Dead transformer in UPS

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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Radio Shack may still sell the fuses.

Have you tried an ohmmeter test?

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 110C=BA, 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.

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.

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.

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. :)

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