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Old 20th Sep 2017, 6:00 pm   #1
Audion_1904
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Question Can you wind your own Repanco DRR2 coil?

I have always wondered if it is possible to wind your own Repanco DRR2 coil, for building a reproduction of the DAF91 receiver found in one of
the late Gilbert Davy's "Fun With Radio" books. In the book's fifth edition ( late 1970s copyright date ), which was available in a public library about 20 years ago ), he stated "unfortunately it has specialised winding techniques and cannot be home made". This I believe is the special "universal" winding design that minimizes capacity and therefore unwanted resonance effects. I can't remember if Gilbert Davy recommended or provided a substitute design that could be home made. Sometimes, DRR2 coils do appear online, but not that often. The price is still reasonable, though.

Also I was wondering if anyone knows of a source for authorized reprints of his Fun With Radio Books. The originals sometimes appear online, but are no longer common.
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Old 20th Sep 2017, 7:03 pm   #2
M0FYA Andy
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Default Re: Can you wind your own Repanco DRR2 coil?

I imagine this coil will be 'wave wound', so hard to replicate without access to an Avo 'wave winder' or similar machine.

Andy
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Old 20th Sep 2017, 7:09 pm   #3
Biggles
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Default Re: Can you wind your own Repanco DRR2 coil?

I'm sure there has been previous discussions on the forum about doing this with some kind of home made machine. Someone on here must have made one before now...
Alan.
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Old 20th Sep 2017, 7:18 pm   #4
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Default Re: Can you wind your own Repanco DRR2 coil?

This site shows a homemade alternative to the DRR2 for use in the Gilbert Davey receiver:

http://www.daveysradios.org.uk/b1v.html

Dave

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Old 20th Sep 2017, 7:23 pm   #5
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Default Re: Can you wind your own Repanco DRR2 coil?

Sorry wrong link, here it is:

http://www.daveysradios.org.uk/conres.html

Dave

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Old 20th Sep 2017, 9:31 pm   #6
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Default Re: Can you wind your own Repanco DRR2 coil?

I recall a thread on VRAT forum regarding the DRR2 and the write up about the DRR2 inductance measurements. Seems it was straightforward for a person with a wave winder,the know how and aso time and patience.

My one adult son asked a couple of years back if we could again build the 2 valve radio we put together using this coil back in 1986,sadly I could not come close to the R.E.P coil.
One is for sale now on the internet auction site with 1 day + to run and it is already at £28.

What happened to cottage industry? I would happily buy a few at sensible price.
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Old 20th Sep 2017, 10:11 pm   #7
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Default Re: Can you wind your own Repanco DRR2 coil?

It was wound with cotton or silk-covered wire (for roughness), and yes, wave-wound, so stayed put. (Enamelled wire is too slippery for wave winding). I've an idea it was Litz, too.

Wave winding is low capacitance, and potentially lower-loss than simple winding, because the self-capacitance of simple winding is rather lossy and this hurts overall Q.

However, as it has a reaction coil, I'm thinking a simple-wound coul would probably work as well. It may need a bit more reaction to compensate, that's all.
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Old 21st Sep 2017, 1:01 am   #8
VT FUSE
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Default Re: Can you wind your own Repanco DRR2 coil?

I remember buying one or two as a kid in which the soldering was poor or indelicate handling led to having to resolder to the tags,don't remember if it was Litz or not and kalee is much more likely to be right on type.
Was going to try and get the initiative up to make a homemade "Gingery" coil winder but many other things intervened and took the time away.Shame,I bought a 250gm spool of 0.08mm (44 SWG) cotton covered enamel wire for the attempt.......
Maybe someone with time on their hands and a wave winder or ability to make one feels like a challenge?
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Old 21st Sep 2017, 4:49 am   #9
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Default Re: Can you wind your own Repanco DRR2 coil?

Useful info here, about halfway down the page:

http://golbornevintageradio.co.uk/fo...id=2606&page=2

You do need to be a member to be able to download the files in the post.

Terry
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Old 21st Sep 2017, 9:50 am   #10
David G4EBT
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Default Re: Can you wind your own Repanco DRR2 coil?

It won't be immediately obvious but I was the originator of that thread as my name on that forum was 'Yorkie' (though I ceased posting there in early 2016).

Non-members of that forum who are interested in the DRR2 coil ('Dual Range Reaction LW/MW') and in winding one, can only read the thread about the DRR2 and the BBC Focus transistor set there - they can't open attachments, so I've attached them here.

Firstly, I should say that building the BBC 'Focus' set designed by Gilbert Davey confirmed my suspicions that it was a waste of a coil that could - at the time (and still could for that matter) - have been put to good use in a one-valve TRF circuit which would have performed far better. The Focus is just a crystal set with a two transistor amplifier using low gain OC71s (typical HFE 50 – 60). As such, it lacked sensitivity and more importantly - as is so with all crystal sets - it lacks selectivity, the effect of which was that though several stations could be heard at reasonable strength they were all on top of each other, with the local MW stations being dominant.

At that time, circuits for one/two valve TRFs were commonplace and would have far outperformed this set, but I suppose there was a desire to try out the newfangled transistors. Even if the set was made to work, I dare say that the end result would have been many disappointed kids and out-of-pocket dads back in 1959.

It was intended to be for a novice to build – perhaps a schoolboy, or a 'lads and dads' project, but in my view, it was very poorly written and the circuit was very poorly drawn, with sketchy and difficult to follow constructional information.

To cap it all, we forget how expensive components were back then in real terms. The article specified Mullard OC71 transistors - 21 shillings each at the time, (which equates to £22.50 each when adjusted to present day for historical inflation), or red spot surplus (out of spec) transistors at 7/6d each, equivalent to £7.50 each in 2017. Yet the instructions gave no precautions or advice about avoiding damage when soldering them. The total cost at today's prices would be about £65.00.

As to winding a DRR2 coil, which is the subject of this thread, I made a 'Gingery' wave winder from metal in my scrap bin and it works well, though some experimentation is called for to adjust it for the wave repetition or the turns are quite air-spaced, making them larger in size for a given inductance. (I wrote a 'home-brew' thread about it on this forum). Those who aren't familiar with the winder can see one in use at this link. If you look at the finished coil at the end of the film, you'll see that the though it looks nice, the turns aren't close-wound:

https://www.youtube.com/watch?v=FIOocMoRsYQ

The 'Repanco' (Radio Experimental Projects & Company) information on the coil was simply a poor hand drawn and written sheet but better than nothing. One problem was that the data didn't show the inductance of the tapping point. I was fortunate to be able to borrow a coil from Alan Beckett, so was able to check the inductances. The MW has a tapping point. That's when I discovered that though the MW coil was 30uH & 70uH, the total inductance wasn't as I'd expected the sum of the two parts of the MW coil - 100 uH, it was in fact 170uH. The reason is as I've explained on the data sheet, that when coils are close-coupled like that, mutual inductance arises. Only when coils in series are separately wound do their inductances add up in the same way as resistors.

Nowadays, with all-in-one inductance/capacitance/transistor testers being so cheap, at least it's easy to check the inductance of hand wound coils.

I've attached a pic of a DRR2 coil, the Data Sheet which I annotated, the 'Gingery' coil winder I made and a 3MH test coil I wound on it. The wire is either silk or cotton covered enamelled copper, 48SWG (.04mm/0.0016"). It's solderable, so no cleaning off of enamel is needed. (The original DRR2 coil was Litz, but I doubt it made much difference to the 'Q'). I've also attached the BBC 'Focus' radio article, and I'd reiterate that I think was of a very poor standard, especially bearing in mind that back in that era, every few weeks there were excellent designs for simple radio in 'Hobbies Weekly' magazine.

So yes, you can wind your own DRR2 coil and you can construct Gilbert Davey designs, but I'm not quite sure - other than nostalgia - why anyone would want to.

I know that Gilbert Davey designs have a keen following, and for reasons that elude me, his 'Fun with Radio' book is much sought after. For those who don't share my view, these links may be of interest:

http://www.daveysradios.org.uk/gwd.html

http://www.americanradiohistory.com/...onics-Davy.pdf

Before anyone asks, I'm not interested in making any more DRR2 coils - the only time I feel like doing anything twice is just before I've done it once!

Hope that's of interest.
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Attached Files
File Type: pdf BBC 'Focus' Transistor radio.pdf (589.7 KB, 204 views)
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Old 21st Sep 2017, 11:05 am   #11
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Default Re: Can you wind your own Repanco DRR2 coil?

Splendid write up David.
From all of your postings your knowledge and ability are readily apparent,the winder you made is beautiful but I would struggle to find work for one here and it would see very infrequent use.
Thought that you summed the whole thing up well esp. the penultimate sentence.
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Old 21st Sep 2017, 12:02 pm   #12
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Default Re: Can you wind your own Repanco DRR2 coil?

I can well believe that a wave-wound coil has lower self-capacitance than a close-wound multilayer coil with its turns closely side-by-side. It also has the advantage of being self-supporting.

However, I do wonder how its self-capacitance actually compares with a random-wound coil such as one that those of us without coil winders might construct using simple card end cheeks. Has anyone made any measurements of the difference?

As kalee20 suggests, the positive feedback of a reaction circuit would in any case be expected to compensate for a Q deficiency.

Martin
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Old 21st Sep 2017, 4:42 pm   #13
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Default Re: Can you wind your own Repanco DRR2 coil?

Just one point. The winding labelled Aerial Coupling Coil, with the mauve and red terminals, is in fact the reaction coupling coil when used in the intended application. The aerial input is the yellow connection when used as originally intended.

The circuit in the attachment has a flaw. There is no dc path to ground for the diode. However it would probably work due to leakage through the electrolytic C2.

I first acquired a DRR2 when I was given a crystal set over 50 years ago. As has been said, it's a bit wasted in a crystal set. Might have a go at a simple TRF when I can find the time.

Keith
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Old 21st Sep 2017, 4:58 pm   #14
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Default Re: Can you wind your own Repanco DRR2 coil?

I'd be tempted to use an old ferrite rod complete with it's MW and LW coils, adding a regen winding shouldn't be a problem, the MW winding could be easily tapped if required.

Lawrence.
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Old 22nd Sep 2017, 12:38 am   #15
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Default Re: Can you wind your own Repanco DRR2 coil?

Thanks for posting the attachments David.

I did actually work out the connection to you when another link in google bought up a previous post by you on the subject in this forum, but it was too late to edit my post by then.

Terry
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Old 22nd Sep 2017, 8:11 am   #16
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Default Re: Can you wind your own Repanco DRR2 coil?

Quote:
Originally Posted by ms660 View Post
I'd be tempted to use an old ferrite rod complete with it's MW and LW coils, adding a regen winding shouldn't be a problem, the MW winding could be easily tapped if required.
This is what I did when I built a regen as a kid. I think the additional winding was stolen out of another old radio and unravelled a bit. Worked pretty well! The design was based on one in a Babani book (all I could get back then)
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Old 27th Sep 2017, 8:01 am   #17
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Default Re: Can you wind your own Repanco DRR2 coil?

Relevant to the DRR2, but not as to how to wind one, is a question I have had for years about these coils. What I assume is the wave change switch appears to short circuit a large chunk of winding, which I would have thought would totally destroy the Q of the coil; but presumably it doesn't. How and why?
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Old 27th Sep 2017, 9:59 am   #18
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Default Re: Can you wind your own Repanco DRR2 coil?

Leaving a coil open circuit, may mean that it resonates with its internal capacitance and absorbs power.

Short circuiting a coil will take some power but will not resonate. The short circuited coil does not short its butty because of the leakage inductance between the two. The PA coils in amateur transmitters have sections that are shorted. The Q is not severely affected if the short has low resistance.

Putting a coil in a metal box can increase the Q as in helical resonators, it keeps the magnetic field constrained.

I have measured the coupling inductance between various coils. It can be quite low.

A better man than me will explain further.
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Old 27th Sep 2017, 10:19 am   #19
David G4EBT
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Default Re: Can you wind your own Repanco DRR2 coil?

Quote:
Originally Posted by trsomian View Post
Relevant to the DRR2, but not as to how to wind one, is a question I have had for years about these coils. What I assume is the wave change switch appears to short circuit a large chunk of winding, which I would have thought would totally destroy the Q of the coil; but presumably it doesn't. How and why?
The LW and MW coils are in series when tuned to Long Wave, but on MW, the LW portion of the coil is shorted to ground, so that only the MW portion is in circuit.

Incidentally, in Gilbert Davey's book Fun With Electronics - available at the American Radio History archive earlier link I posted, there is a neat little one-valve circuit (MW only), with full constructional details, including how to wind an alternative coil to the DRR2 using either a cardboard postal tube or plastic drainage pipe 1.5 inches diameter, though I notice that unlike the DRR2, it isn't tapped. (The book was first published in 1962, and revised in 1972, by which time coils for homebrew sets were starting to go out of production. The original circuit used a 'Teleton' coil, by they'd gone out of business).

I've attached a pic, just of the coil winding instructions. You'll see that to cover LW, (with the wavechange switch open), the two sections section of the coil from green to black are in circuit, between aerial and ground, but for MW only, with the switch closed, only the section from green to blue is in circuit, with blue to black (which covers LW when in series with the green to blue winding), is shorted to ground. The full article on the one-valver can be found in pages 15 - 20 at this link:

http://www.americanradiohistory.com/...onics-Davy.pdf

The performance of the one-valver would have been far superior to the poorly designed and poorly written 'BBC Focus' transistor radio and cheaper to build at the time.

Hope that's of interest.
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Old 27th Sep 2017, 12:38 pm   #20
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Default Re: Can you wind your own Repanco DRR2 coil?

I used to worry about sections of coils being shorted, but it turns out that the different parts of a coil wound on a cylindrical former are only loosely coupled to each other. I just did a bit of algebra (quicker than finding a textbook). If you have two coils L1 and L2, with mutual inductance M (= k x sqrt(L1 L2)) and a short across L2, then the net inductance seen across L1 is then (1-k^2)L1. If k=0.5 (and it could be less than this) then you get 75% of L1. The reduction in inductance will have been taken account of when the coil was designed. There will be some reduction in Q due to losses in L2 but the low coupling again makes these small.

Shorting part of a toroidal coil wound on a ferrite or iron-dust former is not recommended, as here the coupling is much greater. I am not sure about coils on a ferrite rod, as these may be better coupled than air core (like the DRR2) but less than a toroid.
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