The unit doesn't have power. I found two diodes (in parallel) on the
power supply board seem got burnt (burn mark on the PCB). Took the
diodes off the board, one is dead (zero resistance). The other one
seems to work. I replaced the dead one with a light emitting diode
(that's the one have handy, ok :-)). Plug the unit in and it has power
to show display on panel and disc tray works. I put a disc in and
closed the tray, after spinning for a few seconds, the other diods (not
the one I put in) over heat and burnt (smoke). The PCB close to the
diodes is a kind of separated from the board, looks like from
overheating.

One other thing. The largest capacitor has some whitish stuff sticks
to it (it is dry though). I wonder if it came from the capacitor, or
it was put there on purpose. The cap of the capacitor looks flat and
good.

I don't have much electronics repair experiences and hope to get some
guidance here. What should I look for? How to test if the capacitor
is good or bad (I only have a multimeter).

Thanks a lot in advance!