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Old 03-21-2012, 07:44 PM   #15
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Guess I need to study these for a while to see if I can figure something out. That, and trace a few more wires.
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Old 03-21-2012, 09:06 PM   #16
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So Tom, have any idea where I can find one of those 24v transformers? I took the control box out and started tracing wires and I suspect that's where the problem is. "Suspect" because I haven't bench tested it yet. Too late tonight, I'll deal with it tomorrow.

Jim
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Old 03-22-2012, 07:07 AM   #17
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Does the AC run other than the hum?

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Old 03-22-2012, 07:27 AM   #18
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Transformers can be expected to hum when they're supplying a lot of current and the hum may only indicate that some component is drawing too much current. The schematics you've posted don't show the important component very well (the start relay) which makes diagnosis tough, but I'd say that a failed starting capacitor would fit your symptoms perfectly.

You can test the transformer independently by disconnecting the output wires and then measuring the unloaded output voltage. If it is 24VAC (probably higher unloaded) then that's an indication that some other component is drawing too much current and pulling the voltage down. Components which draw too much current get hot BTW and capacitors often make a popping sound when they short out.
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Old 03-22-2012, 02:15 PM   #19
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Quote:
Originally Posted by Jim & Susan View Post
So Tom, have any idea where I can find one of those 24v transformers? ...
Air conditioning parts houses sell 24 volt transformers. Beware, though, that most of these transformers are sold with either 240 or 208 vac primary sides instead of 120 volt like your Airstream needs. Larger parts house sell a universal transformer with multiple primaries - use the wires you need, and tape-off the rest.

You mentioned reading "4 volts DC". Keep in mind that this is all AC voltage.

Your transformer could be bad. But it could be that its output wires are being shorted. Disconnect the transformer's output wires, and reassess. If the hum stops, then the trouble is downstream of the transformer.

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Old 03-22-2012, 05:06 PM   #20
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Well, I finally just took the entire assembly out of the Airstream and took it to the workbench in the garage. The transformer is indeed outputting 25.2v AC with no load on it. Which would be about right if I'm following Crusty's logic. I want to go back and reread his and Tom's posts right quick.

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Old 03-22-2012, 05:18 PM   #21
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Well, I think what I'm going to do is put everything back together and try to start the AC unit.

One thing I do remember is that the transformer always hums when the unit is in use. I keep the thermostat "off" and the temp setting way up to around 90 degrees when it's not in use. Once I turn the thermostat "on", the transformer starts to hum. I'm talking about historical function--it's always done this.

Strange how it just started humming on it's own a few days ago. Maybe that start capacitor simply gave up the ghost and caused all of it. I'll let you know how the test run goes.

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Old 03-22-2012, 05:34 PM   #22
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Us long-time members are here to help

Quote:
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... One thing I do remember is that the transformer always hums when the unit is in use...
The transformer is powered anytime your Airstream is plugged in. When it's powered, there's always the propensity for hum... Unless it is mounted in the finest Airstream ever built - the '67 Overlander.

Keep this thread posted; I'd like to help if I can. I love electrical stuff.

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Old 03-22-2012, 06:57 PM   #23
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Would you believe the dern thing kicked right on? The air is not as cold as I would like it, but that's another item for the check list.
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Old 03-22-2012, 07:16 PM   #24
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If it kicked right on in the shop. You may have a low voltage problem in the trailer. What size is your shore power cord? Do you have a 30 amp dedicated circuit feeding the receptacle for the trailer?
Maybe you had bad connections where the power for the AC connected to the unit.
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Old 03-22-2012, 08:19 PM   #25
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Twinkie, I didn't remove the entire AC unit, just the control box with the transformer and switching devices. I bench tested those and found no issues (that my limited abilities allow).

I have 30 amp run out to the camper. We spent the bucks on that a few years back so that we can use it as a spare bedroom for the frequent visitors we entertain. We have a large family (read: we have lots of kids, that are grown, but still think the 'Stream is cool). Seems somebody extra is always here for dinner. What a great problem to have.

But I digress...it looks like that dang thing is functioning properly. Except for the fact that it needs a little more RJ12. Many, many thanks to all those who helped out. Looks like just a false alarm.

Jim
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Old 03-23-2012, 02:42 PM   #26
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Be sure to unplug and replug all connectors in your 30A cable to wipe the contacts clean of corrosion that may have accumulated.

I suspect that due to the sequence of events, your testing operations moved something which was making poor contact and it started working normally again after the connection was restored.
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