OP

Quoted from TomReed:

I'm trying to repair my neighbor's Simpsons pin and really need some help with a stubborn MPU board. It had suffered fairly extensive battery leakage damage. I removed and replaced a lot of components, IC sockets and neutralized the board and re-applied solder mask to exposed tracks, replaced battery pack with CR2032 lithium battery and changed D25 to a 1N5818 Schottky diode to minimize voltage drop for the 3V battery. Re-installed original CPU, game ROMs and VRAM and everything fired up fine. I returned the game and neighbor was happy for a couple of days until the game crashed and refused to boot again.
I have the MCU board on my workbench connected to a variable power supply set for 5V - the board is drawing 1A. The PIA and 5V LEDs are stuck on, Blanking LED never lights. I read many posts here that were of great help diagnosing what could be the problem. I don't have Leon's test ROM, but I've read his web post many times going through his trouble shooting guide - extremely valuable! I have an oscilloscope and have checked all signal paths. Here's the results....
+5v - ok
CPU
pin 2, 4, 6, 40 high - ok
Clock signal pin 39 -ok
VMA signal pin 5 - ok
E signal pin 37 - ok
Address signals pins 9-25 (not 21) - ok
Data signals pins 26-33 - ok
Data buffer 5E pins 2-9 -ok
Address buffers 6C and 6D pins 3,5,7,9,12,14,16,18 - ok
CHIP SELECT
PIA 5F, 7B, 8H, 9B, 11B, 11D, pin 23 - ok
5B & 5C pin 20 -ok
There is a solid high on Pin 4 (blanking) of 11B PIA. I thought this chip might be the culprit so I ordered a replacement from Marco and at the same time bought new ROMs, 6802 CPU, and 6264LP RAM (I live in Australia and cannot get these parts here, so I just bought what I thought may have been affected by the corrosion and waited 2 months for delivery). I installed all the new chips and the the fault is still there - no change! I lifted pin 4 of 11B PIA (as it is now socketed) thinking maybe it was being dragged down by another component but it remains high.
I buzzed out all bus lines thinking that the corrosion may have eaten a track, but all checks out ok.
I'm out of ideas... any help would be very much appreciated.