Mitsi old unit problems...stumped

Good day,

Please could you advise and/or send a process i can check for a PUHZ-RP5VHA.

Original complaint U6 error so attending engineer ordered power board and compressor. This was replaced and unit work ok for about 6 hours when apparently RCD tripped.

Compressor meggered ok (500v...more than 20megaohms & no shorts to earth).
No power board so replaced and no leds lit on control board (no fault codes) although there is power at the control board test point. Mains voltage good, not tripping....it appears that there is not enough dc voltage to control board.....changed resistor....still no joy. I do have 240v at the noise and input to power board. I have swapped over the magnetic contactor from another unit but still no joy.

I am running out of parts to change on this 14 year old unit.....could a noise board cause this ?

I will be gratefull for a steer...

Thanks very much.