Hi Korlatos, did you ever make an image of the 3.7 firmware on the Control PCB? Or can you share the checksums? My unit also has ver 3.7 and I would like to verify that the firmware is (still) OK. My checksums are 6B33 for U11 and 5CCE for U9. I can replace with the 3.6 firmware that is elsewhere in this thread, but I'm not sure if that is compatible with my unit. Background: My A7550 has no horizontal sweep. I traced it back to the CTC CE line staying high always. This originates from the video processor module, chip U20, which is a decoder chip, driven from the video CPU, U19 (also a Z80). I need to check inputs to U20 tomorrow and trace back some more. I don't have a logic analyser, so it is rather challenging working on a system with two CPU's. I suspect that the Sweep must be activated quite early in bootup sequence, as not even the IFR logo can be displayed without a sweep.