System 80 blowing pop bumper fuse

Trackball

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2 out of the 3 pop bumpers work on this pin. I was convinced it was a bad coil after I grounded the diode on the non banded side and it did not fire. The fuse looked good for that pop bumper but tested bad. I replaced the fuse and it blew again.

pinrepair.com says:

If the Pop Bumper Driver Board's fuse blows every time a game is started (and the pop bumper coil is 2.5 ohms or greater and the 1N4004 coil diode is new), suspect the two chips on the pop bumper driver board. The 74LS121 and/or the 74LS16 can have an internal short which will cause the pop bumper's 2amp slo-blo fuse to fail. This can be tested by using a DMM set to the diode function. Put the black lead on the chip's +5 volt power leg, and test each other leg with the red DMM lead. A reading of .4 to .6 should be seen for each chip leg (except for the chip ground leg).

Looking for some alternative possibilities before I put on my pinball surgical gown and go in.
 
Mine was doing the same thing...I just did the board mod where you remove a diode and replace with a jumper wire.It's on pinrepair...then they worked perfectly !!!
 
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