Wlliams WPC-S Phantom Switch Problem

THE COUNT

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My Williams Dirty Harry has a phantom switch issue. When powering on the machine, it gives an error report. It says switch 51 right ramp. What is happening is when switch 51 is triggered, it is registering it as switch 31, the trough jam opto, as going open (it is normally closed). These two swiches are in the same row of the matrix. Switch 31 still functions normally.

I have checked the following...

All diodes on the row, they test good

All diodes in the "square" of switch 51, they test good

Swapped in another switch in place of 51, no change

Checked switch 51's wiring for continuity between the switch and the CPU board connector, it has continuity

Hooked up a switch (with diode) to the CPU's header pins on J207 and J209 for the two wires for switch 51 (with the 2 IDC connectors unplugged). When the switch is activated, it registers for switch 51, not swtich 31.

Hooked up a switch (with diode) to the CPU's header pins for the two wires for switch 51 on J206 and J208 (these connectors are the same as J207 and J209), leaving J207 and J209 connected. When the switch is activated, it registers for both switch 31 and switch 51.

Repeating the previous test, but this time unplugging the opto board on the bottom of the playfield... results in the switch registering for switch 51 as it should. This leads me to believe the opto board is bad.

Anyone know of anything else I should test or try? Or am I correct in my diagnosis of the opto board being faulty?
 
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