its a hantarex MTC 9300 (27")
yes it worked on that monitor perfectly, but not anymore
ok that's good. the first step is to rule out if the problem is with the monitor or the game Board.
If you haven't already I would see if you can get any other PCBs to run normally on that monitor. you said MK1 had an image but was really bad. do you have other JAMMA PCBs that you can test that DO display correctly? if every board is problematic then the issue is the monitor, if other boards display correctly then the issue is the game board.
it also helps to check/rule out simple stuff too...
-are you getting neck glow on your monitor?
-is the edge connector on your game board clean?
-are the contacts on the JAMMA connector clean and making good contact?
-is the video connector securely plugged into the monitor board?
-is the remote board for the monitor plugged in securely?
-is there any damage on the remote board harness?
-are the video wires free from damage?
-are your voltages adjusted correctly