TECH: Williams System 3/4 and Driver Board issue

Archived from groups: rec.games.pinball (More info?)

Couple of questions regarding the repair of a Williams System 3
(upgraded to a System 4)

So far, I have replaced all Scanbe's, bunch of the switching ICs,
upgraded the reset section, PIA was intermittantly bad, and jumpered
the board to use 2716s.

The board works great with Leon's Test EPROM. And the memory test
passes.

Attach the Driver board, and with Leon's Test EPROM, still works great.
(I haven't tested all the PIAs at output yet, but PIAs at pins 2-17
test fine)

With game EPROMs, I am getting an intermittant blanking signal on
Interconnect Pin 37. Checked the MPU Blanking Circuit, and it's also
intermittant. The Diagnostic test is lighting the bottom LED (PROM
Error), so burned new EPROMs and same thing. All sockets test fine,
both solder side and component side.

This leads to my first question: Does a correctly booted System 4 with
Game EPROMs flash the LEDs once, and click the relay?

And then the natural second question: What is a good method to check
out the blanking circuit to pin down whether the issue is on the Driver
board or the MPU?

Thanks!