Quoted from Chitownpinball:OK, so switch is open. I can close it by shorting the switch, so that means my matrix is good, right? Could it be the diode or the switch itself? The diode reads open going one way and 5.5 going the other. I think the diode is working properly. Can any one confirm?
Adjust the switch.
But most likely your switch is toast after +25 years of service. This switch near the popper does tremble a lot during it’s life and one day it’s just game over and time to replace it. Adjusting it might solve the issue temporarely.