tracing out schematics

From: Jules Richardson <>
Date: Thu Nov 18 06:26:16 2004

Has anyone got any useful tips for tracing out schematics? The SMPSU in
this 'ere NCR Tower is toast, but it's a complex beastie as it's pretty
huge and spread over two large boards.

Is copying down component locations (but in their schematic form) to a
sheet of paper the same sort of size as the PCB a good place to start?
(Which can then be tweaked into a more sensible-looking schematic later)

What about making a list of every component on the board and how many
tracks leave it at each terminal, which can then be used for sanity

I'm not used to drawing out schematics for something this complex, so on
smaller items I normally just muddle through somehow :) At least this
one's only 2 layer...

Aside: the PSU doesn't blow fuses, but is totally dead and doesn't even
attempt to start. I found a dead 5W 10 ohm resistor (open circuit, no
discolouration) in the live feed upstream of the bridge, which seemed
like an odd component failure. I had a spare in the junk pile so
replaced it, and that one immediately went the same way.

The diodes in the bridge check out OK, the main filter caps aren't
shorted, and the large chopper transistors (all 8 of them) seem healthy
enough tested on a meter. Whether the filter caps are on the way out is
another matter - no bulging/leakage though (I've got a homebrew ESR
meter, but it's at the museum at the moment not at home)

Startup for the machine is on the surface pretty complex; the switch on
the front goes into the main backplane (multibus) and from there to who
knows where. Then there are 12 low-power wires going between the
backplane and the PSU (in addition to the main power lines) presumably
for status monitoring / startup signal. Then there's going to be a lot
of charging circuitry in the PSU for the system battery that keeps the
memory alive when the mains power goes out...

Hence the reason having done some obvious checks I really need to trace
out the whole darn lot so I can see how the thing's even supposed to
start up... :)



PERSON 1: That's an awful gash on your forehead! How did you get it?
PERSON 2: I bit myself.
PERSON 1: How the hell did you manage to bite yourself on the forehead?
PERSON 2: I stood on a chair.
Received on Thu Nov 18 2004 - 06:26:16 GMT

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