Still debugging CPU problems…

As I stated in an earlier post, the pinball machine will only work when it is in PROGR mode (DIP switch 4 to ON position on the CPU board).

When the machine is in normal mode (DIP switch 4 to OFF position), al the displays show 0’s and that’s it. Sometimes not all displays will light up at once and it will take up to 30 seconds for all of them to turn on.

Lights on the head and playfield will stay on or off at random (changes every time I restart the machine).

When actuating the Test-Advance button in the door, it will go to test #1 and all the displays will test correctly showing 0’s, 1’s, 2’s, etc… up to 9’s and back to 0’s, but after that, there’s nothing I can do to stop that test, go to the next test or whatever.

Also, when I start the machine on normal mode, the error LED on the soundboard lights. Actually it always lights and never turns off. I noticed it takes a little time to turn off when the machine is turned on in PROGR mode, so I guess it’s the CPU not initialising the sound card.

I’ve tested the IC’s 31, 37, 19 and 8, but they all do what they’re supposed to do…

The PROGR circuit. The PROGR signal comes from DIP switch 4.

What you can see here is that the PROGR signal enables addresses 1C00 to 1C7F (or 1800 to 187F, since AB10 is not used but the software holds it high for RAM access) for writing when it is LOW. When the switch is put back to the OFF position, this memory range becomes read only.

So what’s next? I’m not really sure. There’s a few options:

1. Maybe the TMS40L45-45NP is not a good substitute for the 2114L. I found some people who still have these, so I’ll get me one and see what happens.
2. I write a test program than constantly writes to RAM when the CPU board LED is on and constantly reads from RAM when the CPU board LED is off and follow the R/W signal with my logic probe.
3. Call Mulder and Scully to find the ghost in my machine…

Anyway, as always I’ll keep you updated.