Area 51 trigger issue repair log

P1 gun trigger wouldn't work. I did some digging and the voltage at pin 2
of the gun connector was only 2.1v on the P2 gun pin 2 was 3.5.

I was able to trace the signal to the 74HC14 right next to the gun
connectors.

Pin Signal

3 Gun 2 trigger in

4 Gun 2 trigger out

11 Gun 1 trigger in

12 Gun 1 trigger out

Grounding the trigger input should pull the in low and then the out should
go high. On the P2 side that worked as expected. On P1 it didn't. I used
a jumper from 5V to momentarily touch pin 12 and verify that the trigger
signal was being processed by the board (in the controls test screen).

I removed the suspect IC (using a SMD hot air station) and then checked the
values at pin 2 on the two gun connectors. Both were at 5v (it was possible
the pull-up resistor was bad, this verified that was not the case).

Installed a 74ac14 in the location and the board now triggers on both guns
as expected.

Note: A second set of A51 boards has a 74LS14 at that location, so it is not
a critical part to be in the HC / LS family.

Probably cause of the issue is bad gun wiring, they become worn and can
short signals.

David

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Received on Sat Jan 26 02:58:47 2013