PFAFF 1475 CD sewing machine - motor problem

[MrWifiHifi]

Hello

This sewing machine came to me for repair. It has exactly this problem.

When I step on the pedal, the motor starts up nicely, but as I press more on the pedal, just before the end, the motor starts to full speed and stops. I have the machine disassembled at the moment, on the main board I found transistors that provide control pulses. After measuring with an oscilloscope, I came to the conclusion that the microcontroller turns off these control pulses for the motor. So I don't know if some sensor is to blame or if the microcontroller is faulty. I don't have a schematic of the main board for this machine.

Has anyone encountered a similar problem, or how to proceed further? Or does anyone have schematic for that main board?

[Qw3rtzuiop]

I had some troubles with that machine too. But i dont know if the fault was identical. However you can measure the resistance of the pedal at the connector. Its just a variable resistor. I my case the conductive layer was depleted. There are spare parts available:
https://www.naehzentrum-braunschweig.de/Zubehoer-nach-Modell/Pfaff/Pfaff-1473/Pfaff-Widerstandsbahn-Anlasser::17605.html
https://www.naehzentrum-braunschweig.de/Zubehoer-nach-Modell/Pfaff/Pfaff-1473/Kontaktfeder-Anlasser-Pfaff-ATK-0001-ATK-0033-ATK-0080-ATK-0070::17635.html

[MrWifiHifi]

I measured pedals resistance and it is OK, also measured voltage on pedal when machine is powered up. I suspect there is something wrong in analog part of circuit (there is 1x LM358 and 2x LM339), or worse, the 8051 on board is loosing bits in program memory. Or it might be some dirty sensor or something.

[Qw3rtzuiop]

But did you measure over the whole range of the pedal? If the machine receives one short full speed impulse it does one stitch at that high speed.

[MrWifiHifi]

Yes, I measured it over the whole range of pedal. When pedal is not pressed, the output is open. If I start to push on a pedal, the wiper comes on resistive track. From minimum (around <1kohm) to maximum (around 20kohm). When pedal is fully pressed, the output is shorted.

I measured pedal and also voltage on the pedal when machine was turned on. On pedal there is some kind of ramp signal which is changing amplitude with pedal position. When motor turns of, there is still signal on pedal.

As I said earlier, I measured ot with scope, the microprocessor which is generating pulses for triacs is turning off those pulses, but I don't know why.