Our hobart HL 600,no display in speed and timer,even there a 24vac,supply in timer controller,only bowl up and down are working

If you have 24 VAC to the timer board and still have no display, then you need a new timer board.

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