Teisco S100P synthesizer. Help please!

Hi, I have a Teisco S100P and it has a problem I can't solve. When I play the keys in sequence, I hit a run of four keys which sound they are being modulated in some way or part of the cycle generating the sound is missing. If I switch the octave, the problem simply moves up or down the keyboard depending on whether I select up of down. Given its symptoms I don't think it's anything to do with the keyed or contacts. I think if I could understand how the synth makes the sounds, that is the oscillator section I might be able to figure out what’s going on. Could somebody please explain how the VCO works from the circuit diagram below? Thanks.

Screenshot 2024-12-14 at 20.31.14.png
 
Hi JJasniew, So I checked out your suggestion and the Portamento/Glissando functioned as it should but the problem I am encountering still persisted. Here is a link to a video showing the problem.
https://drive.google.com/file/d/1d5_PkuCReVZR7v4103jfQGb0hxVdym04/view?usp=sharing

When I transpose up of down, the problem moves, but not in a way that would seem logical. Up position; 6 notes in middle of keyboard: C#, D, D#, E F, F#. Normal position 4 notes at top of keyboard F, F#, G, G# affected. Down three notes at very bottom of keyboard C, C# and D.

Help!!!
 
Thanks for making that video! I can clearly hear the "shimmer" on the few notes where it happens and not on the subsequent notes. Something is modulating those, like another frequency present in the circuit that coincides sometimes, sometimes not, depending on the note frequency.

The way the schematic is drawn, how the transpose works is a bit obscure. The VCO clearly has "CV" marked, which I suppose I'd have a look at with an oscilloscope, but I'm pretty sure because it failed the portamento test, there's nothing "riding" that - just on certain voltage values that would cause a modulation.

I'll guess it's a power supply problem. The VCO has 3 voltages going into it which may have degraded due to age of the device. Check those for any possible ripple, with an oscilloscope.
 
Hi, Thanks for taking the time to mull over my problem. I recapped the power supply when I serviced this synth. It had a dead voltage regulator too (+5 i think) and i renewed that. I'll get my scope out and see what I can see. I'll update as and when I make a breakthrough. Seasons wishes, Craig