Marshall Mode Four MF350 crackling at power on

[flashburn]

I bought the 2 yr old MF350 used and it worked great! Then the fans started buzzing.... First I thought it would be an easy fix, just clean the dust off and add some oil (I could always find replacement fans if need be). Well I fixed the fans and put everything back together; screw fans back to heat-sinks and plug them back onto the board. Then I go to turn it back on and it immediately crackles for about 3 seconds then continues to hum. Note the fans are working properly now. The input seems to have no signal to my guitars, also all the lights work. What could be wrong?