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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:

Front:

  • brown spots around the feet of one transistor marked K2778/41

brown spots around the feet of one transistor marked K2778/41 Back:

  • 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)

dark brown spots under same transistor (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!

More photos of the PCB in case they help: enter image description here enter image description here enter image description here

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The transistor is a Sanken 2SK2778 MOSFET. The rest of the part number is irrelevant (a date code, most likely). It's not made anymore (like many MOSFETs, technology has moved on) so the substitutes you can find may be NOS or counterfeits made by re-marking newer parts. They probably will work though.

You can verify that the transistor is blown, even in-circuit, by measuring between the 2nd and 3rd pins from the left with a multimeter. If the resistance is similar to shorting the leads together then it's a goner.

This is a relatively high voltage (100V) logic-level part. Most of the newer parts have double or more the gate charge/capacitances so they are harder to drive with the existing circuit but would have less Rds(on) so maybe it's a wash. If you feel like gambling in a different way, the IRLI540GPBF has better specs (other than almost 3x the input capacitances), comes in the overmolded TO220 like the original, and is available from reputable suppliers.

The discoloration is probably just what happens with time and those paper phenolic FR2 boards, I would not worry about it too much. The pads are fragile however, especially when they've been heated, so take care when swapping the part out you don't lift them.

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    \$\begingroup\$ The mouser.com/ProductDetail/onsemi-Fairchild/… seems similar enough but with a little higher RdsON. I'd recommend a heatsink, and check the temperature after using for a while. The device Spehro found is even better, for a dollar more. \$\endgroup\$
    – PStechPaul
    Jan 25 at 22:02
  • \$\begingroup\$ [1/2] Thank very much for your help! The pointers are extremely helpful. Now, like I said, I know very little about electronics, so I need help making sense of the "gambling" you mentioned. It sounds like you're suggesting 2 options: * a "newer part" of the 2SK2778 * the IRLI540GPBF What risks am I taking with each option, in layman's terms? I admit I don't have the electronics experience to evaluate whether "3x the input capacitances" is a bad thing or whether "harder to drive with the existing circuit" is too risky. \$\endgroup\$ Jan 25 at 23:31
  • \$\begingroup\$ [2/2] So what, in practical terms, should I brace myself for with each option? The machine may not run at all? It may catch on fire? I know it's not ideal to post in this SE without a certain level of EE knowledge, but I'm also hoping not to need an EE course to fix my sewing machine. I appreciate any insight you have! \$\endgroup\$ Jan 25 at 23:31
  • \$\begingroup\$ The part might get hot and fail again is probably worst case (assuming the replacement and reassembly is done in a workmanlike way). I would give it 95% chance of complete success either way, but assuming the transistor is the only thing that failed. There is no way to evaluate the effect of differing parameters without a lot of information we don't have available and you are unlikely to be able to provide in useful form. (switching frequency, drive circuit particulars and so on). Having been burned (not literally) by dodgy suppliers, would probably try the IRLI540GPBF myself. \$\endgroup\$ Jan 25 at 23:43
  • \$\begingroup\$ This kind of thing (a hard to find part that people will pay whatever for to get their gadget working again) is the kind of thing that attracts bad behavior. Like the miracle cures sold at the auto shop for cracked engine blocks and other disastrous mechanical faults, people will give it a shot out of hope. \$\endgroup\$ Jan 25 at 23:48

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