This Solton SM-100 vintage DSO-analog synthesizer was sent to me for repair with a hanging voice/missing, and one output channel not working. Troubleshooting the missing output channel was quite easy. Since there was a hanging note constantly, I didn't had to play any keyboard. I started at the outputs of the opamp which worked both. So I traced the signal, but next was just the audio connector. Testing the solderjoints was fine but still no output. This could only be the connector. I ask the client to use the "pedal" jack, which he said he didn´t use. So I swapped these and second channel came back to life.
Throubleshooting the hanging/missing voice was a bit more complicated, but was located around 4 LS377 ICs. I did test them and one failed the test. Replaing the one marked on the pictures stopped the voicing from hanging notes and all 6 voices could be played.
I thought, "that's it", all problems located and solved. But while testing the presets I still found one missing voice. I did program my own preset to find out exactly whats going on here and I found, that the 2nd DCO on one voice chip wasn`t working. All logic ICs on that voicecard failed testing, so I swapped them all and -voila- now everything is playing correct.
Cleaned the pcb.
Didn't take too many photos this time. If you have troubles with your Solton, don´t hesitate to contact me for repair.
There are lethal voltage exposed in electronic devices. Unless you know what you are doing, don’t try this. If you following along, you doing so at your own risk.
I’m documenting repairs for my reference and yours.
I do synth restorations and repair with a specialty in vintage synthesizers, drum computer and other vintage gear, based in Düsseldorf, GER. Get in touch about your synth situation!