The transformer in my Tripp Lite "Internet750U" UPS appears to be dead.
120v goes into it (measured near where the power cord goes into it) but
nothing comes out the other side.

The sci.electronics.repair faq and http://sound.westhost.com/xfmr.htm
both lead me to believe I should check for a thermal fuse under the
insulating wrapping.

The FAQ says
"There may be a thermal fuse buried under the outer layers of the
transformer which may have blown. These can be replaced but locating one
may prove quite a challenge."

There's no visible bulges in the wrapping; do I just cut into it and try
to find the thing? I'm not terribly afraid to destroy it. If I can't
repair it, I'll want to hack the remains into a standalone inverter and
use a standalone charger with it (and maybe a bigger battery -- say, the
one in my RV).

Are there any other diagnostic tests I should try first?

Rick Onanian wrote:
The transformer in my Tripp Lite "Internet750U" UPS appears to be dead.
120v goes into it (measured near where the power cord goes into it) but
nothing comes out the other side.

The sci.electronics.repair faq and http://sound.westhost.com/xfmr.htm
both lead me to believe I should check for a thermal fuse under the
insulating wrapping.

The FAQ says
"There may be a thermal fuse buried under the outer layers of the
transformer which may have blown. These can be replaced but locating one
may prove quite a challenge."

There's no visible bulges in the wrapping; do I just cut into it and try
to find the thing? I'm not terribly afraid to destroy it. If I can't
repair it, I'll want to hack the remains into a standalone inverter and
use a standalone charger with it (and maybe a bigger battery -- say, the
one in my RV).

Are there any other diagnostic tests I should try first?

If its a metal incased transformer you have to take the clam shell
shield off of it before you begin. if its a open type transformer, your
in business. First check the primary and secondary for continuity.
My guess is you will find the primary open. Measure at the leads
directly coming from the transformer. If the leads measure open, look
where the leads go into the transformer body. There will usually be a
plastic top and bottom of some molded variety. You need to cut away
this plastic carefully with a Xacto knife or such and follow the lead
wires down to the core. Look for a bulge in the core where the wires go
down. This will probably be where the thermal fuse is. If you found it,
very carefully pull it out of the transformer core with needle nose
pliers. Measure the thermal fuse for continuity. if its open, you can
either bypass it (not so safe way) or put in a new thermal fuse. If can
be a challenge to get a replacement thermal fuse back into the core.
then you have to replace the cut away plastic ends the best you can
and secure it. If your real lucky, the transformer core was not
damaged and it will work. If it works, check closely for excess heat
or smell.

Bob

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"Rick Onanian" wrote in message
...

There's no visible bulges in the wrapping; do I just cut into it and try
to find the thing?

Radio Shack may still sell the fuses.

Are there any other diagnostic tests I should try first?

Have you tried an ohmmeter test?

Bob Urz wrote:
in business. First check the primary and secondary for continuity.
My guess is you will find the primary open. Measure at the leads

My diagnosis was wrong, and even some of my observations were wrong. I
thought AC from the wall went straight in to the transformer, but having
taken it completely out now, I see that they merely are routed very
close to it. I thought I had better eyesight than that.

Primary has two wires (black and white). They have continuity.

Secondary has three wires, each of which come out in bare-looking (but
actually covered in thin invisible insulation) twin heavy solid copper
wires. All have continuity to eachother.

One (red) is definitely +12v (goes directly to the battery and to the
circuit board). Two heavy wires (black, white) go to big heatsinks on
the circuit board, to which MOSFETs are screwed.

It is at this point that I can tell you with absolute certainty that I
am out of my league. Monetarily, I have nothing to lose, although I
suppose there could be some physical danger here for me. I'd plow on and
learn something anyway, but having just spent a half hour following
circuits around, I have no idea how to do anything more.

Drat...this won't even get hacked into an inverter. The battery seems
ok, though, so maybe I'll use the battery for something and give up on
the rest. I think with the battery and a tiny float charger I've got, I
could build a DC UPS for my cable modem and router and skip AC altogether...

Still searching for a faulty component in the power circuit, and
nothing seems to be wrong?
Search for a (very) little transformer on the printed circuit board (or
nearby) and for a fuse attached to it. Should one of these two fails,
the whole thing stops working, because this sub-circuit is user for
detecting when the mains power is on. Probably, this transformer has a
built-in thermal fuse that trips when it reaches 110Cº, so check
resistance (its about 50 Ohm for 230 volt mains). The other part to
check is a fuse in series with this transformer, but maybe you have a
hard time trying to identify it, because its encapsulate is sometimes
very similar to a diode bridge, and others seems to be a resistor in a
yellow or green body, but with very strange or no markings at all.
Manufacturers tend to be very creative when choosing this fuse, and its
my believe they do it on purpose.

Hope you can "bring it back to life".
Greetings
Louis.

wrote:
Still searching for a faulty component in the power circuit, and
nothing seems to be wrong?

Well, stuff certainly seems to be wrong, but no component appears dead.
There is some really shoddy soldering on this board, and I did a little
resoldering, but most of it looks functional now.

Search for a (very) little transformer on the printed circuit board (or
nearby) and for a fuse attached to it. Should one of these two fails,

How little, and what kind of package (if any)? There is definitely
nothing that looks like a transformer on the board. There two black
plastic boxes but I googled their markings and they're relays. I ought
to test them.

resistance (its about 50 Ohm for 230 volt mains). The other part to
check is a fuse in series with this transformer, but maybe you have a
hard time trying to identify it, because its encapsulate is sometimes
very similar to a diode bridge, and others seems to be a resistor in a
yellow or green body, but with very strange or no markings at all.

There are a few green resistor-looking things that have common resistor
markings on them. There are diode-looking things that are also marked
like resistors. It's been a long time since I've paid this much
attention to electronic components.

Hope you can "bring it back to life".

I appreciate all the help. This newsgroup is friendlier than I expected.