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 Post subject: Re: Rebuilding an interstage AUTO-transformer
PostPosted: Apr Sun 12, 2020 10:53 pm 
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Lamination material: How would I find out?
the OEM would be audio iron. To discover some other unknown iron, dunno? Take some sort of audio reactance bridge and some sort of calibrated reaction winding. IMHO more trouble than it is worth, unless building transformers from unknown junk. Should be plenty of junk iron that has a known source. Toss out the filament transformer iron, vertical output and choke iron what is left?

Seems 1/2 the winding was good, resistance was measured, wire size is known and the temperature, the resistance would give the length, wind until the foot/feet meter says you got the length, but count turns too, check the resistance if good tap, if not backup or forward, fix, then tap finish the winding and ignore the footage as the bobbin has gotten bigger, now take the turns to get the wind from the first wind and apply turns to the second. Should be in the ballpark. Caveat if the coil that is still good was the beginning wind, most exact, if not, well it won't be off that much...

When I was making braided antenna wire I used a continuous, rotating counter. A iron grooved wheel circumference of 12", product made about 190 deg around the wheel, got my measurement on the fly AND I could back up and save count...

GL

Chas

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 Post subject: Re: Rebuilding an interstage AUTO-transformer
PostPosted: Apr Mon 13, 2020 12:12 am 
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It is really impressive I think how these vintage transformers (or chokes) were would with the perfectly even layer winding. It may have had some disadvantages such as residual salts in the paper & water absorption and green spot corrosion, so like vintage ignition coils it might have been better off with oil than wax. In fact, in this day and age the only coils I know that are would like this are oil filled auto ign coils. Modern enamels, like grade 2 help with jumble wind reliability, but still, I really admire what they did with this old method, it very pleasing to an engineer's mind.

Make sure when you calculate the turns you can fit on your bobbin, to measure the actual wire diameter, including the enamel, with a micrometer and to leave the required space for the outer insulation layers. That is already in you fudge factor I suspect.


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 Post subject: Re: Rebuilding an interstage AUTO-transformer
PostPosted: Apr Mon 13, 2020 12:48 am 
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It is really impressive I think how these vintage transformers (or chokes) were would with the perfectly even layer winding.

they had machines--and operators with LOTS of practice.....

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 Post subject: Re: Rebuilding an interstage AUTO-transformer
PostPosted: Apr Mon 13, 2020 6:09 pm 
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Location: Powell River BC Canada
In the decades that radios stopped being battery operated, the phone companies
were winding coils by the millions.

Apart from ringing bells over chancy paths, (earth, and copper) they seldom
got burned out despite lightning.

The other thing they did was stretch the wires on poles as far as they could
to establish long distance service.

Winding precision is one thing, the other half is the core and what effect heat
treatment, or re-treatment may have on performance.

The 1941Terman, first ed. (already mentioned) and Radiotron's have a lot
on cores.
Attachment:
tr iiii.jpg
tr iiii.jpg [ 370.16 KiB | Viewed 402 times ]

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 Post subject: Re: Rebuilding an interstage AUTO-transformer
PostPosted: Apr Sun 19, 2020 11:02 pm 
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For those watching the gory details....

the inner half is done, and varnish drying. I got the target of 14,000 turns, which is the best guess as to what the original was. I had a small amount of room left, but decided to quit while I was ahead.
One thing I realized is that a square bobbin is not going to give the same packing factor as a round one. I could have maybe done a bit better with more tension, but--with 40AWG--that's dicey.
Pictures will follow once it's done (and working....;) )

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 Post subject: Re: Rebuilding an interstage AUTO-transformer
PostPosted: Apr Mon 20, 2020 11:15 pm 
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I am faced with the same situation on a 36AC and am going to try a Stancor 63C in place of the choke. Will have to wait for you know what to blow over.


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 Post subject: Re: Rebuilding an interstage AUTO-transformer
PostPosted: Apr Wed 22, 2020 12:51 am 
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Quote Lamination material: How would I find out?
Attachment:
123 22.jpg
123 22.jpg [ 552.08 KiB | Viewed 349 times ]



37 pages long. But easily doable .

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 Post subject: Re: Rebuilding an interstage AUTO-transformer
PostPosted: Apr Wed 22, 2020 3:13 pm 
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Again--many thank for all the inputs.

The rebuilt transformer is now complete and I am struggling to understand its performance.

Here is a preliminary synopsis:
--Someone suggested the inductance should be ~70H. It came out around 2,000H
--With some appropriate resistors to simulate the actual usage in the radio, the low frequency rolloff is something like 10Hz, and the high end is ~6KHz

This aside, it appears to do its job. (And, unless another Colonial 36 owner can make a measurement, we may never know if the frequency response is normal.)

Random observations, as I attempt to unravel this:
1. We know that the original had a DCR of about 10Kohms. Random sampling of modern interstage transformers shows that most are at least 10X lower.

2. While the new coil should have inductance similar to the original, the distributed capacitance will be higher (higher "fill-factor") This is consistent with the relatively low HF cutoff.

I'm thinking of throwing together an audio amplifier similar the the original and see how it performs. (Possibly more rewarding than trying to do all the math to understand the parameters---and figure out how to do a different winding to change them.)

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 Post subject: Re: Rebuilding an interstage AUTO-transformer
PostPosted: Apr Wed 22, 2020 3:26 pm 
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I'm sorta interested, Mark, in the outcome of the experiment. I am still looking for Silver Marshal "transformers". These have a tapped auto-former and an internal coupling cap. I have other models of S-M's and learning how they work would enable me to create the model transformer I need from the internals of the ones I have. Unlike the Colonial that is used to develop the 180 phase shift for push/pull the S-M's are tapped and coupled with a cap and an internal resistor is used as a plate load for single ended. The method is supposed to give more fidelity than conventional transformer coupled...

Chas

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 Post subject: Re: Rebuilding an interstage AUTO-transformer
PostPosted: Apr Wed 22, 2020 3:59 pm 
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More as a general comment. One of the members of our Radio Club here, does a lot of rewinding, mainly power transformers; to the point where he has setup a lathe, with counter to do it. What we were warned about, was using the old style wire, even if its NOS. He has found that the insulation used on much of it, will not now withstand actions such as winding, without the insulation failing.

One is better off with new, modern, winding wire.

Marc


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 Post subject: Re: Rebuilding an interstage AUTO-transformer
PostPosted: Apr Wed 22, 2020 4:43 pm 
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+100
I use almost exclusively what is typically called "double-build" insulation--eg Soderon 155 (Essex, sold by Temco:
https://temcoindustrial.com/10-awg-copp ... on155.html

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"Even if you don't understand Ohm's Law, you are still required to obey it."


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