There is a ribbon cable from the CPU to the DMD controller, and then a much smaller ribbon cable from the DMD controller to the actual DMD. Either of them could be a problem. After removing them, check the male header pins very carefully for any corrosion. No idea how it happened on mine (no onboard batteries and no corrosion elsewhere), but I got some header pin corrosion on my DMD controller board, just at the connection for the ribbon cable from DMD controller to DMD. I replaced the header, but still not good, corrosion had invisibly gotten into the cable. Replaced the small cable between the DMD and DMD controller, plus also the longer cable from CPU to DMD controller and everything now works fine.

If you need them, Ed at GPE can sell you the ribbon cables at a very good price. Ask him for 130-214-007 (Ribbon Cable, 14 Conductor, 7", works as DataEast DMD to DMD controller cable) and 130-326-056 (Ribbon Cable, 26 Conductor, 56", works as DataEast CPU to DMD cable). These part numbers don't exactly match (last 2 digits) what you see on Ed's site, but those are the numbers from my order and if you ask Ed he'll make them to these custom lengths for you. Less than $10 plus shipping for the pair. Hope that helps.