[bojan]

Last update:
Lathe works now with repaired control module (original faulty processor replaced with Attiny85).
I did some more measurements on both modules...
Minimal pulse duration (determined by external buffer circuitry) is ~8us, and combined with ~15kHz (generated by Holtek processor) results in quite high duty cycle (~12%), and that means high minimum speed.
My program for Attiny85 generates ~4kHz for PWM and with ~8us PW duty cycle is ~4%, so the minimum speed is considerably lower, ~70rpm, acceptable for threading.

This is one very primitive controller.. obviously made with very low level of expertise in analog design.

For example, they are using diode in series with transistor base in buffer circuit (no idea why.. maybe to have better noise immunity.. but then they have too slow transistor switching speed? And they have no control over duty cycle at low speeds.. except via PWM frequency).
Bypass cap (1nF) across this diode reduces duty cycle down to 1.5%... Or, using series resistor instead of diode) could also have the same effect.

They should have used dedicated Gate Driver IC for FET imo.

No wonder people are buying better electronics (which is not cheap, ~US200-300).
Anyway.. Another project is finished.