hello Johannews ,
thank you very much, I measured the level Ic13 pin 1, the probe, and have in dc -5 to +5. That seems fine.
The IC 20 pin 12 output. I see nothing in ac, but putting the selector in DC, I see -01 to -5 v that I can modulate PWM Speed potentiometer. I do not see the waveform you told me to AC, I changed IC20, c32, ic 15. But still does not work.
Victor