Radical is showing as System 11C. The 4N25 optocoupler should be on the interconnect board.

http://www.pinwiki.com/wiki/index.php?title=Williams_System_9_-_11#4N25_Opto_Couplers_in_the_Switch_Matrix

Unless you have a stacked switch on your flipper EOS (not likely since this is a System 11C) it is likely to be the 4N25 optocoupler on the interconnect board. The flipper switches only complete the +50VDC to ground. There is no signal to the CPU that the flipper has energized. Not even the EOS does this. The EOS only interrupts power to the power winding on the flipper coil. The detection is via the 4N25 optocoupler.

If the game is a System 11C (I'm reading the manual as I've never worked on a Radical) the EOS only interrupts current to the power winding on the flipper coil. If the EOS switch is broken you will probably blow a fuse on the auxiliary power board. The only time I've seen the EOS do anything is if there is a stacked switch off the EOS that closes when the EOS opens. I doubt you will see that in a System 11C.

If the optocoupler reads out-of-spec it's almost certainly bad.

There are 3 optocouplers on the board. 2U1, 2U2 and 2U3. It looks like 2U1 is used to detect A/C relay state. 2U2 shows as the left flipper and 2U3 shows as right flipper. This is according to a manual for System 11C. You can verify which is what by looking for continuity between the connectors and the legs of the optocoupler. The manual usually is correct but I've seen errors in them before so you should double check.

Maverick is Data East. There is no interconnect board and no 4N25 optocoupler. The root cause of your problem will be different.

Try with the switch matrix tests and see what works and doesn't.