Replacing the capacitor and triac on the "accelerator pedal" of a sewing machine

Introduction

Error : Overlock machine (Singer) only knows full throttle, no longer switches off.

Fault: Triac BT136 and possibly also one of the gray capacitors - see picture - was defective and was also replaced.

With a multimeter teh capacity was only approx. 1/10 of nominal value. (0.047μF = 47nF dropped to 15 nF).

For the disassembly, see also the instructions for BT136 here under sewing machines.

ATTENTION : Such circuits without a transformer use special capacitors. When replacing, make sure that any "X2" specifications or similar on the spare part also apply. It is about dielectric strength / voltage peaks in the 230V mains).

Step 1 Capacitor and triac

Replacing the capacitor and triac on the "accelerator pedal" of a sewing machine, Capacitor and triac: step 1, image 1 of 1

Disconnect the powerplug!
Open the device. Compare with this wiki. You may have to look for the catches or the screws with which it is locked. On this model, the pedal is only engaged. Pay attention to the moving plate and the spring.
Expose the circuit board so that you can reach the underside to desolder the components.
This is the triac. Desolder its three legs on the underside. These instructions tell you how to check it. Replace it if necessary.
Here are two capacitors. Read their capacitance. Desolder them and measure them with a suitable digital voltmeter. Replace them if necessary.
Look for the same type, usually MKP, the same dimensions, the same voltage and the class, usually X2.
Fuse. Measure its continuity and replace if necessary. Remember that a blown fuse usually has a reason.