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Old 11th Jan 2020, 2:15 pm   #1
ChristianFletcher
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Default Rewinding Bush DAC90 Audio Output Transformer

Hello Everyone

I am looking at rewinding a DAC90 audio output transformer and I have read about inserting a spacer air gap between the E & I lamination's using a cigarette paper shim 0.0508mm.

The instructions I have read imply that this shim is very very important. Having dismantled the original transformer I didn't find a shim and I am sceptical the original manufacture could have maintained such tolerances as the original part is crudely manufactured and pressed together. I am happy to install such shim as it wont cost anything to install it but is it actually important. My understanding is these are used to prevent core saturation. Do any off the transformer winding guys on here bother shimming the core ? I dont smoke so buying cigarette papers is going to be interresting.

Many Thanks
Regards chris
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Old 11th Jan 2020, 3:05 pm   #2
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Default Re: Rewinding Dac90 Audio Output Transformer

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I don't smoke so buying cigarette papers is going to be interesting.
Often used by me as a "am I near the workpiece" indication, stick on part, move tool 'till paper falls off then I am about a thou away. Doing this I have found at least four different thicknesses of cigarette paper! Having a look at Rizlas website https://www.rizla.com/products/ there are more than four! I assume the "grammage" is grammes per square metre.
 
Old 11th Jan 2020, 5:19 pm   #3
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Default Re: Rewinding Dac90 Audio Output Transformer

That is a job for Kapton tape I suggest.
It is very resistant to being punctured - much more so than cigarette papers.
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Old 11th Jan 2020, 5:37 pm   #4
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Default Re: Rewinding Dac90 Audio Output Transformer

Thanks for the replies John Merlin . My first Thought was Kapton tape but I had incorreclty assumed that it was going to be a lot thicker than a cigarette paper but you have proved not Thanks.

PS: No excuses LOL buy an edge finder your lungs will love you for it.

Thanks Guys

Regards Chris
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Old 11th Jan 2020, 5:41 pm   #5
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Default Re: Rewinding Dac90 Audio Output Transformer

Pleased to help Chris.
Let us know how it works out!
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Old 11th Jan 2020, 6:09 pm   #6
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Default Re: Rewinding Dac90 Audio Output Transformer

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PS: No excuses LOL buy an edge finder your lungs will love you for it.
Do edge finders work on a lathe?
 
Old 12th Jan 2020, 2:03 am   #7
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Default Re: Rewinding Dac90 Audio Output Transformer

Yes the gap is important. For something like a single ended output tranny of a watt or two I use standard yellow Mylar tape used in rewinding transfomers. The precise thickness isnt as important as its being there. You are correct, its to prevent core saturation. Another way to look at it, is to make the transformer much less efficient.

Joe
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Old 12th Jan 2020, 7:59 am   #8
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Default Re: Rewinding Dac90 Audio Output Transformer

Hi Chris, on the many output transformer i have rewound the paper (albeit rusty) has normally been present.
When it has been missing I have use a single layer of type 56 polyester transformer tape.

Ed
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Old 12th Jan 2020, 9:00 am   #9
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Default Re: Rewinding Dac90 Audio Output Transformer

Hi Chris, I smoke like a fish and have boxes of the green slips of cardboard that the papers wrap round as well as about 100 spare packets of green rizzlas, drop us a PM if of any use, I'll bung some int post.

Andy.
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Old 12th Jan 2020, 12:51 pm   #10
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Default Re: Rewinding Dac90 Audio Output Transformer

Thanks guys for all the replies I’m going to go with using the Kapton tape as it’s what I have to hand and is going to be more mechanical robust than the paper. I will probably post a video of my winding attempt wart and all.

Regards Chris
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Old 12th Jan 2020, 5:17 pm   #11
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Default Re: Rewinding Dac90 Audio Output Transformer

Strictly, a gap is necessary. In practical terms, butting-up the lams would be OK. Kapton at 1 thou should be fine. If you were getting it right to the nth degree, you'd set the gapping material to give a specific inductance or magnetising current, possibly with DC bias. But you'd need to do a load of calculations to determine what figures to use.

Quote:
Originally Posted by ChristianFletcher View Post
I dont smoke so buying cigarette papers is going to be interresting
You just buy them!

I'm not a parent, but I have bought babies' disposable nappies! And I'm not a girl or woman, but I've bought sanitary pads! (They both make excellent dressings, large or small, near-sterile, at a fraction of the cost of the 'proper' thing, for wounds to horses).
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Old 12th Jan 2020, 7:21 pm   #12
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Default Re: Rewinding Dac90 Audio Output Transformer

I was joking on the cigarette papers. I once bought 15 large tubes of KY jelly used for acoustically coupling transducer to the side of pipelines. I’m still talked about in the village Boots store nearly 20 years later.

I just happened to have a second transformer and having disassembled it and can confirm it does indeed have a shim fitted. It’s around 0.07 mm. I measured my kapton as bang on 0.05 so it’s going to get a slice of that for the rebuild.

All the old laminations are a bit rusty should I do anything to try and improve them or does it not matter.

Thanks regards Chris
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Old 12th Jan 2020, 7:27 pm   #13
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Default Re: Rewinding Dac90 Audio Output Transformer

I'd just give them a wipe with a (dry) semi-abrasive scouring pad, clean off any loose rust but not strip off any insulating coating.

0.07mm sounds quite credible, about 3 thou'.
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Old 12th Jan 2020, 7:30 pm   #14
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Default Re: Rewinding Dac90 Audio Output Transformer

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All the old laminations are a bit rusty should I do anything to try and improve them or does it not matter.
Do nothing, the insulation helps reduce eddy currents, a smear of oil to make them a bit easier to install is as far as I would go.
Quote:
I once bought 15 large tubes of KY jelly
Done a similar thing with lots of talcum powder for cable assembly, as said powder was 99.9% talc (magnesium silicate) it met the Mil Spec!
 
Old 12th Jan 2020, 8:01 pm   #15
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Default Re: Rewinding Dac90 Audio Output Transformer

Thanks
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Old 13th Jan 2020, 4:26 pm   #16
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Default Re: Rewinding Dac90 Audio Output Transformer

Another question comes to mind?

The primary on the original transform had a thin layer of waxed sticky paper every two hundred turns or so. I guess part of the reason is that the internal former had no end plates and I think the wax paper probably helped to hold the windings together although it could be for insulation.

I am guessing the original transformer would have been automatically machine wound with the windings perfectly side by side etc. I would like to leave out this insulation material between the primary windings as I think my windings will be less neat and take up more space than the original and it was already tight. I am worried I may not get all the windings on with the insulation between layers etc.

Can I leave out the intermediate insulation. I am going to tape at the start and stop of the windings and the primary secondary. I would of thought the modern materials have better insulation than those back in the day.

Thanks Regards Chris
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Old 13th Jan 2020, 8:19 pm   #17
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Default Re: Rewinding Dac90 Audio Output Transformer

Yes, modern enamel is better. (Presumably you are using Grade2)?

You have end cheeks to the bobbin, and you also have a mandrel which supports the ends and sops pressure of the wire rom gradually spreading them outwards. Both good.

Expect to make a couple of false starts, and having to strip and restart.

(Insulation between primary and secondary is still necessary of course!)
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Old 14th Jan 2020, 5:25 am   #18
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Default Re: Rewinding Dac90 Audio Output Transformer

Thanks Kalee

Got the primary wound. I wish it could have been a bit neater but I guess it’s functional as Usual all finger and thumbs covered in glue and kapton tape stuck in my hair. Regards the wire I’m not actually sure what grade it is but it is sold as transformer wire with solder through enamel.

Was chasing the electric drill starting and stopping is the hardest bit. And making sure you have a good joint between the thin enamel and the lead in wire. Unfortunately the end in the centre of the bobbin of new wire is not exposed so couldn’t bell it out.

There is only 76 turns on the secondary and it’s thicker wire so hopefully the worse is over.

Thanks
Chris
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Old 14th Jan 2020, 5:51 am   #19
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Default Re: Rewinding Dac90 Audio Output Transformer

Hi Chris, yes as Kalee has said use grade 2 (double insulated) wire for the pri.
Layers of insulation and then the sec, as this is fewer turns and thicker I would forget the drill and hand wind this.

An old technique, especially with self soldering wire is to wind a few turns of the pri around your fingers and twist it; thus making a stranded lead out wire.
Tin at the correct length and oversleeve before attaching to the tag. This would certainly be appropriate for the DAC90 type of transformer.

Ed
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Old 14th Jan 2020, 10:04 am   #20
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Default Re: Rewinding Dac90 Audio Output Transformer

Hello Ed

Oh - I may have messed up on the wire. The stuff I bought is only single coat but looking at the specifications on the website they don’t carry double coat grade under 0.12mm. The specifications does say suitable for transformers but I will certainly buy double coated next time time.

Will the wire I have used likely fail ? Or is it just that double coated is better. The actual volts per turn is relatively low the primary has 3150 turns. 3150/75

Thanks Ed and thanks for all the comments help and advice.

Chris

Metric Size - 0.10 mm (Solderable)

SWG - 42 / AWG - 38

Length - 14322 Meters (Approx)

Cross Sectional Area - 0.007854 sqmm

Build - Single Coat Solderable

Net Weight - 1000 Grams (Actual Wire mass)

Coating Class - Polyester 180 Single Coat

Temperature Class - 180 Degrees Celsius

Chemical Resistances - Unaffected by all common solvents perchloroethylene, mild acids and alkalis

Typical Applications...
For the rewinding or manufacturing of but not limited to... Electric Motor windings, Transformer Windings, Solonoid Coils, Armature & Rotor Windings, Tesla Coils, Choke Coils, Guitar Pick Up Coils, Aerial & Antenna Coils, Contactor Coils, Metal Detector Coils.
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