Ref: 19900008
Title: Symptoms of a Bad Power Supply in a 3Server386
Date: 12/17/90

Copyright 3Com Corporation, 1991.  All rights reserved.

Incorrect power supply outputs may cause a server to act strangely.  For
example, a 3Server386 would not boot.  It always failed at the first
point that a write to the hard disk would occur.  LASTBOOT showed that an
error occurred when trying to write configuration files to disk.  Strangely
enough, a format and the first part of a tape installation always seemed to
proceed normally.  The drive appeared to be read-only when the server was
booted in server mode.

The strange behavior was due to one of the power supply outputs being
-13.5V instead of -12V.  The +5V and +10V outputs were fine.

Checking the power supply voltages is a simple step that can be taken
when troubleshooting a server.  On 3S/400 and 3S/500 servers, all voltages
can be measured on the connector that plugs into the motherboard.  Voltages
should be +5V (red wire), +10V (orange wire), -12V (white and/or yellow
wires), and Ground (black wire).  All readings should be within plus or
minus .05V.

Measure the voltages twice:  once when the power supply is under load
and again when there is no load.  Voltages should be consistent between
load and no load measurements.

