In article ,
"robb" wrote:

i have a micro-controller board with 3 big 40DIP ICs, 10 or so 16
DIP support ICs. with 32v/8v/5v transformer supply that connects
to two bridge rectifiers {32v, 8v}a fuse links sit between
transformer and rectifiers, no schematic.

i believe the 32v shorted with 5V line and it blew one fuse
link. the fuse link was replaced with same and when powered all
three fuses blown.

So....where does one start to diagnose such a *big* problem ?


.
Only thing working is transformer outputs and it seems like i get
continuity hits everywhere i check
the board was built about 1985 (no SMT) it is about 14" x 5" has
3 x 40 DIP ICs and 10 or so 16 DIP support ICs, handful of
transistors many resistors,diodes etc,

ideas and help would be greatly appreciated
robb

Clearly there is one or more shorts somewhere on the board. Your
mission therefore is to determine what is shorted, and where it is.

If the ICs are socketed, I'd probably pull them (making careful note of
what was where, if it's not clearly labeled on the PCB). This avoids
any (additional) damage to parts that may be quite hard to replace.
From there, checking components in the on-board power supply section
might be reasonable, as would checking the resistance between
permutations of the various power supply rails and ground for shorts
(preferably with something like an ESR meter that won't damage
semiconductors, especially if the ICs can't be removed.) Shorts between
rails could be caused by things like failed de-spiking capacitors. If
everything looks OK with ICs removed, maybe power up the board without
them and see what happens; if the power supply is stable and current
draw reasonable, the problem would seem to be with one of the removed
ICs, or with a circuit driven by one of them.

(It's not a bad idea to figure out what the output interface circuitry
looks like that's driven by the ICs before powering it up without them;
maybe you'd be wise to tie some of the outputs to a fixed level rather
than letting them float.)

What's the purpose of this board? What sort of board-level I/Os does it
have? Based on the atypical power supply voltages, I'm guessing it must
drive or interface with some external devices; maybe those output
circuits are damaged, or the things driven by them or the wiring to them.

Just some thoughts; hopefully they're helpful.

--
Andrew Erickson

"He is no fool who gives what he cannot keep to gain that which he cannot
lose." -- Jim Elliot