I have a APC Back-UPS 280B that has stopped working correctly.
I've had experience in troubleshooting basic electronics and
since the layout of this thing is primarly discreet components
(only one small IC if I remember correctly), I believe that I
might be able to fix it. However, I can't find anything at all
about it on APC's website or anywhere else (schematics, user
guides, nothing).

Symptoms are as follows:

In testing it using a 40 watt light as a load, the output voltage
across the light is normally about 125 VAC. When the plug is
pulled, that voltage drops to about 85 VAC.This was how it was
behaving when the problem was first discovered (power outages
caused computre to go down), so the internal settings were all as
they came from the factory.

Initially suspecting the battery, I checked the voltage on the
battery in the normal running (i.e., charging) mode. Voltage on
the battery was somewhere around 13+ VDC and when the plug was
pulled dropped only slightly to somewhere around 12 and 1/2 VDC
and held steady. I interpreted this as the battery and the
charging circuitry of the 280B were basically good.

The setting on the potentiometer for the output voltage of the
inverter section was physically set originally at about 2/3 full
scale. With a 40watt load and the unit unplugged (output voltage
at outlet around 85 VAC), I turned the pot up to full scale. This
resulted in the output climbing to around 119 VAC.

Although I can (just) get the output up to a "working" level, I
would not trust it, it's stability, or load handling since
something has obviously gone wrong with it. I do not own or have
ready access to a 'scope so I could not examine the output
waveform or frequency (I own a Fluke 27 Multimeter)

It would be nice to have this working again as I do have an
application for it, however:

1) Paying someone to fix it would cost as much as buying another one

2) I can't justify buying another one as I don't have the money
for it

3) If it is only a component or cold solder joint that has gone
bad, it should be very cheap for me to fix, however:

4) I have no schematics or info on this to help locate where the
problem is so I'm a little at a loss, not being familiar with
which levels and where to check.

If I can't fix this I'm pulling the battery and trashing the UPS
which would be shame since I'm sure it is a simple fix once the
problem is located. Any assistance, ideas, or even places I could
look to find literature on this UPS would be appreciated. Thanks!

- Jeff