power inlet, processor & mainboard has solder mask, mounting bolts for LH slideswitches nowintegrated into LH panel with only visible screws on mod/bendlevers. CA3240 replaced with TL082 (LF353 is suitablereplacement) in polyphonic glide circuit.
2c	Power switch moved to top ofpanel, 'RESET' renamed 'CHORD' and center-aligned, rev B voicecards(gate LED added)
3	Bezzle added in hole formod/pitch levers. Gray outline changed to darker scheme with hintof blue. OTAs matched in polyphonic glide circuit for better (butnever perfect) tracking of glide rates between voices.
4	LH panel slideswitches has grayborder, rev B processor board (adds 'Oberheim interface' DB37 connectoron LH endblock), panel boards have solder mask.

So which is the the best 'rev' OB-X to look for? Soundwise,they're the same. If you want one with minimalmaintenance/reliability, there is no better rev - they all have thedesign faults, flakey CMOS ICs, failing solder joints, and badtrimpots. If you want one with minimal restore effort, the laterones are not much safer. top

OB Model Comparision

There's a lot of confusion between the different Oberheim OB models soI built this table to compare them

	OB-X	OB-SX	OB-Xa	OB-8
VOICE CARD	4-8	4-6	4, 6, 8(even only)	8
2x discrete	2x CEM3340	2x CEM3340	2x CEM3340
Modulation	XMOD	XMOD	VCO2 FM byVCF EG	expanded insoftware matrix
yes	yes	yes	yes
VCF (lowpass only)	Discretecascaded OTAs, 12dB	CEM3320, 12dB	2x CEM3320,12dB or 24dB	1x CEM3320,12dB or 24dB
3080 OTA +TL081 buffer	3080 OTA +TL081 buffer	2x 3080 OTA+ TL081 buffer	2x 3080 OTA+ TL081 buffer or CEM3360
Voice volume control	no	no	yes	yes
2x CEM3310VCF & VCA	2x CEM3310VCF & VCA	2x CEM3310VCF & VCA	2x CEM3310VCF & VCA
LFO	one	one	two (fordual timbrality split/layer)	two(software)
yes	no	yes	yes
SYSTEM
# of patches	32	24/48/56depending on version	32/120depending on version	120
Full	Limited	Full	Full
User Storage	yes	no	yes	yes
analog	56-presetversion only, digital	analog	digital
Timbrality	mono	mono	dual	dual
stereo	mono	stereo	stereo
Voicecard Audio Summing	3080 OTA +TL081 buffer	3080 OTA +TL081 buffer	CEM3360	CEM3360
10	10	12	14
MIDI Retrofit	EncoreElectronics	HighlyLiquid UMR2	EncoreElectronics	EncoreElectronics

Fully functional Oberheim OB-X units rarely come up for sale andwhen they do they command some serious bucks. Most casual ownersdon't have the resources to keep them running and resort to 'partingout' valuable stuff like the voicecards. If you can't afford afunctional unit, by all means don't buy a broken on until you arrangethe services of a competent tech for restoration work (and enough $$$for the labor and parts) - unless you're lucky enough to possess an EEdegree for DIY jobs. Luckily you can still get replacementcomponents for the OB-X, with the exception of the 80C98, CA3080, andCEM3310. CEM ICs are getting expensive and hard to source. Unlike the CEM-heavy OB-Xa and OB-8, the only CEM in the OB-X is theCEM3310 EG and those usually are quite robust.
Most people would be crazy to own both an OB-X and Memorymoog in thatbothare service nightmares, but I havebeen restoring and maintaining these things for years and can keep themrunning in top condition. I have loved my versatile Memorymoogfor years, but the OB-X compliments it nicely, fills in somegaps, and produces unique sounds only the OB-X can do. The filter isvery unique and colorful, and despite appearancesof basic architecture the OB-X can produce some wild and crazysounds. Most people identify the OB-X as the sound of the 80s forgood reason. Many classic songs featured this beast, andsomething was 'lost' when the OB-X was replaced with the OB-Xa, OB-8,Xpander.. Oberheim never reclaimed that 'sound'. There aren'tmany of these units left, and people who have good units in theirpossession are not giving them up. top

Oberheim Sem Schematic

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Oberheim Matrix 12