"Jeff Wiseman" bravely wrote to "All" (28 Aug 04 17:52:25)
--- on the heady topic of "APC Back-UPS 280B faulty - looking for ideas"

JW From: Jeff Wiseman

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

JW Symptoms are as follows:

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

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

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

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


I recall that most multimeters are calibrated with pure sinewaves and
their internal precision rectifiers measure the average value of the
waveform. Thus depending on the shape of the BUPS's output waveform the
reading may be inaccurate. The meter may indicate a correct reading
from the normal AC line but then read low when the BUPS is running.
I think an AC RMS meter would be needed for a proper calibration.


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

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

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

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

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

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

JW - Jeff

I would try loading the BUPS to a significant percentage of its
ratings and see how the output voltage is affected. If there isn't
much change, say less than 10%, then I would suspect the reference
voltage or the feedback divider chain. Resistors do go open circuit,
capacitors get leaky, solder joints and connectors can go bad, etc.

A*s*i*m*o*v

.... Transformer designers take turns doing it.