1980s Holz Her 2420 belt sander - Only works when speed control at maximum

[Babbar]

My fathers belt sander from the 80s which i very much would like to continue using has an issue with the speed control. The machine works fine on maximum speed but will not turn or give any sign of life when on any other setting. The speed control is a dial going from 1-6 although implemented as a potentiometer without distinct steps. I took the machine apart and measured each component individually, all components and measurements have been captioned in the attached pictures. The Triac did not get identified correctly using my cheapo component tester, i don't know if this is user error, if the component tester is failing or if the Triac is broken. Also the thermistor? measures open circuit.

Any help appreciated on what you think and what i should focus on!

[stj]

did you check the pot itself?

[Babbar]

did you check the pot itself?

Yep, it is supposed to be a 220k pot and i measured it going between around 12 Ohm and 114 kOhm so should be good.

Just using the symptoms of the machine i.e. it works fine on maximum speed setting but not at all otherwise. Does that point to a capacitor issue most likely or could it as well be the triac not working correctly?

[R_J]

What you call a thermistor is likely a diac and is ok, it should be connected to the triac gate, it could be that the triac is bad, have you checked the other resistors on that board?
When you say it only works on maximum, is that when the control is turned to maximum? have you removed the pot from the board and checked it out of circuit?
Post a picture of the trace side of that circuit board so we can see the circuit and how components are connected.

[petehall347]

resistors look burned next to 220nf .

[Babbar]

What you call a thermistor is likely a diac and is ok, it should be connected to the triac gate, it could be that the triac is bad, have you checked the other resistors on that board?
When you say it only works on maximum, is that when the control is turned to maximum? have you removed the pot from the board and checked it out of circuit?
Post a picture of the trace side of that circuit board so we can see the circuit and how components are connected.

Exactly, with maximum i mean the speed control turned all the way. The trigger switch is just on/off so speed control is implemented via the pot etc. Attaching some more pics, one of the board traces. I didn't do a very good job checking the other resistors, just confirmed that they were not open or short which they were not. If the "thermistor" is actually something else and behaving as intended then I don't know what to focus on, might just try to buy replacements for all the components but would be nice to narrow it down a little bit.

[Babbar]

resistors look burned next to 220nf .

Checked these for open and short circuit, they passed those tests at least but didn't do a proper job checking the ratings. If you think them being bad might explain the symptoms I'll take the machine apart again and have a proper look/measure!