Hi guys. Wondering if you can help. My old Proxxon mini-saw went wrong with the motor suddenly not running to full speed and no speed control. I have struggled to get the circuit right in my head and there are some puzzling voltages. Add a silicon bilateral switch and opto-isolators into the equation and I am struggling. I have attached my drawing of the circuit diagram. Towards the bottom middle right you will see a variable resistor that is used for the speed control. I am getting no variation ion the voltage - yet the resistance is varying. Secondly the voltage comparator (a 393) appears to be gibig a weird result as the voltages measure (shown in blue) dont appear to be right from my understanding of the comparator function. As a base I used the ground on the comparator so the dc voltages reference that.I have checked the SCR by touching the gate and it does conduct. I dont know how to test the bilateral switch easily and finally there appears to 240v at each of the points on that side of the circuit...suggesting no current is flowing. Anyone with suggestions on how to debug would be gratefully received. Fianlly is there any reason why the circuit cannot be tested without the DC motor connected as I suspect it as the cause of the circuit failure (measuring the resistance between windings seem to vary quite a bit 1.1ohms to 3.6 ohms). Cheers in advance

 

Yes substitute the motor for a bulb, check the supply voltage across the zener diode ZD1 and the output of Q1,

 

(measuring the resistance between windings seem to vary quite a bit 1.1ohms to 3.6 ohms). Cheers in advance

It is a likely a brushed Universal motor so It would be normal to get varying reading depending on armature/brush position, etc.