On the lower playfield under the glass, hitting the center 10,000 point target also fires the center ball kickout, so the game thinks the ball has drained and ends lower playfield play. It can seem intermittent.

Lower playfield center target fires the kickout and ends play: switch wired backwards

Cause: A previous repair had reversed the wires on that switch, so its diode was effectively the wrong way round and the closure showed up on the kickout switch as well.

Checks

• Put the game in switch test and see whether closing the center target also registers the kickout switch.
• Check the game settings for the lower playfield (page 4a of the manual) so normal rules are not mistaken for a fault.
• Compare the wiring and diode direction on the target with a neighbouring switch: the diode band should face the same way on every matrix switch. It should be an ordinary 1N4148-type switch diode, not a zener.

Fix

• Rewire the switch the right way round so the diode is in the correct direction (replace the diode with a 1N4148 if it is wrong or damaged).

Systems

• Bally MPU AS-2518-35

Games

• Elektra

Related tips

• Test a playfield switch diode
• Spare diodes for coils and switches
• Settings are lost at power-off even with fresh batteries: bad battery diode D2
• Bench-testing a -133 MPU: swap the R113 diode for a resistor
• MPU LED locked on with no flicker: likely causes
• Testing the MPU reset circuit at U9 pin 40
• Single reset parts that hold the LED on
• MPU boots only some of the time

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