A1304 wont boot. Fan spins, then fan stops, spins again nothing more.

I have two A1304 Airs with the same problem:

Battery is o.K. and fully loaded.

When I try to start, the fan spins up, screen stays black, no sound, no beep. Sleep LED is lit normally. After a few seconds, Fan stops spinning, LED goes of. After one more second, fans spins up again, LED is lit again but no chime, no picture, no startup.

When I disconnect serveral parts as HDD, output bracket, battery or magsafe, it still does the same.

Any suggestions (besides "new mainboard") . May an attempt to "bake" the mainboard (7 Minutes, 200 degrees) in an oven be successful?

The one is a A1304 1,86 GHz, W89361K59A5

The other is a 1,86 Model, too. W89020D25L9