We have no power out of the hl600 motor control board. whats wrong?

There are at least 3 boards on the HL600, the panel control board where the timer and start/stop switch are located, the interface board a little one on top of the output board, this one take the voltage input from the plug, and has the output to the contactor that energize the motor, in order for this last board send the power to contactor must to receive and input voltage from the control panel board trough out the interface board DX+ &DX- terminals if there is voltage present that should be 24 vac or 12 dc and still no output then check the capacitors board, is next to the output terminals, make sure you discharge those capacitors before touching it.

Because of the very high voltage on the output of the Inverter drive, I do not want to be responsible for your injury or death from you servicing this machine without proper training.

Looks like you are familiar with all those Hobart mixers. Hope you can help me too.
So I have a HL600. Got it from a school. Was working prior to remove from the school facility.
A month later plugged it in. Worked for like 20 min and stopped.
Bowl still goes up and down but there is no lights on the display.
And mixer wont start at all. Called Hobart. They sent out a tech. He checked the mixer and said all is good with it just the control panel board is bad. Ok So I got another board. Same thing. Mixer worked like 20 min and stopped. All the same sypthoms. Now since the tech said its the board thought OK. Let's put another board on it. And same thing happened again. Mixer stopped, lights disapeared.
Now we have 3 mixers and 3 bad boards I guess.
But I don't think the board is the problem anymore. What's the chance that working machine when I take out the board and put it on another unit it won't work.
The problem must be something else.
Please let me know if you have any ideas how to get them back working.

Much appreciated.
Regards Stoyan