Oberheim FVS Four Voice System Analog Polyphonic Synthesizer
Last Update
02-05-2021
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
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
Expansion
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
Restoration
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
top
Epilogue
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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