"robb" wrote in message
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"ian field" wrote in message
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wrote in message

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I'm reading this thread with some interest, but I'm more
puzzled as it
goes on, there should be something we are (and the OP is)
missing.

AIUI the OP is trying to find which/how many 5V rail devices on
a heavily
populated logic board are S/C after a short between 36V & 5V
feeds.

A slightly risky strategy would be to use the 5V O/P from an
old (pre-3.3V)
AT PSU, which typically can supply 200A or more via
progressively lower
current limiting resistors, at some point the current will be
high enough to
distinctly heat any S/C components enabling rapid
identification, obvious
pitfalls are S/C Unobtainable Logic Arrays, firmware ROMs the
supplier
refuse to sell as spares, the risk of blowing circuit traces
(unrepairable
in multi-layer boards) and the risk to eyesight from exploding
S/C
capacitors!


Ian has the problem packaged precisely in a nutshell.

but i don't remember the explosion risks better get my safety
glasses.

having said that ian i think i will desolder the rom, ram, and
8031 just in case they are ok and are not replaceable as you
point out.

the board was mfg mid 80's so i am pretty certain it is only two
layer board.

thanks for the nutshell version,
robb

In my experience of situations like this (such as regulation failure in a
SMPSU) its quite common for a small number of devices - sometimes as few as
1, to go S/C and "crowbar" the rail effectively protecting the remainder of
devices. Occasionally a few devices will blow O/C instantaneously after
going S/C, which can lead to further problems after the S/C devices are
replaced - if the Vcc bond wire to the S/C die blows, the I/P & O/P pins cn
the S/C die can load the O/Ps of other chips connected to it, this can be
tedious to trace on bussed logic, its possible to snip the tops of DIL chip
pins to isolate the chip while leaving enough pin on the chip body to solder
the pin back on to (you can always replace the chip later if you want to)
with the pin isolated you can use a pulse injector and scope to find if the
snipped chip or something else on the bus line is still holding it down.