Brad:
Did it turn out to be a problem with the mainboard?
I had this same problem on a RS-640 last week and after 25 minutes of troubleshooting, I determined the board had bad leg on the heat sink (which controls signals to the heads) New mainboard fixed the problem. But I have also seen the transistors be more of a factor from time to time.
Tell us what you did to finally resolve this issue.
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I am editing this post, because your printer has a seperate head driver board, and the printer I am referring to, the mainboard has the head board control integrated. but nevertheless these are places I would be looking. The problem your having is definitely electrical and if you have changed all the parts you have listed, then the boards are really the next option. The flat cables that connect to the print carriage board from the head board rarely ever go bad, but anything is possible. Print carriage boards are simply junction boards, I have replaced one in 10 years of my service experience, and that was because the building was struck by lightening and it burned the head cable and the connection on the print carriage board...very rare to be this
Be sure to inspect the cables for rubs or burn spots. I am also hoping that when you are moving cables around that the machine is off and unpluged. Please dont unplug any cables with the power still connected to the machine. You will cause problems for yourself that will get very costly real quick.
I am sure you are unplugging the machine, but I just want to make sure I say that, because you will be inspecting the cable ends which requires unpluggin them from the boards.
Be aware that a loose or partial connection on any of your boards can make these printers do some weird stuff. I would be inspecting all connections on all boards just to be safe. Sometimes the contacts can oxidize and cause issues, just like a car battery that has corossion on the terminals. I use a brand new pencil eraser to clean these contacts.