I'm trying to repair a DC motor controller that moves a caravan. It turned out that the negative side of the motor control was switched by a pair of IRF3205 MOSFETs on each side (two motors) and all the MOSFETs were damaged. I've replaced them, and one channel works well, but the other channel only works if I use the multimeter to briefly power on the gate line. I read that if a MOSFET is damaged it can damage the drive circuitry for the gate, so I started tracing back from the gates. The board is symmetrical so even though I don't really know what I'm doing, I can compare the working side to the broken side

I've pictured the two sides of the board and overlaid the tracks with the voltages I've measured, with blue for 0v, orange for 0.8v, purple for 5v and red for 12v - key of exact colors in the centre of the image:

[![enter image description here][1]][1]

There's a difference that seems to arise from the transistor with J3Y written on it; would this reasonably be the cause of the problem? I'm a bit stuck for time in getting an exact replacement so I'm wondering if I can raid a PC power supply (of which I have a couple, and appear to contain a few transistors) for something that will work similarly. Here's a pic of what I have (I've a couple of power supplies, this is one):

[![enter image description here][2]][2]

The transistors in the supply aren't surface mount style but I figure I can probably shape the legs so they'll perch.. I just don't know what I've got or how to check it's compatible. In the PSU some are labelled as Qx and others are labelled as Ux, I don't know if this is significant. I'm assuming I'll need to pull all the components and look up the numbers on them to find out more about them (PNP/NPN) etc ,but how do I know if one is a compatible replacement for a J3Y? What figures off the datasheet are relevant?


  [1]: https://i.sstatic.net/iwYN6.jpg
  [2]: https://i.sstatic.net/v614A.jpg