I have the Multigrade 500 system and I had to buy a new controller by RH Designs from SecondHandDarkroom, but I have not seen any items for 600/Multigraphe system. They do have a complete enlarger and head if you have the funds.
When the 500 throws errors or stops working here is my checklist. 1) Change Bulbs. 2) open the power unit and use compressed air to remove dust. I recently had to do the same with the RH controller because a button was sticking. 3) unplug and clean all ports and connections. some of these connectors may be found at a Guitar repair shop if they need replacing. 4) Look for blown fuses, loose wires, and things that look out of place. Sometimes errors are thrown due to fluctuations in electricity. If you live in an area with older mains add a voltage stabilizer.
Beyond that you may have to find someone who works on circuit boards in the music equipment or appliance industry. We had a new board made for an oven that was an odd size rather than remodel a 1960s kitchen.
