Star Wars board stopped booting - SOLVED.

jkoolpe

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Hey all!

I always like to post my trials and tribulations when it comes to fixing board problems for my games when they arise in the hopes that they may benefit others in the future.

So here is my latest for one of my Star Wars boards that recently decided to stop booting and was stuck in watchdogging mode. And in this case, the board would still go into diagnosis mode but would show no errors.

So I initially tried some of the obvious stuff, including cleaning/reseating chips, replacing the CPU chip, and also swapping in a known, working AVG board, none of which fixed the problem. But from using the working AVG board, I at least knew that the problem was definitely with the main MPU board.

So I hooked up my o-scope and began the probing. Followed some of the tips from page 4 of the Troubleshooting Guide and found that the clocks were all good and the HALT, NMI, and FIRQ lines were all high as they should be for the MPU chip. Also checked the RESET and IRB lines and they were doing what they were supposed to be doing (again, as outlined on page 4 of this guide). Note that this board originally had the VL multikit on it which I uninstalled for these testing purposes, but that also meant that I was pretty sure that the instruction PROMs were OK since the VL kit replaces these. I even tried two sets of these PROMs anyway on this board, and again, no go. Also picked up an FPGA CatBox from fcawth and have been learning how to use it (figures it would be a good tool to have and good skill to acquire ;) ), and confirmed that the ROMs and RAMs were all good.

I then began to learn how to do some signature analysis via the FPGA CatBox but wasn't finding any issues. Granted that I am still very much in the learning phase for the CatBox, but it has certainly been interesting. I do highly recommend picking one of these up as it is an extremely useful item to have on hand for any board work. Hats off to fcawth for this most excellent product!

But at the same time, I was introduced to another guy who does board repair by a mutual collector/friend, and after describing my problem and what I had done already to try to figure it out, he sent me an email for some chips to test out via my HP logic comparator and my o-scope. I'll try my best to summarize what went down thereafter (and all of the description below is for the Main/MPU board):

First he had me focus on the IRQ circuit via the chips at 2P, 3R, and 2M. When my comparator came back and said that all 3 of these chips were OK, he then had me look at check pin 37 on the CPU chip to make sure it was hi (which it was).

So then we moved to the outputs on chips at 8H, 7P, 6J, 6H, 5R, and 8E, and here's where it gets interesting. I tested all with the comparator again, and all were good except that when I hooked the comparator up to 6H (a 74LS191 chip), the game stopped watchdogging and I started to see activity (via some o-scope probing) on all of the other chips that had previously stopped doing so!

And I also noticed that when I first would power up the board with the comparator clipped to this particular chip, it would initially give some bad outputs on a couple of the chip legs that would then show OK after just a second. I tried this power off/power on cycle several times, and got the same result each time.

So I strongly suspected that this chip might be my bad apple and I replaced it, and that did the trick.

Board is now fully functional again and happy. My friend speculated that the Matrix processor address and data selector control may have been stuck or wasn't being accessed (from Sheet 10B of the schematics). And I've also been told by another KLOVr that he, too, has encountered situations where hooking up a comparator to a given chip has brought a given board back to life, but we're both unsure as to why this happens. Still, it does provide the clue as to what may be wrong with a given board which is obviously useful.

Anyway, I hope that the above makes some sense, and hopefully might be useful to any and all of my fellow KLOVrs sometime in the future. ;)

Cheers,

Jon
 
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