diy IF transformers: first attempt.

[Amie]

Hi all!



Pictures below are fairly self explanatory, but I've been experimenting with creating a front end for a valve AM superhet radio.

I don't like winding coils much, and had previously seen my dear departed dad, and others, use axial chokes to create coupling transformers, and oscillator coils.

I thought I'd give it a go as there is a distinct lack of available off the shelf choice if you're looking for IF transformers for valve equipment.

First effort shows encouraging results, a possibility for a narrower pass band shape would be coupling the two sides with a transfilter rather than by proximity.

Thanks,
Amie.

[AdrianPH]

I like that. A novel approach to making the IF cans using fixed coils, I have always thought they would need to be on a single core, is one in the anode and one in the grid circuit, just wondering about impedances, but then think tuned circuits are either very high or very low impedance depending on parallel or series connected.
I must get on and build an audio IF section for a TV, that gives me another option.

Cheers

Adrian

[Kalee20]

Impressive - particularly the symmetry above and below the centre frequency.

Isolation between primary and secondary should be absolutely above suspicion with this technique.

One thing occurs to me - and you are ahead of me on this particular topic - if a screening can is wrapped around, I bet it will affect both coupling and tuning a lot less if the coils are arranged longitudinally rather than transverse. Happy to be proven wrong, though...

[Amie]

I like that... ....gives me another option.

Cheers

Adrian

Thanks!  You're welcome Adrian. Just be aware that these are just experiments, though I have hacked one into the 1st IF of an old Sky Captain chassis and it works acceptably as is.

I don't see any reason why the idea wouldn't transfer to higher frequencies for your TV IF's. Moulded chokes are available to suit the values needed.

The components used here are made by Epcos.

Impressive - particularly the symmetry above and below the centre frequency.

Within the bounds of Amīcalabs flaky test equipment, yes! However I did find that placing the chokes axially, that good linearity disappeared like the morning mist, but I don't know why

Isolation between primary and secondary should be absolutely above suspicion with this technique.

Absolutely. One experiment that worked very well was back to back transistor style IF's, I haven't completely exhausted that idea, but as one wise sage pointed out to me, the enamel in those miniature cans coils are good for 50volts at best.

One thing occurs to me - and you are ahead of me on this particular topic - if a screening can is wrapped around, I bet it will affect both coupling and tuning a lot less if the coils are arranged longitudinally rather than transverse. Happy to be proven wrong, though...

The prototypes as pictured were that physical arrangement for ease of assembly, and adjustment. Once made like that I was surprised how easily they coupled, and yes any screening will affect it.

They are 20mm square, and the screening can may need to be twice that, Another option is mount the chokes out of plane with each other so they can be adjusted variometer fashion, and therefore be closer.

I will experiment further if, as I suspect you are correct, and it is badly affected by the screening....

WTS.

Thanks.
Amie.

[boater sam]

I came across this article some time ago, may be of interest.


https://www.antiqueradios.com/forums/viewtopic.php?f=6&t=196404


Sam.

[boater sam]

Getting duplicate posts for some odd reason.

Sam.

[Ed_Dinning]

Hi Amie, I recently repaired an IF for a battery valve set. These were both wave wound but only about 8mm dia. They sat inside a ferrite cup that had the" base" threaded to accept an adjuster. The centerpiece was stepped (small/large/ small) and also of ferrite and the whole lot was glued together with wax.
The whole lot fitted inside a conventional 1/2" square can.

I have seen (I believe US origin) IFTs that had the 2 winding bobbins at right angles to each other, but I think these used an additional coupling winding (and trimmers for tuning)
There are many weird and wonderful ways to make a tuned filter, if you have the time Electronic Engineering from the 40's has some articles on IFT design and details of coil/ can spacing.
There was also lots of work done in Wireless Engineer in the 30's and the patent office library will have quite a bit of detail on this (not sure how accessible that is these days.

I have plenty of IFs of various sizes if you need any to experiment with.

Keep up the good work and innovative thinking

Ed

[Amie]

Thanks Sam, I think that guys posts are what originally gave Dad his ideas to use those axial chokes for IFT's, it's noteworthy that he didn't bother screening his. I believe that he had also seen some similar ideas in one of the MJF projects books, sent to him by his friend Greg in MS USA, that had construction details for kits that MJF enterprises, (Starkville Mississippi) produce.

Dr Ed, I admire anyone who can do that kind of repair, its a little beyond me, best I managed so far is a relatively simple fault of a detached wire in an oscillator coil for a Murphy battery set. My aim with this project is to design a radio using readily available components like the little chokes. Ambition is also to design the front end of a radio to use up the world stocks of EF80's 

....I have plenty of IFs of various sizes if you need any to experiment with....

Thanks for the offer... I'll know where to come for some! I don't like to experiment with stuff that could be used for the repair of vintage equipment,  but would love to find a source of the likes of denco or wayrad components

**********

Project....
I managed to make a (close fitting) copper screening can that I enclosed one of the prototype IFT's with to see the effect, it didn't seem to seriously alter the coupling, but did lower resonance by approximately 25kHz, next I'll try a round screening can to see if that is more tolerable with what ive already made....

Thanks,
Amie.

[Cleggy]

Nice piece of work Amie! I wonder what equipment you used to determine the response of the transformers?

C

[Amie]

Hi Cleggy.

Initial checking done with 1947 Taylor all wave, and Topward oscilloscope.

Envelope traces by Maplin, (Precision Gold) sweep/function generator, Dad built frequency counter, (to check the settings of the generator), and oscilloscope.

Thanks,
Amie.