I have recently gotten a SCI Prophet 600 for repair. During testing I found that it sounded generally ‘wrong’. Further in-detail testing of the various stages revealed that on some of the voices, oscillator B was not producing any sound.
I then proceeded measuring the outputs of all individual B oscillators and found 2 of the CEM3340 VCO’s out of order. I confirmed this by swapping the IC’s with another voice.
Once this was identified I ordered some new CEM3340 reissue VCO’s, which luckily exist today. Fifteen years ago this would have been an issue to source these replacement parts but now in 2023 it’s no more a problem.
When the parts arrived, I replaced them and confirmed their function. To me they sound exactly the same compared to the old ones.
Left is the new CEM3340 (Rev G) probably from 2021, right the old one from 1983.
After that I proceeded replacing most electrolytic caps in the signal path as well as the ones in the power supply circuit as they date from the 80ties and it’s a good opportunity to replace them now that everything is apart.
GliGli P600 CPU modification
Since I had the synthesizer already in the workshop, I also chose to replace the CPU with a Teensy++ and the GliGli (Imogen) Firmware.
This CPU replacement gives the P600 a couple of important improvements, like better potentiometer resolution, more versatile LFO’s, envelopes etc. and is very well documented on following websites:
GitHub – image-et-son/p600fw: GliGli based Prophet 600 firmware upgrade
Prophet 600 firmware upgrade revisited
Program the Teensy++ | Prophet 600 GliGli mod | Adafruit Learning System
Here in the left picture one can see the old Z80 CPU replaced with the new Teensy++ on the right. The Teensy++ needs to be slightly modified with respect to the wiring of the socket connections.
The backup battery is not required anymore after this upgrade.
Once everything was tested to be functioning well and after a warmup period I went ahead and performed the calibration procedure of the initial settings for each voice. This is done by putting the synth into service mode, as documented in the P600 service manual. After the calibration of each individual VCO and Filter I executed the tuning routine on the CPU. This achieved a rather excellent tuning of the oscillators as well as of the filters. The differences between voices became almost marginal if not inaudible thanks to this tuning procedure.
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