I have a PSU that had a D667A NPN transistor that took some damage. I am not 100% sure what its goal is in the circuit, but I have the impression it is used to drive the power transistor mounted on the heatsink you see in the picture.
The psu had a fuse that got blown violently, and there were some shorted components in the PSU (shorted power transistor, shorted electrolytic cap, shorted schottky diode)
The original D667A NPN transistor datasheet mentions a transition frequency of 140Mhz.
In the BD139 NPN transistor datasheet I could not find this frequency, although other sites mention it as being 190Mhz. The other parameters were the closest I had compared to the original one, making me think (perhaps wrongly) that this is a suitable replacement.
Is my thinking correct, or should I replace it with a more suitable transistor. And if so, what would that replacement be ?
Although the fuse is kept intact now, I am not seeing any DC output voltage, so I am wondering if this BD139 might be the cause of that. (it not being able to turn on the power transition properly).