HP CM6030 Color laserjet startup problem

What I diagnosed so far:
- formatter heartbeat led is not blinking 
- when powering on, i hear the ventilator, the scanner moves and the light goes on for a second
- after that I hear the engine starting up, but the control panel display stays dark
- then the startup stops....

I have replaced the formatter board, but no avail
I have pushed the engine test button (on the back) - one time a test page was printed (with vertical lines). This is not working anymore right now.
I removed the formatter and pushed the test page button again, but this doesn't help either,

and now I am stuck...... any suggestions anyone??

Many thanks in advance.