BBC 1959 'Focus' Transistor Radio & DRR2 Coil

[AlanBeckett]

It's a pity Col got shot of his Wavewinder

Alan

[Yorkie]

I've yet to finish my little 'Gingery' hand wave winder, made from scrap - yet another of my 'almost finished' projects. I was intrigued by the excellent youtube video which enthused me to make it.

All I need to do to finish it is to make and fit the wire guide - most of the work has been done. In place of the 'tally counter' I fitted as shown in the book as a turns counter, I might instead use a small pocket calculator. You then use either a microswitch or reed switch so that for each turn it shorts out the '=' key on the calculator. Then to start counting turns, you enter '1' twice for it to become a constant, then each time the = button is shorted, with each turn of the coil, the calculator increments by 1, counting upwards - 2,3,4 and so on. Not my idea, but neat eh - a turns counter for less than a pound. (87p at Tesco - cheaper than Poundland!).

All I need now is to get a 'round tuit' and I'm sorted!

[pwdrive]

Have you got a signal yet David.

Lawrence.

[Yorkie]

Nah - I've checked the wiring, which is fine, and a 1kHz signal injected at the input of the diode comes through loud and clear in the phones, so the diode and transistor stages are working. I therefore think it's most likely the coil because the tuned circuit is just the coil, a tuning cap, the diode and a coupling cap - hardly rocket science. A DRR2 coil will soon be on its way to me from Big Al, so I can give that a spin, and if all's well, can check the inductances of the windings to enable me to hopefully wind a replica or two. I'm quite taken by the simple R.E.P one-valver circuit that Al posted, so I can see me knocking that up ere too long. Yet another diversion from other items on my 'to do' list!

I have a nostalgic fondness for little TRF valve sets - it was listening to one made by the teenage son of the proprietor of a Bed and Breakfast place we stayed at in Mablethorpe as a ten-yr old in 1949 that was the spark that got me into radio. He'd made what I later discovered was a one-valve TRF. Once he'd helped me master the reaction control, I recall excitedly listening to Morse code, and wondering if it was spies. (Maybe it was?).

Also, we listened to the local Coastguard station ship to shore radio, little realising that some 23 years later, I'd be taking my 12WPM Morse test at that station for my amateur radio licence! From the age of 12, I began building little radios with varying degees of success (or failure!), and when I started a newspaper round, I used to dawdle along scowering the pages of 'Hobbies Weekly' magazine for simple radio circuits before delivering the mag - well thumbed by then - through the letter box of the intended recipent!

Happy days.

I'm not going to be around much this next few days, so I'm afraid that tinkering will have to take a back seat.

[Yorkie]

I've finally managed to wind a replica DRR2 coil and got this little set working!

Alan Beckett kindly loaned me a genuine Repanco DRR2 coil so that I could try it in my set (it worked!), and measure the inductances of the various windings, as the diagram on the coil info data sheet is really quite poor, and not sufficient to enable me wind a replica coil. For example, though the diagram shows the inductance of the (tapped) MW coil as being 160uH, it doesn't give the inductance at the tapping point. Furthermore, the diagram infers that the major part of that coil is from pin 1 down to the tapping point at pin 2, with perhaps 10% or so from pin 2 - pin 3. In fact on measuring the inductance of that winding, the reverse is true:

From pin 1 to pin 2 the inductance is 30uH and from pin 2 to pin 3, the inductance is 70 uH, so the bulk of the winding is at the lower end of the winding. The total inductance of that winding is not - as might be imagined, the sum of the two - 70+30 = 100 uH - it is in fact 170uH. The reason for this is that inductors in series don't always behave in the same way as resistors in series. If two windings are wound on a former, one after the other, then L1 + L2 will more or less be the sum of the two, as would two resistors in series. However, when coils are close coupled, 'mutual inductance' arises, which affects the overall inductance. In this case, the two windings of the MW coil are very tightly coupled as they are wound on top of each other in a close-wound coil.

The inductance of the coupling winding (from pin 5 – 6) isn't mentioned on the data sheet, but the sketch of the coil at the bottom LH of the page infers that the coupling coil is smaller than the MW coil. In fact, the coupling coil is almost the same physical size. Hence, it wasn't surprising to find that the inductance of the coupling coil was only a little less than the MW coil, and was 145uH.

The LW coil is stated at 1.5mH.

As I had a coil former identical to the Repanco DRR2, I was able to wind a replica coil based on these findings. I simply wound on what I thought was more than enough turns of wire, measure the inductance, which was in excess of what was needed, and took a few turns at a time off the coil, measuring the inductance at each stage until it was that which was needed.

When I fitted the coil and attached an outdoor aerial and an earth, the set brought in at least six stations at very good volume, but as crystal sets lack selectivity, it wasn't possible to listen to any one station without other stations being heard in the background. That of course was one of the shortcomings of crystal sets that hastened their demise as more and more stations went on air. When there was just one station to be heard within the reception area, this wasn't a problem.

Making this little set was enjoyable and cost me nothing as the bits came out of my spares box. However, it did confirm my suspicions that anyone choosing to build this set back in 1959, would have been much better advised to have built a one valve TRF with the same coil, for no extra money, and would have found it a much better performer. I find it hard to believe that the BBC would put out such a tacky little circuit diagram back in 1959, and wonder how many disappointed novice constructors failed to get the set working, with the end result of putting them off rather than encouraging them to take up radio as a hobby. As I said earlier, building this set would have been quite costly

I've also attached the DRR2 Coil Data Sheet with some annotated notes that might help anyone wishing to wind a replica DRR2 coil. (They occasionally come up on e-bay – the last one I saw went for £21.00, which is frankly ludicrous, but nothing surprises me about e-bay any more).

In addition, I've added the data sheet for two mains valve sets using Repanco coils.

I hope these notes might interest someone who – like me from time to time - finds that nostalgia for lost youth occasionally triumphs over commonsense and sends me off on a frolic to build these little sets of yesteryear. When time permits, I'll knock up the little Repanco one-valver, for which I’ve attached the circuit. Three caps, two variable caps, two resistors, a coil and a valve - what's not to like about that?

I’m indebted to Alan Beckett for the loan of his DRR2 coil.

[pwdrive]

Thanks for the update David, mystery solved then.

Lawrence.

[Keithstv]

In the late 90s I was working in Coventry and we had some inductors wound by Repanco or Repanco-Bartlett as they were by then. We went to visit Repanco on Foleshill Road to discuss some aspects of the coil design. As we were walking through the factory I noticed a data sheet just lying around for the DRR2. I wish I'd asked if I could have had a copy.

I have built a number of crystal sets using the DRR2. One using 2 coils is incredibly selective managing to separate Talk Sport, Radio 5 Live, and Absolute Radio from the Moorside edge transmitter which I can see from my house 8 miles away. A single DRR2 crystal set brings in Talk sport and Absolute radio simultaneously with the tuning control appearing to act as a balance control varying the volume between the 2 stations. Radio 5 live not even audible on that set.

Keith

[Yorkie]

Thanks for reading the thread Keith, and for your interesting comments about your own experience, and about Repanco. They're very much still in business as Repanco Bartlett, and seem proud of their heritage in that they feature their little one valver on their website under 'About Us'. I rather fear that if we were to ask of they'd still make the DRR2 coil, they'd ROTFL!

http://www.repancobartlett.co.uk/index.p...om_content&view=article&id=2&Itemid=3

I really must finsh the little 'Gingery' hand coil wave winder that I referred to in post 12 and use it to wind some coils. It only needs the wire guide to be made and fitted, and some additional cams for various widths of coil, but I'm not happy with the crude 'bevel gear' arrangement, which is just an 'O' ring on a pully pressed hard against a penny washer coted with wet 'n dry, as can be seen in the drawing. I've decided to fit proper 1:1 bevel gears and yesterday ordered a pair of gears, which cost only £2.24 + £2.88 P&P and to my surprise, arrived today. Excellent outfit called 'motionco' which has a wide range of useful gears, shafts, bearings etc, all of which to my mnd seem keenly priced. Worth a look - see here:

http://www.motionco.co.uk/gears-bevel-ge...32_41.html

I'm not happy about the 'tally counter' to count turns on the coil winder, so I'm going to fit a small pocket calculator and microswitch instead. The switch is wired across the '=' button on the calculator, and when shorted out by the microswitch, increments each turn of the coil. The tally counter lever needs a bit too much pressure with each downward stroke to operate it in my view. I know that lots of these little wave winders have been made, and it was a chum who alerted me to the calculator idea, which he's sucessfully used.

All I need now is the fine gauge Litz wire, which I believe is in the same pigenhole as unicorn horns and hen's teeth! I do have some cotton covered wire, which I think will suffice.