We are working on a Bally Fireball 1972 and have an issue with the zipper flipper. It closes, but won't stay closed unless we manually lift the blue mushroom bumper for a second or so.

The blue mushroom bumper switch activates the close flipper relay, and that relay has a switch that will keep itself energized until the close flipper solenoid has finished moving its connected mechanical parts over the locking mechanism. The EOS switches for both the relay and the solenoid then opens and breaks the circuits. The flippers should now stay in closed position until the lock release mechanism (which works fine) is activated.
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On the same stack of switches there are two other, normally open switches for the open flipper relay and coil, but these two closes when the flippers are in closed position. The open flipper relay can then be activated from 4 separate switches including the open flipper relay itself. A malfunction in one of these relays or switches could be causing this type of issue because the coil operating the unlocking mechanism would be activated simultaneously with the closing solenoid and thus never locking the flippers in closed position, but they all seem to work fine when inspected.
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We tried adjusting some switches but the problem continues. Do I read the schematics right, or have I missed something? Could it be a pure mechanical issue?

Thanks for the replies!

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We adjusted the EOS switches as much as possible (even more than in the pictures), but still it's not enough, so I will check the circuits once again to see if the close flipper relay hold-in switch is broken:

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If so, the main circuit will still work when I lift and hold the blue mushroom bumper manually. It will override the relay EOS but not the solenoid EOS, and the bell rings. The bell solenoid keeps buzzing as long as I hold the blue MB up, which also makes sense. But I can't find any relay named "close flipper"! I just saw one reacting when lifting the blue MB, so I assumed that was the correct one. Is it one of those under the playfield, or in the bottom of the cabinet? (I'm familiar with EM circuitry, but new to pinball machines...)

(1) Yes, but it slips back before the lock catches on (2) The lock catches on only when I lift the blue MB and hold it for about a second. This may be caused by a broken relay hold switch, as the short pulse from the MB switch when touched by a ball isn't long enough to allow the solenoid to move all the way along before the metal part locks on to the nylon ramp, but holding the MB up for a second overrides the possibly broken switch so that the solenoid have enough time to move all the way. So I think it's an electrical problem, but not the EOS switches. I need to locate the "close flipper" relay to check all its switches and connections. My multimeter doesn't react fast enough to register very short pulses, so I may have to use an oscilloscope.

WE GOT IT WORKING!

I located the close flipper relay, which is (mis)labeled as "M-B C.F.". Also, it's located on the left side of the cabinet, while the open flipper relay, labeled with its schematic name, is located on the right side.

1. The contacts of the relay holding switch was cleaned, adjusted and measured. To get correct readings we had to hold in the #2 roll over lite relay, which controls a NC switch.
2. The spring on the mechanical locking mechanism was adjusted so it applies a little more force to the lock.
3. The EOS switches were adjusted again, so that they cut the current as late as possible.

After this, the zipper flipper has only malfunctioned once during test play.