Electromechanical pinballs with weak AC flippers likely have wear or
mechanical problems. If you fix all the problems and the flippers are still
weak and/or noisy you may wish to convert them to DC. DC coils run much
quieter and are stronger. However, some game components (plastics, drop
targets, etc.) will be damaged by coils that are too strong. Be careful
when trying this modification.
Buy a bridge rectifier rated at 50V or more for each coil. If your flipper
circuit runs more than 50V you will need to use a higher rated rectifier.
You will also need a short length (5" or so) of wire. Use the following
diagram to make all your connections. Note that the coil has three
terminals and they are NOT alike. Notice particularly which terminal
has the light (thin) wire connection.