Finding shorts in boards - test-drb@ccmp.vtda.org

1 Dec 2004

On Dec 1 2004, 13:21, Jules Richardson wrote:
...

 I've got a board here with a short between ground and the +5V rail
 (actually not quite dead - I'm getting around 10 ohms between the rails)
...

 Any useful tips for finding the fault? It's a large board, multi-layer,
...
  lots of silicon on it unfortunately :-( 
That's only about 500mA at 5V.  Sure it's a short?  Of course, it may
draw lots more than that under power, if your meter is not using enough
voltage to turn on junctions.

...
  Are there any particular components that are likely to
fail in this way
...
  that might be found across PSU rails? (decoupling
caps, certain ICs,
 crystal modules etc.?) 
Decoupling caps, TTL, and some MOS memory chips all can draw excess
current when faulty.  I usually put them on a hefty 5V supply and feel
for what's hot (you can sometimes burn out the short in a decoupling
cap that way -- they often go bang).  Point of reference: a BBC B board
normally draws about 2A at 5V, but the last one I had with a
particularly nasty set of faults drew over 10A at 5V -- and the faulty
TTL chips got hot enough to feel without actually touching them.

...
  Given a suitably sensitive meter is it sensible to
assume I can try and
...
  home in on the short location a little? (I've
found readings between
 GND/supply on various LS chips of anything between 9.5 and 12 ohms so
 far) 
You really want a current tracer.  HP made a really nice one, and I'm
still annoyed I didn't bid high enough on the last one I saw :-(

-- 
Pete						Peter Turnbull
						Network Manager
						University of York