Oberheim FVS Four Voice System Analog Polyphonic Synthesizer



Last Update 02-05-2021

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This handyman’s specimen landed in my lap as part of a trade deal with a studio owner.  A friend of his gave up the tour lifestyle and gave his Oberheim Four Voice Synthesizer (FVS) to the studio owner, but it didn’t work at all.  He sent it to my tech friend, who found it beyond his skills.  To be fair, the schematics that are available are pretty poor.  My tech friend knew of my restoration success with Oberheims so he gave my name to the studio owner.  I managed to get it to make sound so I could access what works and what doesn’t.  It needed a LOT of work, more than the studio owner was willing to invest.  So he offered it as a trade for one of my vintage keyboards.  I could have pointed him to a new reissue but he insisted on vintage, so I had a dead Memorymoog I bought from a friend that I offered to restore to full functionality in trade.  I delivered, and I had a FVS and a DS-2A sequencer both with Anvil road cases (which needed new foam).  I knew the Memorymoog would be much more user friendly to the studio owner.

The backup battery - over forty years old - had leaked and damaged some components and circuit board traces.  My tech friend had already removed the battery, and I neutralized the acidic corrosion of the leakage to prevent further damage to the circuit board traces.  The keyboard controller and programmer needed some real help.  I spotted many 1st generation RCA CMOS logic ICs that are notorious for dying in legacy Oberheims, as well as 1458, LM301, and LM324 opamps that aren’t any better.  Luckily the not-available-at-any-price concentric ball bearing reduction VCO tuning pots were fine, and some SEM malfunctions were easily remedied with new regulators and opamps.  I managed to partially restore the keyboard controller, so with all SEMs working I had a working FVS sans programmer (whose extensive repairs would be addressed later).

The FVS has been a popular item at various AHMW events, where synthesizer enthusiasts bring their vintage and modern synthesizers for show and tell.  There were a couple of FVS units brought by members in the past, but mine was the first one that wasn’t DOA.

Polyphony & Programmabilty - Tough Nut To Crack

The Oberheim Four Voice Synthesizer (FVS) is not the first polyphonic synthesizer; but it was the first one that mattered.  In the 1970s, monophonic synthesizers were the rage.  But they could only play one note and musicians longed for a synthesizer that could play chords.  Polyphonic synthesizers back in the 1970s were a tough nut to crack, and varying designs did not always meet what the musicians wanted.  To be fair, there was a communication gap.  Few musicians could translate what they wanted into terms that synthesizer designers could understand.  The university intelligentsia were not much help either.  It was really an empirical process until gold was struck.

To establish convention in this discussion: “voice” is a unique pitched source; “note” is a key pressed on a keyboard.

Initial polyphonic synthesizer designs featured paraphonic systems; despite the number of voices, a single voltage controlled filter (VCF) covered the instrument.  Some designs were a monophonic synth with a keyboard that could produce two pitch CVs per VCO but the remaining VCF, VCA, EG, LFO was shared between the two voices.  Not terribly flexible, could not play chords with more than two notes.  Some designs were a top octave divide-down system similar to electronic organs, either with single VCF or none at all.  Although every note produced a voice the sole VCF, lack of individual articulation (IE variable envelopes per voice), and lack of wave shaping was very limiting.  Usually these were pre-configured string machines.  None of these comprised what the musicians wanted in a polyphonic synthesizer.  The idea was floated about bunching individual synthesizer voices under the control of a keyboard, but the missing technology was a polyphonic keyboard controller and a programmer.

Tom Oberheim had a good business going building guitar pedals for the Norlin conglomerate under the Maestro brand; Norlin also owned Gibson guitars, Pearl drums, Lowrey Organs, Moog Synthesizers, and other well known music brands.  Oberheim was also an ARP dealer on the west coast which gave him his first exposure to analog synthesizers - and plenty of interfacing with customers.  Customers desired a sequencer for their ARP products; Oberheim had design experience with NCR corporation which built cash registers, so he designed the DS-2 (later DS-2a with double the memory) sequencer to meet their demand.

The DS-2a sequencer was a successful product.  Oberheim noticed that when customers interfaced the DS-2a to their synthesizer, they couldn’t play on it any more.  So Oberheim designed the SEM module (Synthesizer Expansion Module) as an accessory for the DS-2a.  The SEM was a compact fully equipped analog synthesizer voice without a keyboard that could be interfaced to the DS-2a so the customer could still play along with the sequence playing on the DS-2a.  Tom Oberheim’s expertise was in logic design, so he employed Jim Cooper (later of JLCooper products) as his analog designer, and consulted with Dave Rossum of Emu on design advice.  Back in those days the collaboration between synthesizer companies on the west coast was generally friendly (with some exceptions), and Emu negotiated a royalty agreement based on the sale of SEMs in exchange for consulting services.

Tom Oberheim never thought of his business as anything other than an accessory maker - until there was a slump in the musical instrument market.  This slump caused Norlin/Maestro to cancel orders for Oberheim’s products - Tom had to come up with a product that could produce revenue, and fast.

Tom was aware of efforts to build polyphonic synthesizers, and from his customers who bought ARP synthesizers he learned what they wanted.  By this time, Dave Rossum and Scott Wedge of Emu had built their polyphonic keyboard controller for their modular systems.  Tom saw this as the ideal controller to pair up with his SEMs.  His idea was to bunch together multiple SEMs then control them with Emu’s polyphonic controller.  So a royalty agreement was negotiated to use Emu’s design, and the FVS was born. 

Emu’s design is a voice assignment system, meaning it could assign multiple pressed keys to individual synthesizer voices and was intelligent enough to know which voices were not used and thus available to trigger.  While you still couldn’t play every note on the keyboard (which would had been prohibitively expensive under such a design), even a limited number of voices was an acceptable compromise.  Emu’s design was hardly the first; the first known voice assignment keyboard was the German Warbo Formant organ of 1937, designed by Christian Warnke (war-) and Harald Bode (-bo).   Yamaha had their monstrous GX-1 Electone organ shortly before the FVS, which was essentially an analog polyphonic synthesizer with a set of voices under control of a voice assignment system.  But none of these were mass market products; Emu products and the GX-1 were out of reach of most working musicians, and the Warbo organ didn’t reach commercial success due to world events and any units were likely destroyed during the war.

The FVS mated four SEM modules to the Emu polyphonic keyboard and was first revealed at the June 1975 NAMM show.  The FVS finally gave musicians what they wanted in a polyphonic synthesizer.  All four voices were true individual synthesizer voices and the polyphonic keyboard actually worked.  It was much lighter and much less expensive than the US$70,000 GX-1.  The FVS was a true multitimbral polyphonic synthesizer in that each SEM - thus each voice - could be configured as a different sound.

The Oberheim Two voice was a similar system released at the same time; early two voice machines were just a FVS case with two SEMs and sequencer, and used the Emu polyphonic controller.  It was later revised to a smaller case and a two voice keyboard circuit that was not the Emu controller.

The FVS wasn’t cheap at US$4,295 in 1975; and that didn’t include the programmer!  Shortly before the FVS was released, Tom realized that his system in a performance setting would take too long to manually change settings so the need of a programmer to store and recall patches became vital.  Even sans programmer, the FVS was eagerly embraced by professional musicians.

By the time the programmer was available in 1976, a complete FVS system would set you back US$5,690.  Larger packages were available; a six voice polyphonic at US$8,685, the massive eight voice (EVS) at US$10,185, or the goliath dual manual EVS at US$12,500.  This was all in 1979 dollars!!!

Not exactly within reach of the gigging club musician.  But Oberheim owned the polyphonic synthesizer market between 1975 and 1978, and the company grew fast during that time.  Other competitive products were offered but failed to make much of a dent to Oberheim.  The competitive products had compromises that fell short of the FVS feature set, and Oberheim guarded its secrets from copycats.  Patent protection also deterred copying the all-too-important programmer and polyphonic controller.  Yamaha had a voice assignment polyphonic controller but it was unique from the Emu system.  The only challenger to the polyphonic throne was the SCI Prophet-5 in 1978, which was much smaller much lighter and could store ALL the synthesizer parameters.  The Prophet-5 (and its rare cousin Prophet-10 single manual version) was able to exploit the power of microprocessors as their prices were falling.

The last published advertisement for SEM-based systems was May 1979, shortly before the OB-X was released to compete with the Prophet-5.  It is estimated that 800 FVSs were built; my unit dates to 1978 per the datecodes on the ICs and its serial is #6xx, and SEM-based instruments were discontinued in 1979 so the estimate of 800 units may well be accurate.  It is unknown how many six voice or eight voice systems were produced.  Only ten of the dual manual EVS were ever made.

There was talk of a twelve voice system custom made for Patrick Moraz, but this is a typo.  First off, a twelve voice system is not possible as the keyboard and programmer electronics only supported a maximum of eight voices.  Second, I saw a picture of Moraz’s setup in the studio and there was both a FVS and a dual manual EVS in the studio.  Someone saw four plus eight and called it twelve voices… <insert facepalm here> top


The SEM is a brilliant design combining a compact 9x7.5 inch panel with flexibility.  It is a full monophonic analog synthesizer module with two hard-syncable VCOs each with ramp and variable pulse wave shapes; a 12dB/oct voltage controlled state variable filter (IE “SVF” or “multimode”) with lowpass, highpass, variable notch, and bandpass filter modes; VCA; LFO with sine wave shape; two ADS envelope generators with shared decay/release control ala Minimoog.

To keep the panel compact, Oberheim used concentric ball bearing reduction tuning pots for each VCO.  Early SEM units had such a control for the VCF cutoff (early programmers had them for VCO tuning but they were unnecessary with their non-fine resolution).  These controls were a dual shaft assembly with lower and upper knobs; the lower knob was coarse tuning, while the upper knob was fine tuning.  The assembly used a single potentiometer wafer.  The ball bearing mechanism reduced the rotation of the upper knob, which enabled fine tuning control.  While those were an elegant solution, Tom was never fond of those controls and today they are no longer available and NOS are extremely rare and $$$.  The only other synth that was known to use them are Memorymoogs.

Each VCO pitch or pulse width can be modulated by an EG or LFO with variable depth, and the filter can be modulated by LFO or the filter EG in either positive of negative polarity.  The SEM has expansion ports that allow additional CV or audio inputs.  The 12dB/oct SVF was deliberately chosen as an alternative to the popular 24dB/oct VCF at the time, and has proven to co-exist nicely alongside most 24dB/oct equipped synths such as Moogs and ARPs without being redundant. 

One feature I miss is keyboard tracking of the VCF, although it is possible using the expansion ports.  The SEMs is not a “pure” modular as the audio path is fixed.  There’s no “master” LFO in a SEM polysynth; the LFO on every SEM is independent and they can offer modulations that weren’t possible anywhere else until the advent of soft synths.

The SEM was designed primarily by Jim Cooper and Dave Rossum.  The VCOs were a derivative of the Emu VCOs which were designed by Dave Rossum.  They are very good VCOs, mine stay in tune very well (they don’t even need a “warm up” period like most analog synths).  Oberheim products were finished in an attractive cream matte finish, which doesn’t reflect blinding stage lighting like a white finish.

The OBX voicecard is a SEM-on-a-pc-board, which is one of the reasons the OB-X sounds so good. top

The Programmer (n=number of SEMs)

The programmer was a godsend at the time - you have to remember that this was an era where if you wanted to change sounds on a synthesizer, you had to set switches and knobs manually.  Doing that with a monophonic like a Minimoog or ARP Odyssey on a dark stage was a challenge, and not likely to result in exact duplication of your sound.  Asking that for a multiple of SEMs in a polyphonic was clearly too much.  The programmer was patented under US 4,185,531 and is recommended reading for service techs.  If you’re an AES member there is a paper presented by Tom Oberheim that discusses the operation of the programmer down to the timing diagram.

It was the first device that could store user patches electronically.  Preset synthesizers that predated it only programmed the unit using logic switches and predefined resistors, and while the presets could be user-modified the modifications could not be stored.  The Yamaha GX-1 could store user patches but required the optional programmer which involved a tedious procedure of setting miniscule controls until their position matched the setting on the user panel.  Some synths used computer punchcards (!) to recall a patch; you could create you own patches if you very carefully punched out the right holes manually...

It can store/recall 16 patches.  Eight buttons recall a patch; a slide switch configures patches 1 through 8 or 9 through 16 for the same 8 buttons.  Each button has a lamp that turns on for the active patch.  There is no compare mode, once you store a patch there’s no real way to compare against another patch.  You can’t edit a patch either.  Patch archiving required a separate cassette tape interface accessory and a tape recorder (this was long before MIDI SYSEX archiving).

Above each patch button is a slideswitch with positions for MEMORY, MANUAL, and WRITE.  These slideswitches control EACH SEM for the current patch.  This confuses many first time users.  The programmer can program up to eight SEM modules.  With a switch in MEMORY position, that SEM is receiving its control from digital memory.  Note that changing the knobs on the programmer does nothing - you cannot “edit” a setting in memory.  By changing the switch to MANUAL, then the SEM is receiving its control from the knobs on the programmer.

It is not an intuitive device.  The programmer doesn’t “program” the SEM modules, it provides a CV offset to the knob settings already on each SEM.  The best way to exploit the programmer is to “initialize” each SEM to a default setting; then any CV offsets from the programmer is “added” to the default setting.  Since the SEM was not initially designed for complete CV control, not all settings on a SEM can be controlled by the programmer IE there is no CV control for VCF resonance so the SEM settings for resonance cannot be controlled by the programmer.  In order for the SEM to be controlled by the programmer, certain switch settings had to be made on the SEM.

No programmer control of VCF resonance, VCO pulse width, hard sync, VCF input mixer, filter modes, modulation source or depths, LFO rate, or EGs.  You had to set these manually between patches.  While this is pretty cumbersome with four SEMs, most owners of eight voice systems found it to be too much.  Dennis DeYoung of Styx was one of the first high profile users of the FVS, which is all over their breakthrough album Grand Illusion.  By the next album DeYoung had purchased an eight voice system but at the end of that tour he abandoned it in favor of two FVS synthesizers (both seen in the Kilroy Was Here concert DVD).  It appeared that one FVS was delegated to lead synth while the other to chordal work.

The programmer supplies a duplicate “panel” from which you manipulate your settings, then store your patch.  You can program pitch of each VCO, VCF cutoff, EG ADS settings (separate from the EGs on the SEMs), VCF EG Depth, LFO rate (separate from the LFO on the SEMs), and LFO depth (to both VCOs only).  Oberheim had newcomer Doug Curtis (soon to be famous for his CEM line of synthesizer ICs) design and fabricate custom ICs for the programmer.  These custom ICs saved a lot of PC board space and had an ADSR EG (wired shared D/R on the programmer!) and an OTA for voltage controlled amplitude.  They are not compatible with any of the CEM ICs and there is no direct replacement.  Each voice used two of these ICs to comprise EGs for VCF and VCA, and the OTAs provided programmed amplitude of LFO depth and EG depth to the VCF.  But you could still use the EGs on each SEM for modulation sources.  The technology had not yet advanced in one area: the transient times with respect to CV was not consistent from IC to IC so Oberheim had to hand test each IC to test the timing, then bin them into sets so a matched set of ICs went into a single polyphonic.

The resolution of the programmed controls was pretty coarse - back then RAM memory was expensive and were low density.  As an example, VCO pitch resolution in the programmer is only semitones (half steps).  The cutting edge technology in 1975 offered a whopping 1024 bit RAM IC, so bit depth was limited for each control.  While VCO pitch bit depth was 6 bits, with semitone resolution it did cover a five octave range.  Sadly there’s no “detune” control with finer resolution.  There is no microprocessor in any of the SEM-based polysynths, they were a discrete computer built from low density logic ICs (microprocessors in 1975 were not yet common and they cost a few hundred dollars).  Tom Oberheim and Jim Cooper designed the programmer.

But the REAL power of the programmer is each SEM can have independent settings within a patch.  You “store” a patch by engaging the spring loaded slideswitch to the WRITE position (while simultaneously engaging the WRITE slideswitch on the far right).  Why are there EIGHT sets of slideswitches?  Because each slideswitch is for independent SEMs for the current selected patch!  If you have less than eight SEMs, only the first four or six slideswitches have any effect.  You CAN program a homogenous (patch all SEMs producing the same sound) if you want to; simply program SEM#1 and store it, then program SEM#2 and store it, etc.  Or you can program each SEM to a different sound within a patch. I was not aware of this feature until I had a FVS in my hands.  The slideswitches have a MANUAL position that bypasses the stored patch and routes the “duplicate” panel setting directly to the SEMs.

Some argue that a pair of Oberheim MiniSequencers in place of the programmer was more practical.  FVS units have been built with this configuration. top

The Polyphonic Keyboard Controller module (n=number of SEMs, x=number of pressed keys)

The assignment architecture of the polyphonic controller was the key to the SEM-based synthesizer success.  So much that the system was patented under US 3,986,423, and the ICs on early keyboard controllers had their markings sanded off to deter infringers.  The patent is also a good companion to the schematics which are woefully hard to read.

Like the programmer, the keyboard controller is a discrete computer built out of CMOS logic.

The keyboard controller configures the keyboard in its various polyphonic and monophonic modes.  It has no “rollover” or “steal” mode; if your system has four SEMs and you play four notes all the SEMs will trigger; while still holding those notes, playing additional notes will not “steal” a voice.

The controller offers four polyphonic modes, whose operation isn’t crystal clear in the owner’s manual.  I try to clarify it here:
Modes (with REASSIGN set to ON):
CONT = continuous = cyclic, with each successive note press the SEM triggering cycles from #1 to #n.
R/C = reset/continuous = while the 1st note is held, SEM#1 is triggered; playing successive notes cycles through the remaining SEMs.
RESET = reset = when x notes are played, SEM#1 through #n are triggered in the order the notes are played; while holding down notes, additional notes played will trigger the remaining SEMs; once you release all notes then 1st note “resets” to SEM#1

With REASSIGN set to OFF, it overrides the other modes and becomes “cyclic with memory”.  Similar to cyclic action of CONT but any note played repeatedly is assigned to the same SEM.

Unison mode puts the keyboard in monophonic mode with multiple triggering.  You haven’t heard analog until you’ve heard four or more SEMs firing at the same time!  Using the mixer you can select fewer SEMs in monophonic mode if desired.  A shame there is no global vibrato LFO or control for lead work.  The only facility for pitch bend is the Tune section.

Split mode splits the keyboard at the middle at the C key and is always cyclic mode.  In a four voice, you could configure each half of the keyboard with 1-3, 2-2, or 3-1 SEMs.  Six or eight voice systems offered more voices in a split mode (although the two octave keyboard range was short).  Split mode also worked with unison mode.  There was no “layer” mode, although you could simulate monophonic layers using “2-2” in unison split mode.

Portamento (actually “glide”) was polyphonic on all the SEMs.  In split mode you could enable portamento on either or both halves of the keyboard.  The maximum portamento time isn’t very long.

Freeze was intended for configuring a single SEM module, but does have a performance purpose.  Say you wanted to repeatedly trigger a single SEM without triggering the others, so you could fine-tune its configuration.  Every SEM has an LED that lights when it is triggered.  So you pressed a key to cycle through the SEMs you could see the LEDs cycle around, and before triggering the desired SEM you enabled the FREEZE switch then only that SEM would be triggered.  It could be argued that the same function can be achieved using unison mode and disabling the other SEMs.  But the FREEZE function can be a performance feature.  You could play a monophonic solo with FREEZE enabled; you could have other SEMs configured for a different solo sound, and cycle through them using the FREEZE switch between notes.

Sometimes changing modes can cause weird behavior like unison mode even though polyphonic mode is enabled.  I’m not sure if there is a fault in my unit or if this was a common “bug”.  I can usually correct that “bug” by reverting to the previous mode and “roll” through a four note chord.

The Tune section provides master controls for VCO and VCF.  There is also an octave switch for shifting the keyboard up or down an octave.  The VCF master control is handy for shifting the VCF cutoff frequency of all the SEMs at the same time.  The VCO master control also shifts the VCOs on all the SEMs up or down an octave, and is intended for master tuning or pitch bend.  If your band tunes at other than A440 you can always tune the SEMs instead.  The VCO control has a center detent to locate center pitch.  It’s a damped control, meaning that rapid pitch changes aren’t easy.  It’s not a very usable control for pitch bend (although great for slow pitch dives).  I have seen at least one FVS with an aftermarket pitch bend lever added.

While the stock FVS had a four octave keyboard, there was an option for a five octave keyboard.  I’ve never seen a FVS with five octave keyboard, which presumably would have to sacrifice the programmer to make room for the larger keybed.  The keyboard controller electronics were already ready for five octave control.  The keyboard is not velocity sensitive and has no aftertouch. top

Output Module

Not an exotic function - it has a mixer for the audio outputs of the SEMs.  Each SEM had its own volume and pan control.  Yes the SEM polyphonic can be stereo (sort of).  Left/Right outputs are on the mixer panel although the same outputs are on the tip-ring-sleeve AUX OUT jack on the rear panel; I prefer the rear panel TRS jack as you can’t straddle cables across the front panel jacks without blocking the mixer or programmer controls.  Two headphone jacks are provided and they can easily push 600ohm headphones.  Then there is a master volume control - done.  This mixer does not overdrive or distort, and the output can be really high level (I think it is hotter than +4dBm!).

Rear Panel

CV/Trigger input and output jacks allow an external device to control SEM#4.  The DS-2a sequencer is a good accessory for SEM#4.  The DS-2a can record CV and timing of a sequence pattern, then play it back.  A bonus is that when the DS-2a is not playing back a sequence, the CV/trigger inputs are “passed through” so that you can play your synth normally.  So on the FVS you want to patch CV/trigger outputs AND inputs between the DS-2a.  While the DS-2a is playing back a sequence you are reduced to three SEMs for live use.  When recording a sequence, just make sure the keyboard controller is configured to SEM#4 using the FREEZE function.

There’s a jack for connecting a sweep pedal for sweeping the cutoff frequency of all the SEMs.  AUX IN only patches audio to the mixer, it is not an audio input to a SEM filter.  As previously mentioned, AUX OUT is the left/right output of the mixer.  The extra SEMs of six or eight voice systems used these AUX jacks to route to the “master” mixer. 

As for the four holes with plugs… no idea.  They were probably reserved for customization.

A multipoint connector for the cassette interface accessory was standard; other multipoint connectors were available for mini sequencers, for routing CV/trigger/programmer signals to extra SEMs, etc. top


Today it would be difficult to add SEMs to a FVS.  In order to accomplish expansion, you not only need the SEMs but you also need another output module, expansion case, and addition circuit boards for the keyboard controller and the programmer.  You have to expand in sets of two SEMs at a time because a circuit board for controller and programmer accommodates two SEMs.  Unless someone clones them, finding surplus circuit boards is going to be very difficult.  The additional case with power supply is necessary because the stock power supply cannot supply enough current for additional SEMs.  Oh, and you need the multipin ports to route control voltages from the programmer and keyboard, along with the cable to connect them… top

Goliath - Dual Manual Eight Voice

The dual manual eight voice system had two keyboards - four octave and five octave, each with their dedicated controller module.  The keyboards were in one large case, and the SEMs were in another large case.  It is a huge system.  The four octave keyboard could control four SEMs while the five octave one could control eight SEMs.  Additional modes were developed such as dividing SEMs between the two keyboards, IE 4/4 or 6/2 or all 8 on the five octave keyboard.  Only one programmer for the system, but there was a panel with CV/trigger outputs for all eight channels.  Quite why they brought out all eight CV/trigger outputs to a panel, I don’t know.  Yes you could trigger external monophonic synths, but eight of them…?!?

Oberheim only made ten of the dual manual systems.  Maybe the US$12,500 retail and its massive size was prohibitive to anybody but successful professionals.  Patrick Moraz owned (and toured!) the first production model, which he sold in 2009. top


My FVS had a bunch of malfunctions: programmer was really messed up, polyphonic controller was wonky, some keyboard contacts weren’t reliable, various malfunctions on the SEMs.  Master volume on the output module was crackly.  The programmer was a major project, so I initially bypassed it to simplify troubleshooting the remaining system.

The programmer is a piece of brilliant packaging engineering of layered circuit boards.  The boards are divided into logical functions.  The lower two boards comprise the basic operational circuits for patch selection, while SEM-level memory storage, demux’d S&H, EGs, LFOs, CV routing, and support logic ICs are on other boards.  A single SEM-level board supports two SEMs.  A complete programmer is comprised of these layered circuit boards, and one of these for an EVS resulted in a pretty deep assembly.  The deep assembly is the reason why the FVS/EVS case is so deep.

Likewise the polyphonic keyboard controller is also a layered circuit board assembly.  This approach kept the panels compact regardless of number of SEMs.

First target was to tackle the SEMs.  One had a faulty 723 regulator, others had defective opamps.  Those weren’t too hard.  You could swap SEMs around on a working key channel to enable troubleshooting.  Once all the SEMs were functional, the polyphonic malfunction was narrowed down to a defective CMOS chip.  By then I had a working FVS sans programmer.

Keyboard contacts had to be adjusted, and one of the CMOS chips was defective.  Servicing the keyboard is a major disassembly effort.  The mounting screws for the keyboard are UNDER the metal frame for the FVS, so the frame must be removed from the case which means ALL the modules must be removed.

The leaking acid from the battery on the programmer posed a major repair challenge.  The acid got to one of the programmer patch select buttons which destroyed it.  Those buttons are no longer available, but I had another derelict electronic piano that used the same buttons.  All I had to do was swap the button cap and lamp bulb into the scavenged one, and that was resolved.  The acid got to an interconnect header which had to be replaced due to corroded plating on one of the pins.  I had to replace a pot damaged by acid by scavenging one from a dead OBSX, swapping wafers as the weird PC board footprint was no longer available.  The acid also corroded many circuit board traces which required jumpering.  While doing that work I took the liberty of drawing my own schematics which are much clearer than the originals.

Troubleshooting the programmer is a bit of a challenge.  Most techs don’t like to work on them, but the effort is much easier by removing the programmer completely from the FVS and using a bench power supply supplying power to it.

...in progress, check back later


Will the FVS/EVS sound like the OB-X/Xa/8? Yes and no. The FVS/EVS sounds great but slightly different.  To start with, the voice summing circuit is different.  On the later OBs the voice summing circuit was based around the not-so-high-fidelity 3080 OTA whose mild distortion did give them a nice organic edge.   The FVS/EVS has no OTA in the voice summing circuit.  The OB-X has the discrete SVF but only in low pass mode, and none of the succeeding OBs has the discrete SVF.  The SEMs in the FVS/EVS feature some modulation tricks not possible on the later OBs though.  You could get those modulations on Xpander/Matrix-12 but not the wonderful discrete SVF.

The SEM-based Oberheim polyphonic synthesizers are true vintage classics, and units trade hands at very high prices.  MIDI?  Only with an aftermarket modification, and don’t expect much other than note-on/note-off and filter control.  The provisions for MIDI volume, key velocity, aftertouch, mod wheel, pitch bend, program select, et al simply aren’t possible on a stock FVS.  Unless you have the rare cassette interface accessory, you’ll need to record programmer patches with pen and paper.

Measuring at 42x18x11 inches, the FVS is bulky.  Deep, tall, and long.  It’s freaking BIG.  Extra SEMs in a six or eight voice system required another case 42x10x11 inches.  The extra case presented a placement challenge - place it behind the base unit, and adjusting the SEMs could strain your back and/or your eyes.  Placing the extra SEMs to the side consumes floor space.  These will be a squeeze in a bedroom studio.  For a keyboard stack, you can’t stack much above these bulky machines unless the devices on top are really shallow, because you can’t block the controls on the SEMs.  In my studio I wanted a stack of keyboards on both sides of my master MIDI controller, but the stack had to have a low profile so that the sound field of my monitors would not be interrupted.  There was just no way to arrange the FVS in a stack without blocking the monitors.  I had to put my FVS in a taller stack behind the main keyboards.  Since the monitors were near field, the stacks at the back weren’t in any danger of interrupting the sound field so that was a compromise I could accept.  Despite the tall 60 inch keyboard stack with tiers, only the FVS and Minimoog were practical to place on it.

Be prepared for an expensive restoration project on an unrestored Oberheim.  SEM-based polyphonics are more expensive to restore than the later products like the OBX/Xa/8 polysynths.  The components alone for my own restoration exceeded $800, and it went beyond that as I restored the programmer.  The labor alone can get REALLY expensive.  Leaking batteries can destroy the programmer.  The pots soldered to the PC boards use a weird footprint that isn’t available anymore.  If any concentric ball bearing reduction VCO tuning pot needs to be replaced, they are very rare and very expensive (Tom admitted that he never liked them).  The knobs are still made by Elma but they are not available from any low quantity source like Mouser or Digikey.  Sellers of NOS knobs demand a premium for them.  Knobs on later Oberheims are not compatible with ones on the SEM units - later knobs were D-shaft and matte finish while SEM and earlier were 1/4” collet fastener with glossy finish.  The interconnects on the SEMs and power supply do not age well and can break; unfortunately there is no direct replacement as their pin pitch was non-standard.  The “tropical fish” capacitors tend to crack with age and fail.  Opamps and regulators can fail.  The logic ICs are first generation RCA CMOS 4xxx series which have a history of failing due to metallization issues on the substrate and zero overvoltage protection on the input pins.  Replacing these CMOS parts is tricky because the electronics were designed around the timing profiles; replace one with a different CMOS family and the circuit may no longer work.  Did I mention that almost none of these logic ICs are socketed?  These are not machines that novices should be attempting DIY repairs on - this is one best left to the experts.

The schematics are hand-written, not complete, and not free of errors.  There is no known “service manual” and no illustration of component layout other than the SEMs.  I collected various internet resources to build a “theory of operation” for the VCF and VCOs on the SEM.  Grasping the operation of the keyboard controller and programmer can stump many repair techs.  The closest thing to a “service manual” is the patents for the programmer and polyphonic keyboard, which is recommended reading for any service tech.  Use any patent search tool using key words “oberheim” and “rossum”.

If you desire nothing more than a homogenous polyphonic (all voices producing the exact same sound), these are not ideal machines as it is a lot of work to configure each SEM exactly the same.  The SEM settings wouldn’t be the same from day to day, and you had to normalize them again.  The programmer is limited and there is no “autotune” feature.  Some players won’t like the limited four octave keyboard.  This is a studio machine; only the brave dared to tour these things back in the day.  The few who tried touring the eight voice found it to be unmanageable.  Today I know of groups touring vintage P5s, CS-80s and OB-X/Xa/8s but I can’t think of a single group touring a SEM-based polyphonic.  I may be crazy enough to gig my Memorymoog or OB-X, but even in my book the FVS is just not a practical unit for the stage.

If you like big fat analog sounds and are ambitious about sound design and exploring multitimbral polyphony, these machines can deliver like no other.  Put a FVS in unison mode and pan the SEMs across the stereo field for the definition of PHAT.  Dennis DeYoung always said his favorite lead synthesizer was the FVS (Grand Illusion has some brilliant FVS leads).  No amount of tweaking can get the SEMs to put out the exact same sound, and that can be valuable as it gives them an organic quality.  The general consensus with the SEM units is while they can emulate traditional sounds they are better for new alien sounds.  For emulative use, they have an uncanny ability to excel at pads and fat brass sounds thanks to their SVF.  Brass is an Oberheim specialty with all their legacy instruments.  The modulation capabilities of the SEMs can offer complex sounds that weren’t possible on later instruments like OB-X/Xa/8.  One of my favorite tricks is routing the VCA envelope to VCO pulse width to emulate an attack transient such as guitars, brass, and reeds.  Routing VCF envelope to pitch of VCO2 (but not VCO1) can impart a quick pitch bend that produces fat attack transients. top

Reverse Color Panels - short lived experiment

Towards the end of SEM production, Oberheim experimented with reverse color panels - black panels with cream labels, some with black knobs, some with white skirts on the knobs, some with white knobs and skirts.  The system frames were also black.  These resulted in a VERY sharp looking machine, albeit Tom did not share the appeal (although he did make reissue Two Voice systems in reverse color!).  By then sales of SEM systems were dropping off due to the successful Prophet-5, and Oberheim’s OB-X was the new kid on the block.  Most black panel systems wound up with Oberheim employees through an employee purchase plan.  Jan Hammer and Roger Powell had their six voice systems with black panels, which they played from their customized Probe “keytar”.  Some two voice and four voice systems were black panel, and at least one dual manual eight voice is all black panels (egads!). top


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