Speed control problems

[pointer2null]

Looks like the old cebora has a problem with the speed controller.

I could do with a bit of help with locating the problem.

I've had a nose around with the scope and here is what we got...

Along the top the three traces are as follows:
1st - trigger off
2nd - trigger on min wire speed
3rd - trigger on max wire speed.


Playing about with the speed control it seems the 50 - 100% part is ok, but the 0 - 50% isn't working - i.e. 50% is as low as i can go.

 

[R Kraft]

One of the op-amps/comparators should have a trigger level bias pot you might try adjusting.
Robert

 

[pointer2null]

There are only two pots - one isn't connected with the motor speed section and the other is glued in position. (one more is shown on the schematic but has been replaced with a fixed resistor on the pcb).

oh and P1 is the speed control pot from the front panel.

Also meant to say the top traces are the collector of the PHP drive transistor to the SCR's

 

[pointer2null]

solved - more leaking transistors. Back to leaning to weld.