I wouldn't bother with a switch tester just yet -- hopefully it's a broken off wire or diode at one of the switches on that row of the switch matrix. You want to check the wiring and diode at *all* of the switches on this horizontal row, 6 in total (including the ball-roll tilt on the left side of the cabinet!):d90f177284cfb98f87c74057233eea4dd3375371.jpg

Gently tug each wire and each diode leg at each of those switches--hopefully you'll find one loose or broken etc... Looking at the schematic, you'll want to focus on the white/red wire(s) at each of those switches.