16-03-2012, 07:23 PM
(This post was last modified: 17-03-2012, 04:16 PM by Robert Darwent.)
Continuing with the project from post #7;
Winding the SW coils
Fortunately, I had a head start when it came to making the four 'new' coils required. I had scrap Ekco A23 and A28 sets from which to scavenge parts. The A28 chassis in particular was most useful, having several bandspread short wave ranges and sixteen coils in its coil-pack. The coils are of the same overall size and shape as those used in the A22, so it was possible to use some of those as 'donor' coils rather than start completely from scratch with a bare coil former.
I found three donor coils that gave the inductance values I had calculated simply by rotating the alignment slug in or out, the value needed being comfortably within the coil's existing range. For the fourth, the best match I had was a donor coil that was slightly higher in inductance, but by removing a couple of turns of wire from its winding I brought the value down into the required range.
It was a simple matter to remove most of the wax sealing this coil's winding with a soldering iron, unsolder one end and take off a couple of turns. I was careful to ensure that enough wax was left on the winding to keep the other turns firmly in place whilst carrying out this procedure.
Each of the four coils now needed a coupling winding adding over the main inductance winding. From dismantling a similar spare coil, I determined that this coupling winding consisted of approximately five to six turns evenly spaced on top of the main winding. Using a similiar gauge enamelled copper wire, I added this coupling winding to each of the four coils in turn. The wire ends were terminated by bringing them out through unused holes in the top of the former, later permanently secured by soldering to small solder-tags.
Below is one of the coils resealed with wax after the addition of the coupling winding;
Next came the task of stripping the existing A22 coil-pack. Below are images of how it looked originally;
After careful removal of all the components I was left with the following unpopulated board;
Note; the addition of an extra trimmer capacitor to the board. This was necessary due to all three coils in the aerial section requiring a tracking trimmer. The coil-pack used in an original A22T also has this extra 'sixth' trimmer, compared to only five found in a standard A22 set.
The additional trimmer was scavenged from the scrap A23 chassis. I found that I could modify the mounting arrangement used by the group of two existing trimmers to securely hold it in place. This leaves the sixth trimmer standing a few millimetres higher than the others, but since its fixing is mechanically sound, I considered it acceptable 'tacked' on to the end of the board in this way.
Winding the SW coils
Fortunately, I had a head start when it came to making the four 'new' coils required. I had scrap Ekco A23 and A28 sets from which to scavenge parts. The A28 chassis in particular was most useful, having several bandspread short wave ranges and sixteen coils in its coil-pack. The coils are of the same overall size and shape as those used in the A22, so it was possible to use some of those as 'donor' coils rather than start completely from scratch with a bare coil former.
I found three donor coils that gave the inductance values I had calculated simply by rotating the alignment slug in or out, the value needed being comfortably within the coil's existing range. For the fourth, the best match I had was a donor coil that was slightly higher in inductance, but by removing a couple of turns of wire from its winding I brought the value down into the required range.
It was a simple matter to remove most of the wax sealing this coil's winding with a soldering iron, unsolder one end and take off a couple of turns. I was careful to ensure that enough wax was left on the winding to keep the other turns firmly in place whilst carrying out this procedure.
Each of the four coils now needed a coupling winding adding over the main inductance winding. From dismantling a similar spare coil, I determined that this coupling winding consisted of approximately five to six turns evenly spaced on top of the main winding. Using a similiar gauge enamelled copper wire, I added this coupling winding to each of the four coils in turn. The wire ends were terminated by bringing them out through unused holes in the top of the former, later permanently secured by soldering to small solder-tags.
Below is one of the coils resealed with wax after the addition of the coupling winding;
Next came the task of stripping the existing A22 coil-pack. Below are images of how it looked originally;
After careful removal of all the components I was left with the following unpopulated board;
Note; the addition of an extra trimmer capacitor to the board. This was necessary due to all three coils in the aerial section requiring a tracking trimmer. The coil-pack used in an original A22T also has this extra 'sixth' trimmer, compared to only five found in a standard A22 set.
The additional trimmer was scavenged from the scrap A23 chassis. I found that I could modify the mounting arrangement used by the group of two existing trimmers to securely hold it in place. This leaves the sixth trimmer standing a few millimetres higher than the others, but since its fixing is mechanically sound, I considered it acceptable 'tacked' on to the end of the board in this way.
