umm correct me if I'm wrong but doesn't that mean that the red gun(monitors=RPB...three collerd guns make the colors) on the monitor is out? I think that's a tube issue and you need a replacement to fix it.(then again I'm an arcade repair rookie)
Could be lots of easy stuff, cold solder on the R line on the chassis, broken wire to the neckboard, bad trimmer pot, bad R transistor. If it's the gun a rejuvenator would probably bring it back.
Game looks damn clean for $195, I wouldn't hesitate to grab it.