Blew another damn transformer on my Trane XB80

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On 4/13/2011 3:28 AM, The Daring Dufas wrote:

Ever seen: Salvadore Dali's Chaos and Creation possibly the first example of ... an abstract painting made using a motorcycle, popcorn and Pennsylvania pigs
I believe it had a similar disclaimer.

I have a friend who is working on a haunted house. These things take months to make. He's in charge of the special effects and an ex alter boy. I'll suggest it to him. The house, I believe, is in the Bible belt so it may appeal to the locals.
Jeff

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FYI, for those following this thread (and perhaps thinking I've abandoned it): The only thing I have to report is that I was expecting the replacement transformer to arrive on 04/15 (Friday), but it never did. The last status on the UPS website (at 7:35am on 04/15) was "Mechanical failure occurred.", then 30 minutes later "Arrival Scan". From the UPS website:
"Mechanical Failure: This scan indicates that an exception has occurred due to a mechanical issue, which may cause a delay in delivery. The mechanical failure may have occurred within a UPS facility as a result of equipment breakdown, or while your shipment was transported by a tractor-trailer, airplane, train, or other vehicle."
Perhaps an air traffic controller fell asleep on the job.
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wrote:

More likely this transformer burned up the truck with it.
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On 4/17/2011 9:24 PM, snipped-for-privacy@optonline.net wrote:

LOL!
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Ok, *finally* some news to report!
First, I'd already procured the fuse holders and various fuses, and whilst waiting for the replacement transformer to arrive I decided to run some innocuous experiments on my other *working* A/C unit. Several days ago I installed (as recommended by several) a 1/4-amp fast-acting fuse on the 115V input side of the transformer, and the unit has been running fine ever since. So I know that a 1/4-amp fuse will carry the load on a properly working system; what I didn't know for *sure* was if that fuse was rated too high to protect a transformer in the failing system...
The replacement transformer arrived this evening, so I got to work installing connectors and testing the two circuits for continuity. I installed a 1/4-amp fuse in the 115V primary side and (once again, as recommended) a 1.5-amp fuse (both fast acting) on the 24V secondary side. I also hooked my meter up in series on the secondary side to measure the current draw (I taught myself how to do this earlier using my little Dremel tool; it draws about 1/2 amp when spinning freely). All set, I closed the cover interlock switch to apply 115V to the system... The red LED on the circuit board lights up momentarily, then "piff"; the 1/4 fuse blows. Didn't get any chance to measure the current on the meter.
Some people mentioned possible shorts in the wiring leading to the thermostat, so my next move was to to eliminate that as a possibility. Once again, the wiring diagrams can be found here:
http://www.trane.com/webcache/un/furnaces%20%28furn%29/product/22-1666-07_04012009.pdf
I removed all wires from the thermostat ("YWRGC") connector block on the circuit board. The wiring diagram doesn't show it (at least it's not obvious to me), but the wires that lead to the external A/C unit also connect to this block; I removed them all (after I marked them :-) ). I also disconnected the meter from the secondary circuit just to remove that from the equation. Once again, I closed the cover interlock switch, and once again the 1/4-amp fuse on the primary side opens up.
At this point, I'm 95% sure there's something wrong with the logic board, and in anticipation of this I already had one on order; it's scheduled to arrive tomorrow. I've toyed with the idea of swapping logic boards between the two blower units to see if the problem follows the board (I used this approach to isolate a similar problem on a Trane unit at my previous residence about 10 years ago; it helped, one of the boards was bad), but at this point I'm calling it a night and will pick it up again tomorrow.
Your comments are welcome, and hopefully by tomorrow evening I'll be back in business!
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wrote:

Sounds like it was many of us thought, ie that something was drawing too much current. I assume you didn't have the system turned on, so the theory that it could be caused by a surge on the AC line from the blower motor is gone. So is the possibility of the AC contactor, as you say you had that disconnected too.
Did you look around the controller board and see if there is anything that looks like it's been running hot? Sometimes the AC to DC power section uses parts, like a full wave rectifier, that are easily identifiable and can be checked. If it were something like that it might be possible to repair it using a part from Radio Shack. On the other hand, if the new board is reasonable price, that will most likely solve it.
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On 4/18/2011 9:41 PM, Steve Turner wrote:

http://www.trane.com/webcache/un/furnaces%20%28furn%29/product/22-1666-07_04012009.pdf
Dammit, I missed a step! One thing I forgot to do was verify that I was getting 24V on the output side of the transformer when 115V was applied to the input side. I just came back from testing for that, and now I'm confused again. I removed both wires from the output side and connected my meter to check for voltage, closed the interlock switch, and "piff", the 1/4-amp fuse (my last one) blew! Apart from the fact that the input voltage comes to the transformer via output connectors on the logic board (which are just tied directly to input connectors on the board as far as I can see), the fuse is blowing and the logic board isn't even a part of the equation... What do you make of that?
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wrote:

The first thing I would do is replace the fast blowing fuses with slow blowing ones. I thought all along using fast ones was a bad idea. If its an overload that's causing the problem, the slow-blow will still fail long before the transformer. And if it's some mysterious voltage spike that causing it, which I doubt, even the fast acting fuse may not prevent damage. It's possible the fast acting ones are getting creamed by the initial start-up current.
I'd also consider useing a fuse temporarily thats 1.5 or 2X the current expected. The transformer should be able to handle that for more than the minute it takes to get a current measurement. And if you measure voltage on the secondary, if it's an overload, I'd expect you'd see less than 24V, as the excessive load pulls the voltage down.
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On 4/19/2011 8:47 AM, snipped-for-privacy@optonline.net wrote:

Sure, I can try some different fuses (but I have to go make a run to buy some; that might not happen until later today). But now with the connectors removed from the 24V output side of the transformer, haven't I removed all the load? Wouldn't that mean there is something wrong with the input side of the circuit?
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On Tue, 19 Apr 2011 06:47:55 -0700 (PDT), " snipped-for-privacy@optonline.net"

work on the furnace that works - which is basically identical - so there is NO REASON the same fuse should not work on this one. Half amp is already about 50% oversized for the transformer at full rated output.
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On Apr 19, 8:06pm, snipped-for-privacy@snyder.on.ca wrote:

But he's using a 1/4 amp fuse, not 1/2 amp. He just posted further down in this thread that the new logic board has a 3A 24V fuse on it. In which case, it would seem his 1/4 amp primary fuse is too small. I don;t see the harm in at least temporarily going to a larger fuse so he can get the damn thing going enough to take some measurements.
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On Tue, 19 Apr 2011 17:17:59 -0700 (PDT), " snipped-for-privacy@optonline.net"

Missed th 1/4 amp - yes, I'd go to 1/2, or possibly 3/4.
Thetransformer is 40va, from previous posts. That's 1/3 amp at full load. It SHOULD hold on 1/4 amp with no load, but he SHOULD fuse it at 1/2 amp.
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On Apr 19, 10:31pm, snipped-for-privacy@snyder.on.ca wrote:

And obviously it actually won't hold at 1/4 amp at even ZERO load. That caused most of the confusion near the end of the story, where the transformer blew the fuse repeatedly at the furnace without any load at all attached to it. I thought from the beginning it was a bad idea to use fast-acting fuses close to the limit, at least for testing purposes, to be able to get it running for current measurements.
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On 4/20/2011 6:38 AM, snipped-for-privacy@optonline.net wrote:

Yes you are correct; it did cause some confusion, and a bit of tail chasing on my part. I would like to have tried the slow-blow fuses, but I couldn't *find* any rated anything less than 2 or 3 amps, and I tried two different Home Depots, two different Radio Shacks, and a Walmart.
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On Wed, 20 Apr 2011 04:38:04 -0700 (PDT), " snipped-for-privacy@optonline.net"

disconnected. He still had wires connected - that went to the AC unit, where the wires were shorted.
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On Apr 20, 6:02pm, snipped-for-privacy@snyder.on.ca wrote:

Wrong again. He made it clear he had the transformer installed in the furnace with only the primary side connected and the secondary side open and it still was blowing fuses. That apparently was because he was using the 1/4 amp fast acting fuses. Your statement makes no sense because the transformer supplies 24V to the logic board, which in turn supplies 24V to the shorted AC contactor circuit when it's time to turn the AC condenser unit on.
He clearly stated:
"One thing I forgot to do was verify that I was getting 24V on the output side of the transformer when 115V was applied to the input side. I just came back from testing for that, and now I'm confused again. I removed both wires from the output side and connected my meter to check for voltage, closed the interlock switch, and "piff", the 1/4- amp fuse (my last one) blew! "
You really need to follow the thread more carefully.
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On Thu, 21 Apr 2011 05:17:58 -0700 (PDT), " snipped-for-privacy@optonline.net"

OK. Why did it work on the bench????? He was so vague with what he was doing/checking that NOTHING really made sense.
Main thing is, he found the problem and FIXED it without blowing more smoke.
Half the responses to the problem WERE blowing smoke.
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On Apr 21, 9:35pm, snipped-for-privacy@snyder.on.ca wrote:

Probably for the same reason the 1/4 amp fuse held sometimes on the furnace with just the transformer connected. It was marginal, so sometimes the inital current caused it to blow and other times it just barely held.

I didn't have any problems following what he was saying and telling us he was doing. For example, from the very start he posted the wiring schematic. Taking a look at that it's obvious, for example, that with the logic board removed as he described there was no connection to the shorted AC contactor circuit. Yet there you were, long into the discusssion, claiming that the shorted AC contactor circuit was still involved with the logic board removed.

Yes, like a few of yours.
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On 4/22/2011 9:55 AM, snipped-for-privacy@optonline.net wrote:

trader4, look me up if you ever come to Austin. I'll buy you a beer. :-)
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On Apr 22, 10:19pm, "Stormin Mormon"

I found and provided the circuit schematic for the poster with the heat pump problem in the other thread. What did you provide? And a little premature for bashing about the heat pump thread, isn't it? Last time I checked no one yet has an answer to the fan issue.
Let's look at a sample of your posts in THIS thread:
"It does sound like there is unidentified problem. As the fellows on alt hvac are often heard to say "replace the thermostat".
"Glad you noticed. I hadn't looked at the pictures that closely. I hope your trip charge isn't too much, in this case? "
"As a heating and AC installer and repair tech, I assure you that David's writings sound fancy, but he's not competent to comment. "
"You should contact the manufacturers of furnaces, and tell them that they are doing it wrong. They will thank you. Don't take no for an answer. "
There is plenty more bloviating like that, where you did everything but try to help Steve. Sounds like you're just pissed off because someone other than you HVAC "pros" helped Steve. So get lost, fuckwit.

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