Bad news : after replacing the burned transistor, the machine doesn't works back :/ the fans and leds turns on for few seconds, then the power cut off (I assume it is a security cut off by the power supply unit). When I turn on with the screen removed, the machine starts normally (at least it seems to do things and after few seconds I got a bip). I checked my transistor fix, it seems correct, contacts are good, no short circuit afaik. But when I try to start the machine with only an external screen, the screen doesn't turns on (but the machine seems to start normally, with the bip after few seconds). Maybe I could try with the external screen adding an ISA graphic card, but that will be for later ! Any advice you could have would be welcome.