Holding the flipper button, the upper right flipper fires only now and then, sometimes stays up for a few seconds while humming loudly. It had been getting weaker first.

Judge Dredd flipper fires sporadically and hums when held: dust in the flipper opto

Cause: Dust in the flipper button opto made the switch flutter.

Checks

• In switch test, hold the flipper button and watch whether the flipper opto flutters.

Fix

• Blow the dust out of the flipper opto and recheck it in switch test.

Systems

• WPC DCS

Games

• Judge Dredd

Related tips

• Indiana Jones to Demolition Man games have two LM339 opto boards

Source: Pinball Repair help group!! (Facebook group)