Oberheim FVS-1 restoration project


A nice, but completely not working Oberheim Four Voice came to the workshop some time ago.

First of all, the power supply had to be repaired and converted to 230V mains voltage by using a toroid transformer.
It has been mounted to a small sheet of Al metal to avoid drilling additional holes into the Obie.

After repair and installation of the power supply PCB, the main part of the restoration might begin

(PSU photos to follow)

Every module on the front panel will undergo a thorough treatment: all potentiometers will be removed, opened, cleaned, lubricated and tested. PCBs will be cleaned, checked for cracked solder connections, switched will be cleaned and lubriatced. Electrolytic capacitors will be replaced, and as this model used a special type of film capacitors which tend to crack (sometimes called tropical fish caps due to their color stripes indicating their capacitance value), also the film capacitors will be replaced. Actually several have broken apart from their leads by the slightest touch, so they are no way reliable for continued service. Poor fish.


The polyphonic keyboard assigner

This module scans the 61 key keyboard and assigns the generated CV and gate signals to the 4 SEM modules according to the adjustments made on the front panel.

Actually, it should do so, upon arrival, no single gate pulse was seen, nor a correct CV.
A defective 723 caused most of the logic to be without supply. But once energized, a hand full of dead CMOS chips paved the way to a working assigner.

[Photo to follow]

The Programmer

A special module of the four voice is its programmer. It allows to set up various parameters and store them into an internal memory with 16 places, separately for each SEM.
It also features two ADS envelope generators for the SEM’s VCA and for modulating the VCF.

The parameters are:

  • frequency setting (offset to other sources) for both VCOs and the VCF
  • A-D-S for both envelope generators
  • Frequency of an internal LFO, individual to each SEM (called “vibrato”)
  • Amount of LFO to OSCs (1+2 simultaneously)
  • Amount of 2nd envelope to VCF frequency (called “modulation”)

The voltages are converted by a 6 bit A/D converter and then into a bit stream to be either stored into 2 1024 bit SRAMs or bypassed around the memory in manual setting. Due to this, testing and troubleshooting of most parts of the circuits is possible in real time.

Every of the 2 channel boards of the programmer features 4 proprietary envelope generator ICs which are said to be a predecessor of the famous CEM 3310. It is far away from being compatible though, and the custom Oberheim ICs also have an additional VCA integated to the envelope chip.

This photo shows the programmer stacked together, with the channel 3&4 board on top. Some CMOS ICs as well as several LM324 had to be replaced here. The cermet presets are kept in place, while the rarely used carbon track presets are to be replaced.

One potentiometer on the programmer’s front panel unfortunately had a severe crack in the resistive track, so I had to mix up parts of the original potentiometer with the track of a brand new 24mm Alpha brand pot.

A set of new T1 3/4 light bulbs completed the programmer.

The output mixer

The Four Voice has an active output mixer with individual volume and pan controls. Apart from cleaning the potentiometers, two of the used µA741 op amps were defective, one had zero output, the other a large offset.
I decided to use NE5534 with approproate compensation here, whereas all other op amps remain original for sound reasons.


The hearts – the FVS has four of them – is the famous Oberheim SEM synthesizer module. Once intended for enthusiasts as an add-on to their modulars, it has all relevant connections on Molex connectors on its back side. Several of them are internally routed to other modules, but a modification of this FVS is also in preparation, making it into a modular hybrid.

First of all, the SEMs need to be restored to a working state. Potentiometer cleaning is a bit more complicated because of the special pots used for coarse and fine setting of the VCO frequency.
Here’s how this potentiometer looked when disassembled:


After potentiometer and switch maintenance the SEMs will be “re-capped”, tested and repaired where necessary.

I got one working SEM in a break-out case in addition to verify things which was told to sound a lot better than the SEMs of a Two Voice that I have not looked at so far.
So I compared the SEMs from the FVS with the stand-alone SEM and found that the single SEM was lacking a 150nF capacitor in the gate input circuitry that were present in the FVS SEMs.
I wonder which impact this capacitor might have on sound – it clearly slows down the slope of the gate signal to the trigger circuit and envelope generator.
Will it affect the “snappiness” of a short attack time to an audible extent? Hard to say from the circuit, probably from simulation, but to make things clear I will do some testing once the first SEM is up and running.

Here’s the additional capacitor (no comments on my artwork!)

SCI Pro One – another approach to MIDI

Back in 2007 I developed a replacement PCB for the Pro One’s CPU, not knowing of parallel work on the other side of the ocean.
I planned it to be mounted below the main PCB, just where the old CPU was, but on the other side, but accidently made the PCB too big so that I had to use flat cable to install it.

Some five years later I got another Pro One in need for a new CPU but also for MIDI, so I re-designed the PCB, still with a DIP CPU so that it still needs to be mounted below the mainboard, but now with the correct dimensions.
Keeping in mind that there a much smaller plug-in replacements on the market, I left it this way to allow for DIY building of those daring to solder SMD resistors, but not fine pitch processors.

Here’s the result:

Except from emulating all of the original Pro One’s functions, it can…

Normal and Retrig Mode:

  • use keyboard and MIDI IN in parallel
  • send every key press via MIDI OUT (also in Normal mode, where only lower notes will sound due to low note priority)

Arpeggiator Mode (which has priority over Sequencer Mode in this version, by the way):

  • use keyboard and MIDI IN in parallel during normal and latch mode
  • send the note on/off’s via MIDI OUT as they are played by the arpeggiator, including additionaly hold notes in latched mode

Sequencer Mode:

  • use keyboard or MIDI IN for sequence entry while recording, transpose with keyboard or MIDI IN while playing
  • sends notes via MIDI OUT when local keys are pressed while recording and the notes as they are played while playing
  • offers two independent 100 note memories
  • sequencer memory is non-volatile (saved once leaving the record mode)

The MIDI channel is set by holding down one of the first 16 keys while turning the Pro One on. Every other key will set omni mode.

For the MIDI note assignment I relied on the user’s manual of the Pro One showing the keyboard scale from C0 to C3.
Therefore, the lowest C corresponds to midi #24.

While working on a Rev.1 Pro One one should always keep in mind that the PCB-mounted mains tranformer is much too heavy for the PCB and that the track clearance on the primary side is much too low to comply with any safety standard or electrical code. In this particular case, the transformer was already damaged and had to be replace anyway. This photo shows a possible solution: screw terminal for power cord, protective earth securely bolted to case, fuse in series with mains, double-pole switch with double isolation, toroid tranformer bolted to chassis. The original power connector to the main board has been rewired and re-used.

Restoration of EMT 250s

The model 250 digital reverberator from EMT is quite special. First of all, it looks like the oil radiator from Uhura’s boudoir. In germany the EMT 250 is usually called <Weltraumheizung> (space radiator) because of that.

As the developers of the digital circuit, Dynatron, feared that their design could be copied they removed all markings from the ICs – and there are many – of the logic board. Only the RAMs and some static shift registers used for pre-delay are usually untouched.

The main logic board is completely wire-wrapped, something not seen that often outside of vintage industrial control, aircraft and military equipment.

(Images an some info on the first 250 I worked on will follow…)

The second 250 was hit much harder. Some kind of heavy corrosion destroyed about two dozens of ICs. Their pins look rusty and broke off partially while trying to pull the ICs from their sockets. All of them seem to be from one manufacturer, other ICs with a different looking body do not show any traces of corrosion.

Further investigation with a microscope immediately after removing an IC revealed that some pins were already broken some time before the attempt to pull the IC:

The next task is now to remove the remainings of the pins, clean the sockets and re-populate the board with ICs.

Beware of the flux

While troubleshooting a Sequential Circuits Prophet 5 synthesizer I came across a quite odd phenomenon: several control voltages, generated by means of multiplexers and some dozens S&H circuits, showed a sawtooth waveform instead of a clean DC voltage. While a falling sawtooth would indicate some ground leakage, I also had some outputs showing a rising sawtooth – so there must be some stray current from a higher potential.

Visual inspection revealed a white, wax-like substance between the pads of some multiplexer IC sockets, obviously flux residue from a previous repair attempt. As both PCB sides were affected, all sockets were removed, the PCB cleaned with isopropyl alcohol. After replacing the parts all control voltage were clean again.


Sliding again

As if disassembling and cleaning all linear pots  of this ARP 2600 is not enough to deal with, several levers were also broken and missing, together with the caps.

Amadeus from drucke3d.de made a 3D CAD model from the parts of a broken slider pot, evaluated the best production method and had some new levers sintered for me.
The caps could be made on a typical 3D printer.

All parts cleaned, waiting for assembly

Eveything lubricated and put back together

Good sliding action and no dropouts anymore. Mission accomplished 🙂


Wave Thinking

Today I analyzed a PROZ84 board from a 2.3 with some “odd” sounds.
There was audible distortion, varying with the pitch and the partial wave number.

The oscilloscope revealed (sorry, no image here) that in cases of highest distortion levels only every other sample appeared on the DAC outputs, while the others were at ~0 volts. Half of the partial wave samples having a value of zero is hardly a problem of the wavetable playback, but most likely to be caused by wrong data in the wave RAM.

It turned out that one of the 74S257’s multiplexers outputs in the write circuit had a stuck low output on RAM address line A0, so only even samples were actually written into RAM, with the odd locations containing what is left in the DRAM’s capacitors – in this case, 0.