Holding the button makes the flipper drop and rise repeatedly.

Pre-Fliptronics: flipper flutters while the button is held

Cause: The normally closed EOS switch is not making good contact (misadjusted, burnt, too little blade tension, or a broken wire), the hold winding has broken away from its lug, dirty or weak cabinet switches, or a badly worn coil stop.

Checks

• Look for a thin hold-winding wire broken off a lug and measure the coil.
• Check EOS blade tension and contacts.

Fix

• File and adjust the EOS switch; adjusting with the button held and power on can be more accurate, but avoid shorting the high-voltage EOS to anything.
• Clean and tension the cabinet switches.
• Replace a worn coil stop.
• Replace the coil if the hold winding is broken.

The pre-Fliptronics EOS switch carries coil voltage.

Systems

• WPC Alpha-Numeric
• WPC Dot Matrix

Related tips

• Pre-Fliptronics flipper coil smokes when held up
• Pre-Fliptronics: a flipper does nothing
• Flipper buzzes and vibrates when held
• Pre-Fliptronics: flipper lane change does not work
• Flipper buttons not seen by the game (pre-Fliptronics)
• Pre-Fliptronics flipper coil wiring
• Pre-Fliptronics: weak flipper
• Pre-Fliptronics games need a driver board with the flipper relay

Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part Two, Clay Harrell (pinrepair.com)