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.

Thanks for the link on how blanking works, I hadn't seen that one. According to that service bulletin, my therory is correct, there needs to be a 2.8ms signal on pin 4 of PIA 11B to stop the 555 from activating blanking. The normal state of "high" is from the output of the 555, I'm looking for the T.G. signal.

Ballypinball - I checked PSPA but they've been out of stock on the 6802 CPU and PIA for months. They also don't have the game ROMS for The Simpsons. The other Aussie pinball parts suppliers don't have much in the way of IC's. Who do you recommend?

UPDATE - I'm getting a bit further with this fault but not 100% fixed yet.

I started to remove the PIAs one at a time and substitute with my new PIA from Marco. When I changed 5F (coil PIA) I finally saw a 3ms signal on the T.G. line (PIA 11B pin 4) and the PIA LED went out after half a second. But the blanking LED still wouldn't turn on. Turns out that the blanking LED was faulty. Replaced the LED and it lights up now.

But the board is still not happy. Intermittantly it fails to boot correctly and the PIA LED stays lit and the blanking LED either stays on or off. It's a bit random. Another (maybe related) issue is when the board boots and I get the correct LED sequence, the coil drive resistors R66-R81 get very hot - seems like they are all switched permanently on. Data on pins 2 & 3 of the 74LS139 (8E) look good, but the CS2 output on pin 5 looks a bit brain dead (see image). I think the 74LS139 is kaput.

Can anyone with an oscilloscope post what the CS2 signal on pin 23 of PIA 5F (or pin 5 of 8E) should look like?

74LS139 pin 5 - CS2 signal for PIA 5F (resized).png