30 Nov
2004
30 Nov
'04
11:21 p.m.
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 :-(
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.?)
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)
I've checked the board for particles of anything that might be causing
the problem, plus I'm halfway through ruling out any of the socketed
ICs. Of course if it's a problem with the multi-layer PCB itself... eek!
(do these ever fail in such a way though? It's a commercial board which
used to be fine)
cheers
Jules
1 Dec
1 Dec
12:43 a.m.
>>>> "Jules" == Jules Richardson
<julesrichardsonuk(a)yahoo.co.uk> writes:
Jules> I've got a board here with a short between ground and the +5V
Jules> rail (actually not quite dead - I'm getting around 10 ohms
Jules> between the rails)
Jules> Any useful tips for finding the fault? It's a large board,
Jules> multi-layer, lots of silicon on it unfortunately :-(
10 ohms means about 0.5 amps -- sounds like a perfectly respectable
amount of current for a board with "lots of silicon" to draw.
So... are you sure there is a short, rather than just circuitry?
paul
1:21 a.m.
On Wed, 2004-12-01 at 09:43 -0500, Paul Koning wrote:
Hmm, you could well have a point there. There's around 150 ICs of LS-
logic complexity on the board, then around 30 or so more complex
beasties. Perhaps that figure isn't so unreasonable.
If I can work out which pins on the euroconnectors provide power then
maybe I should hook a known-good 5V supply up to it and see if that's
happy powering things...
cheers
Jules
>>>> "Jules" == Jules Richardson
<julesrichardsonuk(a)yahoo.co.uk> writes:
Jules> I've got a board here with a short between ground and the +5V
Jules> rail (actually not quite dead - I'm getting around 10 ohms
Jules> between the rails)
Jules> Any useful tips for finding the fault? It's a large board,
Jules> multi-layer, lots of silicon on it unfortunately :-(
10 ohms means about 0.5 amps -- sounds like a perfectly respectable
amount of current for a board with "lots of silicon" to draw.
So... are you sure there is a short, rather than just circuitry? 
2:35 a.m.
On Wed, 2004-12-01 at 09:43 -0500, Paul Koning wrote:
If I can work out which pins on the euroconnectors
provide power then
maybe I should hook a known-good 5V supply up to it and see if that's
happy powering things...
Measure from the +5v and 0v pins of one of the 74LS chips?
You could even apply current there, but be careful.
Gordon.
2 Dec
2 Dec
12:37 p.m.
At 03:21 PM 12/1/04 +0000, you wrote:
On Wed, 2004-12-01 at 09:43 -0500, Paul Koning wrote:
> >>>>> "Jules" == Jules Richardson
<julesrichardsonuk(a)yahoo.co.uk> writes:
>
> Jules> I've got a board here with a short between ground and the +5V
> Jules> rail (actually not quite dead - I'm getting around 10 ohms
> Jules> between the rails)
>
> Jules> Any useful tips for finding the fault? It's a large board,
> Jules> multi-layer, lots of silicon on it unfortunately :-(
I'd be willing to bet that the short is in a decoupling cap. Get one of
the HP current tracer probes. You should be able to track down the short in
minutes with it.
Joe
1 Dec
1 Dec
6:32 a.m.
On Wed, 1 Dec 2004, Paul Koning wrote:
10 ohms means about 0.5 amps -- sounds like a
perfectly respectable
amount of current for a board with "lots of silicon" to draw.
So... are you sure there is a short, rather than just circuitry?
I agree. Put the ohmmeter on both ways, see if the "short" acts
like a diode. If so, likely normal load. Might be symmetrical
though.
10:56 a.m.
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 :-(
10 Ohms is not a short! It might even be the right resistance. Have you
tried powering it up?
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.?)
Most times something shorts (particulalrly a decoupling capacitor) it
goes dead short (<< 1 ohm).
-tony
7093
days inactive
7093
days old
6 comments
6 participants
participants (6)
-
ard＠p850ug1.demon.co.uk -
gordonjcp＠gjcp.net -
julesrichardsonuk＠yahoo.co.uk -
pkoning＠equallogic.com -
rigdonj＠cfl.rr.com -
tomj＠wps.com