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.
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.
Regards, David.
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
