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Old 19th Aug 2019, 7:10 pm   #1
Chris55000
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Default Making A Transistor Radio LB Book R.F. Choke Details?

Hi!

Just a quickie one!

Does anybody recognise the make and approximate inductance of the small ferrite–core axial lead R.F. Choke suggested in the LB book, "Making A Transistor Radio" please?

Would one of the modern colour–coded axial chokes about 10mH do, or will this t.r.f. need an "old fashioned" type of wave–wound R.F. choke you could get in the 1960s?

Chris Williams
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Old 19th Aug 2019, 7:19 pm   #2
ms660
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Default Re: Making A Transistor Radio LB Book R.F. Choke Details?

The stock item for the RFC back in my day was a 2.5mH choke (Repanco I think)

Lawrence.
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Old 20th Aug 2019, 11:05 am   #3
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Default Re: Making A Transistor Radio LB Book R.F. Choke Details?

As long as the new choke dc current rating is OK, it should work. Check that its primary resonance is higher than 1.6MHz for best results.

In the LB reflex regen circuit you would probably get reasonable results just using a 1k resistor instead.

A version of that circuit I built as a lad using an OC140 (NPN, similar RF performance to OC45 which I didn't have) worked very well when tweaked to just not oscillating.

Edit: SRF of 10mH likely to be ~350kHz. 2.2mH would be a better bet!
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Old 20th Aug 2019, 1:54 pm   #4
David G4EBT
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Default Re: Making A Transistor Radio LB Book R.F. Choke Details?

Quote:
Originally Posted by ms660 View Post
The stock item for the RFC back in my day was a 2.5mH choke (Repanco I think)

Lawrence.
Yes, though the late Rev George Dobbs didn't specify the make or inductance of the choke in the book, back then, the standard RF choke for simple transistor radio projects was the Repanco 'CH1' which was 2.5mH 'High Q' 10 Ohms, 100mA. There was also the 'CH5' which was 1.5mH 7.5 Ohms. I've attached a picture below of a CH1.

Farnell did a 2.5mH Bourns choke, but unfortunately, they've stopped making them:

https://uk.farnell.com/bourns/6302-r...RoCTfAQAvD_BwE

You could use a modern 2.5 mH choke but it would look like a modern resistor so if authenticity was desired, it wouldn't tick that box.

Or you could wind your own using something like an old 1Watt carbon resistor of any value as a former. If you used something like 42SWG enamelled copper wire, that has a resistance of 2.18 Ohms/Metre, so you could 'scramble wind' say 5 Metres onto the body of the resistor and solder the ends of the coil onto the resistor leadouts. The resistor has no function other than as a suitable former around which to wind the coil, which of course shorts out the resistor anyway. If you have a cheapo Chinese multi-tester that would indicate the actual inductance of the choke.

Someone may come to your rescue and find something suitable in their spares box or something might pop up on ebay.

It's not that critical a component.

Hope that helps a bit Chris.

Back when George wrote the Ladybird book in the early 1970s, (before he was ordained, hence as author was referred to as G.C.Dobbs), I knew him quite well as he and I live in Grimsby at the time. He told me that just as the Ladybird book was going to print, publication was halted as he'd originally written it to be soldered together on a breadboard as a 'Lads and Dads' project, with a father supervising his son in using a hot soldering iron. (Back then boys still did metalwork and woodwork and girls did cookery and needlework - now, nether gender does either).

However, the litigious society was beginning to develop and likewise, concerns about health and safety. Ladybird became nervous about the prospect of hot soldering irons in inexperienced hands, so halted publication and asked George to redesign the project on a 'solderless' basis for which he developed the 'screw and cup' technique. However, the first two pages of the book were about soldering techniques and clearly featured a Henley Solon 25 Watt iron, which was the standard iron of choice back then and those two pages remained in the book.

I built the project in 1974 with my then ten year old elder son and it worked quite well. I still have the book with his name inside. (The cover price for the 53-page hardback book was 15pence!). Despite my best endeavours, neither of my sons (now aged 51 and 55) ever showed the slightest interest in amateur radio or electronics (except of course consumer electronics!).



Good luck with the project.
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Old 20th Aug 2019, 3:04 pm   #5
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Default Re: Making A Transistor Radio LB Book R.F. Choke Details?

Interesting. I tried making old style RFCs earlier this year. Used 10K 1W carbon composition resistors (as they are fairly big!) and the DC resistance trick as David pointed out and some wire from scientific wire co. I glued a small plasticard disk (cut out with scissors) at each end of the resistor. Then held one leg of the resistor in an electric drill chuck and tried to weave the windings on in zigzags as per original items. Quite difficult. After managing to snag one and tear the lead of the resistor, then snapping the wire I gave up for a couple of days. Third time lucky it worked. I used superglue to secure it, left it overnight then snapped the plastic former off and soldered the wires to the leads. Looked quite good!

Inductance was around 0.9mH when measured with a 1% silver mica in parallel and poked with a pulse generator through a 100k resistor (and measured on digital scope). Way under the predicted 2.5mH!

Alas the first thing I stuck it in, I cocked up and shorted my power supply across it and burned it out so I gave up. Hrumph

Epcos still sell cheap axial chokes which are pretty good but SRF is down at 800KHz: https://uk.rs-online.com/web/p/leade...ctors/8807262/ ... I've used them above that quite happily though just avoid 800KHz!

Edit: They go up to 10mH too: https://uk.rs-online.com/web/p/leade...ctors/1911204/

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Old 20th Aug 2019, 7:04 pm   #6
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Default Re: Making A Transistor Radio LB Book R.F. Choke Details?

Just for interest, I recently built a regen radio using some old (2.5mH?) chokes I bought recently from J Birketts. While I was testing I did try various options but didn't find anything modern which worked as well as they did. They were the multiple bank wave wound type which did look perfect for the age of design too.
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Old 20th Aug 2019, 11:29 pm   #7
Chris55000
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Default Re: Making A Transistor Radio LB Book R.F. Choke Details?

Evening David!

Thank you for your time to look up that choke in the Ladybird Book circuit for me!

I certainly have come across Repanco's products in both P.W.and R.C. circuits, R.C. magazine tending to use it more than P.W. did!

Whether I'd strike gold with one at Birkett's these days I'm not sure but when pocket–money permits I'll have to have an excursion there!

I suspect that winding it on a carbon resistor wouldn't give enough inductance, so it looks like Repanco actually wound it on a ferrite tube with the leadout wires cemented into it's ends – the inductance factor AL of the ferrite obviously contributed to part of it's total inductance!

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Old 21st Aug 2019, 10:23 am   #8
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Default Re: Making A Transistor Radio LB Book R.F. Choke Details?

Old slug tuned IFT's can be a good source for regen RFC's.

Lawrence.
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Old 21st Aug 2019, 10:30 am   #9
short wave
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Default Re: Making A Transistor Radio LB Book R.F. Choke Details?

For people that don`t have the book here`s a link...

https://www.americanradiohistory.com...Books-1972.pdf
regards S-W
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Old 21st Aug 2019, 11:31 am   #10
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Default Re: Making A Transistor Radio LB Book R.F. Choke Details?

Quote:
Originally Posted by Chris55000 View Post
I suspect that winding it on a carbon resistor wouldn't give enough inductance, so it looks like Repanco actually wound it on a ferrite tube with the leadout wires cemented into it's ends – the inductance factor AL of the ferrite obviously contributed to part of it's total inductance!
Definitely. I had to wind a ridiculous number of turns to get anywhere near 1mH. The winding was about an inch across and 8mm thick on mine with hair fine wire.

These look like a possibility but I gave up before then: https://uk.rs-online.com/web/p/ferrite-rods/4674043/

You can drill with carbide drills very slowly to make pits for the leads.

Edit: I figure a Peak LCR45 component tester is a good bet to drag along to radio rallies. You can test variable capacitors and chokes etc then!
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Old 21st Aug 2019, 11:37 am   #11
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Default Re: Making A Transistor Radio LB Book R.F. Choke Details?

The book is also on the sister site to this forum at https://www.vintage-radio.info/download.php?id=737.
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Old 21st Aug 2019, 1:02 pm   #12
Ian - G4JQT
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Default Re: Making A Transistor Radio LB Book R.F. Choke Details?

There is a great deal about this book and its radio on the web. I've just had a dig around, but unfortunately can't find the website I have seen that has a collection of the Ladybird Transistor radio made by various people.

But it is out there somewhere!

The choke I think is 2.5mH, but not particularly critical. There is a minor mod putting a 1k/22u R/C combo in the emitter of the first OC71 as its operation tends to depend on the gain of this transistor.

I expect many of us here now were influenced by this book...
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Old 21st Aug 2019, 2:56 pm   #13
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Default Re: Making A Transistor Radio LB Book R.F. Choke Details?

I worked on a project with a Swedish guy, on a security gate product. His background was RF (he was a ham), and his rule of thumb for inductor design was "first decide how much you want to pay for the copper"

This is the guy https://www.youtube.com/watch?v=2T9jGP3K_o0 one of a series on RF.

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Old 21st Aug 2019, 5:50 pm   #14
David G4EBT
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Default Re: Making A Transistor Radio LB Book R.F. Choke Details?

I've had a poke around in my spares box(es!) and found a 2.5mH (7.5 Ohms) R.F. choke which might suffice.

If you feel it would suffice, it's yours FOC Chris - just drop me a PM with postal details.

A couple of pics below.
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