14 Apr
2010
14 Apr
'10
7:45 a.m.
From: snhirsch at gmail.com
---snip---
As you surmised, the problem was on the 12V rail,
which measured 0.5 ohms
to ground. It runs to quite a number of chips, but was surprising easy to
follow around and through the various via holes. I dragged out the dremel
tool and did a binary search by slicing traces. Took the full logN tries,
but on the last cut I nailed a 4.7ufd / 16V tantalum cap with a dead short
:-).
Hi
Ahhhh!
I have a none destructive method of locating such shorts. It requires
a bench power supply and a good DVM.
You set the supply on a low voltage and current limit at about 2 amps.
Place the outputs of the supply from end to far end of the 12V rail
( not through the short ).
A small voltage will drop across the trace from end to end. With
a DVM on a 200 mv scale, one should be able to see this drop.
Place one meter lead on the ground. With the other lead, probe
along the 12V rail.
When the voltage on the DVM goes to 0, you've reached the location
of the short.
To develope enough voltage across the length of the trace, you
may need to increase the current. Check the trace after each
increase to make sure it is not gettin to hot. Power rails can usually
take quite a bit.
If the rail has a number of fingers, first connect the supply along the
main rail and locate the shorted finger. Next move one of the supply
leads to the end of that finger and repeat the probing.
This method can be extended to power planes ( I've done this )
but it is a little hard to describe in text and easier to show.
I have used this metode many times and never failed to locate
the short. It works better with a 5 digit voltmeter but I've used
a 4 digit in a pinch.
Dwight
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