This article is from the Pinball FAQ, by Andy Oakland sao@REMOVETOREPLY.mit.edu with numerous contributions by others.
If your flippers seem feeble, have a look at the contacts on the buttons
and the coils themselves.
The flipper coils are actually two coils in one. One is the relatively
high-current one to initially fire a flipper, and the other is the lower-
current one for holding a flipper up. The high-current coil is supposed
to cut out at the end of a stroke, leaving the lower-current coil to hold
the flipper up. If the high-current coil isn't firing, the flipper will
move very feebly. Conversely, if the high-power coil is constantly
energized, you're likely to fry the coil or blow a fuse.
How this is done depends on the age of the machine. On older machines,
(Pre-Dr. Dude, 1990) it's done in hardware with a normally closed end-of-
stroke ("EOS") switch which opens at the top of the stroke and puts the
low-power coil in series with the high-power one, reducing the total
current and protecting the high-power coil. If the contacts on this switch
are bad, the high-power coil won't get full power, and the flipper will be
feeble. If the switch opens too soon, the flipper will be deenergized
too early. But if it doesn't open at all, you risk burning out the coil.
Cleaning and adjusting these contacts, as well as the contacts in the
flipper buttons, will fix many flipper problems. See the directions for
contact cleaning under "General Cleaning Tips" below.
Most modern machines use "solid state" flippers, which use software to control
the strength of the flip. The most important difference is the fact that the
end of stroke switches are normally open, and close when the flipper reaches
the end of stroke. When the player presses a flipper button, the flipper
controller board energizes both the high-power (50-volt) and low-power (25
volt) coils. When the flipper closes the EOS switch, the controller board
shuts off the 50 volts, leaving the 25-volt coil to keep the flipper up.
The practical upshot of all this is that the switches, being low current, do
not need as much care. Also, the flipper buttons may be replaced by optical
switches, again reducing the necessary maintance.
A sluggish flip may also be caused by a dirty flipper sleeve. Remove
the sleeve and clean it and the plunger. DO NOT USE LUBRICANTS on
the flipper sleeve; they will pick up gunk and eventually clog things
back up again. Replace the sleeve if it looks really worn.
A melted sleeve should warn you to check the EOS switch and make sure
the high-current coil is cutting out on cue.
You may also have a worn coil stop or plunger, causing the flipper
to pull in too far. And eventually, the end of the plunger will
"mushroom" from hitting the coil stop thousands of times, making the
end fatter and causing friction as it moves through the sleeve.
Best bet here is to replace the plunger.
If you need new contacts, sleeves, plungers, or whole coils, you can order
replacements from the sources listed below.
Flippers in many electromechanical (EM) machines are driven by AC, so there
tends to be some buzzing associated with them. This is normal.