Re: Star Wars CPU D3 short

Can you adjust the resolution of your ohmmeter? Even a change in milliohms would point you in the right direction without damaging the board! Also, might you have access to an analog Ohmmeter? You might be able to eyeball a slight variance in ohms as you get closer to the short.
 
Also, assuming the remaining chips are common 74LS/S type chips, you should clip the legs as opposed to cutting the traces...start with any bus driver ICs first....I always find them toasted on Asteroids boards. If you do cut traces, you should try to take the divide and conquer approach...ind the midway point between all endpoints, cut, and check....repeat for each half!
 
You should see how much fun you can have fixing I, Robot boards where someone plugged the CPU in upside down :)! Same problem....tons of "grounded" data & control lines.
 
Ed

Mark Hooks <markhooks@earthlink.net> wrote:
Matt,
 
Thanks for the suggestions. The short reads 1.1 to 1.2 ohms between D3 and the ground buss. It seems to be the same at most any location. I even tried measuring differences in voltage with the board running but had no success narrowing it down. I also tried the hot chip search and everything is running cool except for the vector PROMs. I've tested them in a good board so I know that they are not the problem.
 
Unless someone has some other ideas I'm going to cut the D3 line on each chip until I track down which one is causing the short. I just hope there isn't some hidden solder short because clipping connections won't find that.
 
Mark
----- Original Message -----
From: Matt J. McCullar
To: vectorlist@vectorlist.org
Sent: Wednesday, June 08, 2005 12:26 AM
Subject: Re: VECTOR: Star Wars CPU D3 short

I'm working on my backup boardset and I've spent hours trying to track down a short to ground on data line 3. Has anyone experienced this before? If so, do you recall what part was causing it? The background is that I've tested all the socketed parts in a known good CPU. They all work fine, but the boardset doesn't pass the buss test using my 9010A. The sound and AVG board work fine too. Any suggestions on how to track this down would be appreciated.
 
How much of a short are we talking about? Ten ohms or less?
Do you have a good set of schematics? If you're sure about which data line is shorted, you can try two things:
 
*) Make a list of all components that connect to that data line. Then remove the socketed chips to eliminate those as possibilities.
 
Now you can concentrate on the chips that remain. With power applied, feel around for one chip that feels somewhat hotter than the others.
 
Or, with the power disconnected, use your ohmmeter and measure the resistance between that data line and ground. You'll know you're getting close when you find one pin just a skosh lower in resistance than the others that are common to that line.

                
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Received on Wed Jun 8 23:11:38 2005