UPS Turnon Brief Short: Help?

[trberger]

A UPS problem:

When reset (AC line power connected or not) the Battery Power comes on
for 800ms then goes off.

I have checked the ESR on all electrolytics. They're fine.
I have two different batteries and the problem with each
installed is the same. Both batteries are of the correct size
and at full charge.

I have attached two waveform files for the AC output. They should be
a clue. Something briefly shorts the waveform but then stops partway
through each cycle. This problem occurs on both the positive swing
and negative swing of each cycle.

After 800ms the UPS gives up and shuts off.

The UPS is a Powercom KIN-625AP UPS.

Any help?

Thanks
tom
.

[trberger]

Re: UPS Turnon Brief Short: Help?

Proximate cause:

Watching the FET gates shows a good pulse. So the problem is on the output
side. There is a transformer winding I assume is feed back. It goes to a bridge
rectifier. This rectifier is "fused" with a 1/4 watt (that size anyway) resistor.

This 2.2 ohm 1/4 watt resistor used as a fuse was toast (and open too).
I believe it was 2.2 ohms but the bands were pretty burned. It is labeled F1. I replaced
it. The UPS seems to be working now. With a 60 watt bulb my battery stayed up
2 minutes before the "low" battery sounded. At 5 minutes I shut it off and it was still
chugging. The battery is old so I assume this is good.

At rated current the 2.2 ohm could take 1/3 amp. I assume it would toast at
something above 1/2 amp. The ultimate in a slow blow fuse. The rectifiers are
the usual size for 1 amp but I don't recognize the number on them.

But what is the ultimate cause? Why did that "fuse" blow?