Electronic Repair

Here is an Oberheim FVS-1 from around 1976 that I fixed up. I worked on it for over a year! When it arrived to my shop, only two of the four SEM modules somewhat worked. My customer purchased it sometime in the 1990s, and who knows when it was last fully functional.

This FVS-1 is an analog synth super computer. It features four 2 oscillator synthesizer modules (SEMs) controlled by a single keyboard, and very early digital logic that can store up to 16 presets! There are no microprocessors. You can control all 4 SEMs with a digital/analog Programmer module. It can play four notes of polyphony, or in unison mode as an 8 oscillator mono-synth! It sounds HUGE!

When it arrived, SEM 3 and 4 were sort of working, but had lots of problems and definitely not scaling correctly. SEM 2 was completely dead. SEM 1 seemed to work, but both oscillators only produced a “tick” every few seconds. Also a ton of small problems throughout. My client wanted me to recap everything, and replace all of the old trim pots with new multi turn pots. I replaced every electrolytic capacitor in the instrument, and replaced every tantalum cap with an equal value electrolytic. Then I calibrated each voice using a very vague process photo copied from a service manual using the new 25 turn trim pots.

SEM#2 wasn’t working due to the gate signal from the programmer not flip-flopping on. Once I replaced that chip, the gate was modulated at an audio frequency whenever it was on. After hours of troubleshooting, I found a NAND gate on the board had one bad output. SEM#1 had a bad CA3046 transistor array that caused the oscillators to operate at only very low frequencies.

Eventually, I got everything working to spec. As I was putting everything back together to be ceremoniously picked back up exactly one year later, SEM#3 developed a new problem with Oscillator 2. I eventually found a transistor with a broken leg that was making just enough contact to work. After that was sorted out a few weeks later, SEM #1 developed a gating problem. Luckily, it was a similar problem as what I found on SEM#2.

Finally, SEM#4 decided to give up Oscillator 1. It was a super intermittent problem. The oscillator would act fine, and then suddenly the frequency warbled, and finally the waveform stopped. It didn't seem temperature dependant.. sometimes the problem would happen immediately after powering, sometimes it took hours after working perfectly. To troubleshoot, I ended up migrating suspected components between the two oscillators hoping to hear the failure. Finally, after several weeks, the problem moved to the other oscillator. I then worked in reverse, but the problem was so intermittent that it behaved all the time. Since it wasn't too many parts, I decided to just replace all those components with new ones. It was a few NPN and PNP transistors, a couple of FETs, a few resistors, and a polystyrene capacitor. One of those was the culprit, but which one? They all tested fine out of circuit.