18-09-2016, 08:07 PM
I may, perhaps, be able to claim some modest expertise with the B18T/LV20, having now restored quite a good number of them.
As the circuit shows, the B18T/LV20 was designed to be used with a balanced feeder. I believe that there was an official Pye mod which was issued to convert it for use with a coax feeder, but I've never seen it. I assume that this will have involved replacing T1A.
What I have seen are all sorts of bodges from the past. Suffice it to say that some of these were more hygienic than others.
After quite a bit of thought and experimentation, the conclusion I eventually arrived at was that the only really satisfactory way of using one of these sets with a coax feeder is, having checked that the mod. done is not the official Pye one, to undo whatever has been done in the past, put the circuit back to "as designed" and use an external balun.
This is easily made by making two windings each of 10 turns of a convenient gauge of ECW (I find about 28 or 30 swg is a convenient compromise between ease of handling and fragility) on a miniature VHF ferrite toroidal core. If the windings are spaced diametrically opposite each other, this overcomes the problem that mains is more than most wire enamels are designed to withstand. The resulting coupling isn't, perhaps, as tight as is ideal, but this isn't a problem in a modern setting where, usually, more than enough signal is available from the r.f. source. Superglue is very useful to stop the windings "coming undone". It is important to use a grade of ferrite which is specified for VHF applications, however. HF ferrites are usually more lossy at 45MHz than is ideal.
I used to make mine in the plastic boxes which 35mm stills film cassettes were supplied in, though anything similar will do.
I believe that there is a problem of a similar nature with the BV20 (receiver strip tuned to Sutton Coldfield).
As the circuit shows, the B18T/LV20 was designed to be used with a balanced feeder. I believe that there was an official Pye mod which was issued to convert it for use with a coax feeder, but I've never seen it. I assume that this will have involved replacing T1A.
What I have seen are all sorts of bodges from the past. Suffice it to say that some of these were more hygienic than others.
After quite a bit of thought and experimentation, the conclusion I eventually arrived at was that the only really satisfactory way of using one of these sets with a coax feeder is, having checked that the mod. done is not the official Pye one, to undo whatever has been done in the past, put the circuit back to "as designed" and use an external balun.
This is easily made by making two windings each of 10 turns of a convenient gauge of ECW (I find about 28 or 30 swg is a convenient compromise between ease of handling and fragility) on a miniature VHF ferrite toroidal core. If the windings are spaced diametrically opposite each other, this overcomes the problem that mains is more than most wire enamels are designed to withstand. The resulting coupling isn't, perhaps, as tight as is ideal, but this isn't a problem in a modern setting where, usually, more than enough signal is available from the r.f. source. Superglue is very useful to stop the windings "coming undone". It is important to use a grade of ferrite which is specified for VHF applications, however. HF ferrites are usually more lossy at 45MHz than is ideal.
I used to make mine in the plastic boxes which 35mm stills film cassettes were supplied in, though anything similar will do.
I believe that there is a problem of a similar nature with the BV20 (receiver strip tuned to Sutton Coldfield).
There is hardly anything in the world that some man cannot make a little worse and sell a little cheaper, and the people who consider price only are this man's lawful prey.
John Ruskin
John Ruskin
