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Old 16th Oct 2021, 4:25 pm   #1
Rishtonite
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Question AJS choke - laminations ?

Long term project, restoring a 1920s wireless. (See pic)
Uses 3 x DER and 1 x DE6 valve.
The AJS choke (see scan) and grid leak resistor were open circuit.
I've repaired the resistor (2Mohm) and rewound the coil (11,000 turns of 42 swg, 2400 ohms dc)
Unfortunately when dismantling the choke (years ago ) I didn't make a note of how the laminations were arranged.
As it will be carring DC I've read that there should be an air gap.
If I assemble as per photo there will be NO air gap.
Would appreciate some help from experts on here.
Thanks
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Old 16th Oct 2021, 10:30 pm   #2
joebog1
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Default Re: AJS choke - laminations ?

Arrange ALL the laminations one way, so that they sit exactly on top of each other. If you are really worried you can actually stack all the laminations on top of each other, then place a layer or two of yellow Mylar tape in the gap area. Careful trimming with a razor blade will get rid of any surplus.
Obviously you will have to take each half stack and load them into the bobbin at once. The last few will have to go in one by one to repack the bobbin. The small magnetic short that this causes should not be a problem.

The Mylar tape causes the airgap, and it's OK if all the lams don't match exactly.

Thats how I would do it anyway.

I hope that helps

Joe
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Old 18th Oct 2021, 10:55 am   #3
Rishtonite
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Default Re: AJS choke - laminations ?

Quote:
Originally Posted by joebog1 View Post
Arrange ALL the laminations one way, so that they sit exactly on top of each other.
Many thanks for the advice. I took it to bits many years ago and other projects started to take priority. I'm now more careful and keep notes, diagrams and photos.

I have some Kapton tape (used on the bobbin rewind) which I can use to
a) hold the stacks together and b) insulate the gap where the two stacks are closest.

I know "normal" laminations are often impregnated with varnish to reduce mechanical vibrations, but I'm guessing that's not needed with DC flowing through the coil.

Barry
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Old 18th Oct 2021, 10:34 pm   #4
joebog1
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Default Re: AJS choke - laminations ?

It can still rattle if the laminations are loose. tighten the bolts properly.
Capton tape will be fine.

Joe
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Old 20th Oct 2021, 10:48 am   #5
Herald1360
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Default Re: AJS choke - laminations ?

Isn't the airgap provided by the dogleg in the centre limb?
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Old 21st Oct 2021, 10:47 am   #6
Rishtonite
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Default Re: AJS choke - laminations ?

Quote:
Originally Posted by joebog1 View Post
It can still rattle if the laminations are loose. tighten the bolts properly.
Capton tape will be fine.

Joe
I will do that. Thanks

Barry
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Old 21st Oct 2021, 11:01 am   #7
Rishtonite
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Default Re: AJS choke - laminations ?

Quote:
Originally Posted by Herald1360 View Post
Isn't the airgap provided by the dogleg in the centre limb?
That depends
If you look at my photo, if the pair bottom right and laid on top of the pair bottom left, the L shapes overlap at the centre.
If all layers are EITHER as bottom left OR bottom right there will be an "S" (or "2") shaped gap.

The gap would be very small, but at least there would be no deliberate overlap.

Barry
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