30-10-2012, 01:24 PM
A chum wishes to build the two-transistor radio which featured in the BBC ‘Focus’ magazine in 1959, which used a Repanco DDR2 MW/LW coil. Having looked at the article, which I assume was intended to be for a novice to build – perhaps a schoolboy, in my view, it was very poorly written and the circuit was very poorly drawn, with sketchy and difficult to follow constructional information. It says ‘any headphones or a ‘deaf aid’ earpiece will do’ but I suspect that only high resistance headphones would be satisfactory if connected straight off the OC71 collector, or a crystal earpiece rather than a magnetic one. I think that to use low resistance headphones it would need a small output transformer such as an LT77.
We forget how expensive components were back then and at face value, they seem cheap by today’s standards but they most certainly weren’t!
The article specified Mullard OC71 transistors - 21 shillings each at the time, (which equates to £21.00 each in 2012 when adjusted for historical inflation), or red spot surplus (out of spec) transistors at 7/6d each, equivalent to £6.70 each in 2012 – yet it gave no precautions or advice about avoiding damage when soldering them. Very irresponsible of the BBC at the time in my view. Excluding the transistors and headphones, the rest of the components came to 19 shillings – about £18.00 at today’s prices. With a pair of red spot surplus transistors, the total cost would have been £1.14s. 0d – about £33.00 at today’s equivalent or with a pair of OC71s, £3.1s.0d - a staggering £59 at today’s prices!
See this website for historical inflation calculations:
http://safalra.com/other/historical-uk-i...onversion/
Basically, it’s just a crystal set with a two transistor amplifier using low gain OC71s (typical HFE 50 – 60), so won’t have been very selective, and probably not very sensitive. 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!
Right, to cut to the chase, once more, nostalgia triumphed over commonsense and to help my chum I found a suitable coil former on which to wind a coil along the lines of the DDR2, but having wound one, because the info on this coil is very sketchy, I decided to knock up the radio to test the coil. I had all the bits in my spares box to make the radio, so it cost nothing. I knocked up a simple PCB, with no attempt at miniaturisation. (The set is about 6" long, 3.5" high and 3" deep).
The only data I can find on the DDR2 coil (which is quite helpful) shows the layout of the coils on the former, and the inductance of both the MW and LW coils (the LW of course being in series with the MW one when switched to LW, or when switched to MW, the LW winding shorted to ground). What it doesn’t give, is the tapping point on the MW coil, or the inductance of the aerial coupling coil. I made an assumption that the tapping on the MW coil would be about 10% from the bottom, and by comparison of it’s smaller size on the diagram as compared to the 160uH MW coil, that the aerial coupling coil would be about 50uH. I wound all of the coils in the same direction so they’d all be in phase. Hence, by reference to the diagram, pins 1,3 and 5 are the start of the three windings – the MW coil being at the top, the aerial coupling coil in the middle, and the LW coil at the bottom.
The amplifier section of the set works (I’ve injected a signal at the base of TR1), but there isn’t a peep out of the set. I’ve used a decent outside wire aerial, and tried several germanium diodes (1N34, OA90 etc), but to no avail.
Anyone got any thoughts about my assumptions on the likely inductance of the coupling coil, or tapping the MW coil at 10% from the bottom? Ideally, what would be most helpful is in anyone has a DDR2 coil and the means to measure the inductance of the windings.
Incidentally, though the original coil will probably have been wound with Litz wire, which might improve the ‘Q’ very slightly, whereas mine is would with cotton covered enamelled 48SWG wire, I’ve measured the inductance of the MW and LW coils and they’re 160uH and 1.5MH respectively. I could perhaps try reversing the aerial coupling coil to see if that brings the set to life.
I’ve attached a copy of the DDR2 coil data, such that it is, and the original BBC2 article can be found here, which rather makes me wonder why people go into raptures over such things unless they’re wearing rose tinted specs:
http://www.daveysradios.org.uk/bbcfoclta3.pdf
A couple of pics of the radio I’ve knocked up are also attached. Like rather too many of my projects over the last 60 years or so, it looks reasonably workmanlike, but is just ornamental rather than functional! (The front panel is scrap PCB material sprayed white – the markings on the ‘dial’ have no significance other the degrees of radius of course).
So, if anyone has read down to here, the bottom line is: Any thoughts about the replica DDR2 coil please?
We forget how expensive components were back then and at face value, they seem cheap by today’s standards but they most certainly weren’t!
The article specified Mullard OC71 transistors - 21 shillings each at the time, (which equates to £21.00 each in 2012 when adjusted for historical inflation), or red spot surplus (out of spec) transistors at 7/6d each, equivalent to £6.70 each in 2012 – yet it gave no precautions or advice about avoiding damage when soldering them. Very irresponsible of the BBC at the time in my view. Excluding the transistors and headphones, the rest of the components came to 19 shillings – about £18.00 at today’s prices. With a pair of red spot surplus transistors, the total cost would have been £1.14s. 0d – about £33.00 at today’s equivalent or with a pair of OC71s, £3.1s.0d - a staggering £59 at today’s prices!
See this website for historical inflation calculations:
http://safalra.com/other/historical-uk-i...onversion/
Basically, it’s just a crystal set with a two transistor amplifier using low gain OC71s (typical HFE 50 – 60), so won’t have been very selective, and probably not very sensitive. 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!
Right, to cut to the chase, once more, nostalgia triumphed over commonsense and to help my chum I found a suitable coil former on which to wind a coil along the lines of the DDR2, but having wound one, because the info on this coil is very sketchy, I decided to knock up the radio to test the coil. I had all the bits in my spares box to make the radio, so it cost nothing. I knocked up a simple PCB, with no attempt at miniaturisation. (The set is about 6" long, 3.5" high and 3" deep).
The only data I can find on the DDR2 coil (which is quite helpful) shows the layout of the coils on the former, and the inductance of both the MW and LW coils (the LW of course being in series with the MW one when switched to LW, or when switched to MW, the LW winding shorted to ground). What it doesn’t give, is the tapping point on the MW coil, or the inductance of the aerial coupling coil. I made an assumption that the tapping on the MW coil would be about 10% from the bottom, and by comparison of it’s smaller size on the diagram as compared to the 160uH MW coil, that the aerial coupling coil would be about 50uH. I wound all of the coils in the same direction so they’d all be in phase. Hence, by reference to the diagram, pins 1,3 and 5 are the start of the three windings – the MW coil being at the top, the aerial coupling coil in the middle, and the LW coil at the bottom.
The amplifier section of the set works (I’ve injected a signal at the base of TR1), but there isn’t a peep out of the set. I’ve used a decent outside wire aerial, and tried several germanium diodes (1N34, OA90 etc), but to no avail.
Anyone got any thoughts about my assumptions on the likely inductance of the coupling coil, or tapping the MW coil at 10% from the bottom? Ideally, what would be most helpful is in anyone has a DDR2 coil and the means to measure the inductance of the windings.
Incidentally, though the original coil will probably have been wound with Litz wire, which might improve the ‘Q’ very slightly, whereas mine is would with cotton covered enamelled 48SWG wire, I’ve measured the inductance of the MW and LW coils and they’re 160uH and 1.5MH respectively. I could perhaps try reversing the aerial coupling coil to see if that brings the set to life.
I’ve attached a copy of the DDR2 coil data, such that it is, and the original BBC2 article can be found here, which rather makes me wonder why people go into raptures over such things unless they’re wearing rose tinted specs:
http://www.daveysradios.org.uk/bbcfoclta3.pdf
A couple of pics of the radio I’ve knocked up are also attached. Like rather too many of my projects over the last 60 years or so, it looks reasonably workmanlike, but is just ornamental rather than functional! (The front panel is scrap PCB material sprayed white – the markings on the ‘dial’ have no significance other the degrees of radius of course).
So, if anyone has read down to here, the bottom line is: Any thoughts about the replica DDR2 coil please?
