Hello everybody,
this is my first visit in the forum!
Please take a moment to read and possibly help me.
I have on the bench an old Micro Synthesizer that behaves quite strangely:
when powerd on it takes 30 seconds or so for the effect to come alive and, if a guitar is played, it comes on with a huge sweep. Maybe related to that is the fact that the sweep does not work properly: or better it’s not working at all. Start and stop frequencies appear to work but I can hear a sweep only when I’m moving the start and stop sliders. The rate slider changes the character of the sound but no sweep comes on. I managed to troubleshoot all of the other features (it was completely in pieces, not a wire was soldered to both boards) but not this one.
Where should I look at?
I replaced U9 and A13, as well as Q3 and C28. D16, D17 and D18 appear to be ok.
I forgot to mention that I must set the trigger slider at its minimum since pushing up the slider makes the pedal silent. Maybe this has to do with my problem.

Is there any critical part to look at?
Thanks in advance for your attention.