Leon Borre WPC Shorted Switch Diode

Repair of a switch matrix with shorted diode.

Using the test possibilities of the WPC machines tells us a lot about the switches . At start up the machine give us a test result , wich tells us witch switches do not work properly.

In case of a shorted diode on one of the switches it will signal several faulty switches all in the same row or collum.

Once we are at that point the only solution is to measure with an ohm meter every diode of all these switches to trace the bad one. There is another way to establish wich one is the sorted one.

When we explore the switch matrix in an inversed way normally all signals are blocked by the diodes , if one is no longer a diode but became a jumper ( by the short) the signal stil passes.

How to proceed? Just plug in a small adapter that guides the output signals coming from connector J206 ( male) of the cpu board to J208 ( female) going to the playfield , and the signals now coming back from the playfield at connector ( female) J206 are plugged in to J208( male) at the CPU board.

Here the adapter.

Back side: this side is plugged in to the CPU board

Frontside ; Here the female connectors J206 and J208 are plugged in.

This is the wiring.

The connector placement on an empty CPU

The adapter in place

Users rules.

Once you have at start up a report signalling several switches from the same row or collum being bad, go to the test of the switch edges ( T1) . unplug connectors J206/J208 and J212 on the CPU board.. Plug in the adapter at J206/J208 and plug the connectors J206 and J208 coming from the playfield back in at their places on the adapter board, J212 stays unplugged.

On the display will appear this image:

A blank design of the switch matrix.

Activate the suspected switches and only the one with a shorted diode will show up in the design of the switch matrix.