Monster Bash right ramp flipper chattering and not holding ball

Flipper chatter (rapid buzzing) on a WPC game like Monster Bash is almost always caused by a misadjusted or failed EOS (end-of-stroke) switch on that flipper. Here's what's happening: when the flipper fires, the coil uses its high-power winding to snap the flipper up. When the flipper reaches the top of its stroke, the EOS switch is supposed to open and switch the coil to its low-power hold winding. If the EOS switch never opens (or opens and immediately closes), the coil's power winding kicks in repeatedly, causing the chatter.

Since you had the playfield up recently, there's a good chance the EOS switch got bumped out of adjustment. With the game off, look at the small leaf switch mounted near the upper flipper mechanism on the underside of the playfield. When the flipper is in the down position, the EOS switch should be closed (leaves touching). When you manually push the flipper to the up position, the switch should open with a clear gap. If the gap is too small or the leaves are bent, the switch may not open reliably. Carefully bend the switch leaf to restore the proper gap — about 1/16" when the flipper is fully up. If the switch contacts are pitted or burnt, replace the whole EOS switch — available at Marco Specialties or Pinball Life.

Also check the flipper coil's solder connections on the small board or direct wiring under the playfield — the upper flipper coils on Monster Bash can develop cold solder joints from playfield vibration. If the chatter has been going on for any length of time, inspect the coil for overheating damage (discoloration on the coil wrapper or a burnt smell). A coil that's been chattering may have weakened and should be replaced preventively even if it looks okay externally.