Disclaimer: I know very little about electronics, but I'm willing to learn as I go. My experience consists of playing with Arduinos and fixing a lamp by soldering a replacement switch. Please be gentle and explicit in your comments. Thanks!
My sewing machine (Singer Quantum Stylist 9910) runs as soon as the power is switched on, instead of when I press the foot pedal. It does this even with the foot pedal unplugged, so the problem is definitely in the machine. I hypothesized that there's a short in the power supply circuitry. I opened up the power supply and found the circuit board in the photos. Here are the things I noticed:
- brown spots around the feet of one transistor marked K2778/41
- dark brown spots under same transistor (circled)
- light brown spots in the middle under (best I can tell) two resistors marked R5 and R6 (circled)
These observations led me to hypothesize that the transistor shorted out. I read that transistors can be used as part of a power switch, so I think a shorted transistor would cause a closed circuit and manifest itself as a runaway sewing machine.
My questions are:
- is that a reasonable hypothesis?
- how do I find the right replacement transistor? I can only find transistors marked K2778/5ZKS, not K2778/41. I read the Wikipedia section on transistor identification but I don't think any of it applies to the K2778/41 label.
- should I be concerned about the other brown spots on the back on the PCB? There's no other sign of damage on the front, either around the corresponding areas or anywhere else.
- what could have destroyed the transistor? More importantly, how do I prevent it?
- the transistors (both the blown one and an intact one elsewhere) are mounted crooked. Is that a normal manufacturing quirk, or another sign of damage?
- anything else I need to know to fix this machine? It's a very good machine; I'm very motivated to fix it.
Thank very much for your help!