Panasonic PRO II NE 2740 oven F84 error

Hello to All

My friend in restaurant has the Panasonic NE 2740. After a start the Oven switches itself off and it is showin the F84. I have checked and the special fuse 7 A (upper one if You look at the front) is blown off. I have changed it but it is happening again. So i read the manual and there is a few steps to perform:

a) Check line fuses and replace if needed      DONE

b) Check door for proper alignment, wear and tear, and excessive movement    LOOKS GOOD BECAUSE THE OVEN IS STARTING

c) Check door hooks for wear and tear (it may affect the switching sequence)    NOTHING SUSPICIOUS

d) Check latch switch assembly (WHICH EXACTLY IT IS OR WHERE IT IS LOCATED) and make sure that all switches operate properly. Make sure they turn on and off when the door is open and closed.  Confirm there are free of grease and debris. NOTE: Replace all switches even though only one may appear malfunctioning.

e) Troubleshoot relays (on the low voltage board) HOW CAN I CHECK IT, VOLTMETER?

f) Check controller (DPU) board for voltages necessary to actuate relays (WHERE ARE THE MEASURING POINTS TO NOTICE IF IT IS WORKIN CORRECTLY)

g) Check controller (DPU) board for cracks, poor connections, or conductive residue on the component side. Clean, repair, or apply technical bulletin  (NE1018-1757)

h) Replace low voltage board (only if confirmed defective)

i) Replace controller (DPU) board (only if confirmed defective).

I know there is a lot of questions but if there is any chance to repair this oven and not to buy new one, I will try.

Thanks in advance for the help.

Witek