Weird Williams MPU issue

YellowDog

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Donor 2011, 2013
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I started playing around with a logic comparator (HP) to see if it would help fix an issue with a Williams Rev D MPU board. When it boots up it gives a memory error randomly 1-2-x where x is randomly 1 through 7, mostly 1 but never 8.

Stuck a 74153 chip in the comparator and put it on one of the 74153s on the motherboard. Rebooted and comparator showed no problem, but no memory error either. Picked a different chip. Same thing. Reboot without the comparator, 1-2-x error.

It seems that when I watch the board with the comparator, it won't mess up. When I boot without it, it always messes up.

Anybody ever heard of something like this? Any ideas (other then mounting the board with the comparator, which I would do if I was sure it really would fix the problems)?

ken
 
Any ideas ...?

coupla wild ideas:

1. installation of the logic comparator clamp could be "fixing" a mechanical fault with the on board 74153 or associated solder joint, socket, trace

2. logic comparator is adding some additional circuit loading (should not be significant normally) that maybe pulling up an input or output of the on board 74153
- test 74153 chip select signal drive - try temporary 4.7k-10k pullup resistors at each of 3d, 3e, 4e, 4f pins 2,14 in turn and see if that changes the error number (maybe faulty chip at 6k or 6e)
- test mem bus inputs - try temporary pullup resistors at
-> pins 6 & 10 at each of 3d, 3e, 4e to isolate PAB (3g) as fault source
-> pins 4 & 12 to isolate the vab (5cdef) as source
-> pins 5 & 11 at 3d, 3e, 4e, 4f to isolate the ab (1D 2d) as source
-> pins 3 & 13 at 3d, 3e, 4e, 4f to isolate the vpab (3c) as source

Cheers!
 
Thanks Andy. That is something along the lines that I was heading. I was hoping that one of our electronics gurus would be able to provide a more concrete explaination and a way to fix it. I guess I will just have to try replacing the other 74153 chips and see if that fixes it.

ken
 
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