AK Model 42 Transformer Ground?
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I'm hoping to find someone who's intimate with Atwater Kent 40-series power supply units--specifically with the power transformer.
I have all of my Model 42 parts ready to reassemble, but I just realized that the power transformer diagram shows a ground connection which I ain't got. I'd like to know for sure that there actually was a ground connection before I start peeling the wrapping from the transformer to look for a broken connection..
[Image: https://lh3.googleusercontent.com/-XbhiG...520206.jpg]
(I posted this question on VideoKarma. Didn't get a solution, but ruled out a few things.)
I have appropriate voltages from the 12 connections which I found, and I determined with an ohmmeter that there is no continuity from the rectifier plates to the iron core or to any of the other connections. The cloth strip in the picture below? I saw a photo somewhere that explained it, but I forgot what is was for and can't find the photo again, but there is no conductor in it.
[Image: https://lh6.googleusercontent.com/-B7m6l...520207.JPG]
Do we have an Atwater Kent guru who can help me with this?
Thanks.
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I can't speak for Atwater Kent Model 42s but in anything with a full wave dual diode tube rectifier, and a transformer, there has to be a center tap on the high voltage winding someplace, if there is not the power supply will NOT work as the center tap is the return path and the most negative point in the B+ circuitry. The only way it can work without a center tap is if the rectifier was wired in as a half wave rectifier, which A.K clearly did not do in this case. So either you haven't located the center tap or the connection has snapped off somehow.
Regards
Arran
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Yes. It used to go up to one of the 2 posts that hold the power supply board in place. Without it you will have no HV return (ground). It must jump into the filter cap section to provide grounds for the caps between the chokes as well.
Use this drawing:
http://www.atwaterkent.info/TechData/Dra...800sch.pdf
The HV connections that go to the rectifier plate will have continuity to it somewhere around 150- 300 ohms, I don't remember right off. It is just another tap on the transformer. So it looks like all of the others. As I recall it is on the same side as the other HV wires. CAUTION, this transformer can easily make 850 volts.
"I just might turn into smoke, but I feel fine"
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(This post was last modified: 11-22-2014, 02:57 AM by Phlogiston.)
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Thanks, guys.
When I first removed the connection panel, after tagging all the wires, I noticed that there was nothing soldered to the two ground lugs on the metal dividers. I could have unknowingly broken the wire then, but I don't recall seeing any evidence of a broken wire--and the lugs still look like they've never had anything soldered to them.
So I'll start peeling layers off the transformer and hope I can see the end of a broken copper wire or foil tab. Resistance across the the winding is 314 ohms, so I'll be looking for ~150 ohms to either side of the plate connections.
If you hear a "Eureka!" emanating from southwest Idaho, you'll know that I found something.
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1. What the previous experts said.
2. Sorry but the winding looks charred. Is it tar or is it really burned? My AK I am working on right at this moment has dark honey colored impregnation but not that ptch blak.
Have you checked the transformer yet?
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Those transformers are potted in black tar. His looks pretty clean, considering that.
It is a good idea to run a no load test before you get into it much (more). Remember the 850 volts though.
"I just might turn into smoke, but I feel fine"
http://www.russoldradios.com/
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The black is tar. I have appropriate resistance across each of the windings and have appropriate voltages with 120V input (using isolation transformer and variac).
The ground wire should have connected to one of these lugs, probably the left one. The underside is perfectly flat, and there's a drop of solder on each lug, but no indication that any wire was ever soldered to it--and certainly there's no hint that a wire was broken off.
In the meantime, I'm probing the perimeter of the outermost winding, which I think is the secondary winding for the rectifier plates, and I'm finding nothing of interest. The cloth strip seems to be between the primary and the aforementioned secondary windings, but I can't find anything of interest there, either.
[Image: https://lh4.googleusercontent.com/-oewZa...520210.jpg]
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Judging by the sch there should be only three lugs from the wining: two ends and the centertap. All three should exhibit finite resistance, probably in hundreds of Ohms, and the middle should be roughly in the middle resistance-wise. So if the two of the Hi Boltage ends are already soldered, the only one left is the centertap and it should have resistance to each end.
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True. I'm looking for a so-far hidden connection which would show ~150 ohms to either end of the secondary, rectifier plate winding. I just can't find it yet. The two high voltage connections emerge from opposite sides of the outermost winding, Of course, due to my level of experience, it could be something very obvious that I can't see or am miscommunicating (which is why I keep posting photos).
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I've never worked with this xfmr so this is just a thought...any chance the center tap is internally connected to the mounting bracket so that it is connected to chassis ground when mounted on the chassis?
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It doesn't have a mounting bracket. It sits snugly in compartment of a metal box. The compartment is lined with cardboard and it's filled with tar. So the ground connection to the winding is either a separate wire grounded to the metal box or connected to the core which would then be grounded to the box.
Question: Roughly what voltage would be expected at the (80) rectifier plates? I'm going to take some time to check the transformer voltages again.
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Referring back to Phlogiston's reply, "Remember the 850 volts," I ran a no-load on the secondary HV windings.
With 110VAC in, I get only 350 volts out. Are my connections actually to one end of the winding and to the center-tap ground? I have a lot of Atwater Kent data, but I haven't found anything about this transformer.
Is it likely that I should have 600 ohms across the HV winding and 700 volts output? If so, then I'm still looking for a missing connection, but to one of the ends of the winding rather than to ground.
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God guess. YEs you should have about double that so you still need one connection.
The resistance will not be exact same as one half of the winding is longer than the other.
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Yes you are missing one end of the HV, UNLESS the winding is shorted. Have you run it at the full 115V for around 15 min? No smoke or sizzle? The outer winding is usually the filament winding for the rectifier or the other tubes.
"I just might turn into smoke, but I feel fine"
http://www.russoldradios.com/
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Russ
If there is a short like this there will be no voltage close to what he is seeing and there will be smoke well before 15 minutes.
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