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.