[600MRG] MF and the BC-453

[Kenneth G. Gordon]

I think the members of this list will find the following of interest. I sent it 
originally to another list.

Ken W7EKB  /24

On 11 Sep 2013 at 8:10, Mark K3MSB wrote:

> "I have all three IF transformer rods pulled up to their loosest
> coupling. The "other side of zero-beat" isn't even there."
> 
> What does that mean Ken?

The BC-453 (and the R-23/ARC-5) have a two-stage variable-coupling
feature in each IF can. These receivers are the only models of the
SCR-274N or AN/ARC-5 receivers which have this feature.

The top coil in each IF can is able to be moved between two "notches"
by pulling up on or pushing down on a fibre rod which protrudes from
the top of each IF can.

The bottom "notch" puts the top coil closer to the bottom coil, while
the top "notch" allows the top coil to be moved at least 1/4" further
away from the bottom coil, decoupling them and making the IF passband
considerably narrower. Gain is NOT reduced, BTW.

Each IF can has a fibre rod under the little "cover" that is screwed
to the top of each can. Removing that cover will reveal the rod.

One must be more than a little careful how one tries to move those
rods, as the coils inside the cans are covered in bees wax, which in
many (most?) cases has hardened to make the coils difficult to move. I
have broken the rods off while trying to move them.

Heating the can carefully will often soften the beeswax enough to
allow the rods to move the coils.

However, if the rods are damaged, it is very easy to remove each IF
can: they are held to the chassis by a 3-48 binder-head screw at
diagonally opposite corners, and plug into a special chassis-mounted
"plug" at their bottom. Also, they are color coded with a colored dot
on the top of each can at a corner so if you are awake, it is
impossible to get them in the wrong order.

Then 4 other smaller screws hold the cover on each can. These are also
easily removed, and one can then move the top coil to its top "notch"
by hand.

It is always much easier to work on the IF cans by first removing the
receiver's outer "case". You must loosen but not remove the screws
around the front edges, then remove the 8 screws near the rear, 4 on
each side. Then the cover lifts up at the rear, and slides off.
Originally all the screws were "color coded" in that those to be
removed were bright, while those to be either loosened or left alone
were anodized black.

As I indicated above, moving that coil very noticeably reduces the
width of the IF passband, making the selectivity of the whole receiver
actually too narrow for good SSB or AM reception, but almost perfect
for the narrower modes.

The Army service manual for those receivers tells the tech to pull all
the rods up to their most selective position when aligning the
receiver's IFs (85 KHz, BTW), then pushing the end two IF can's rods
back down for normal use, leaving the center IF can at the selective
position. Doing this flattens out the double-hump in the IF passband.

For OUR uses, having all three rods at their most selective position
is better.

Does this answer your question OK?

BTW, after WWII, AirCraft Radio Corp experimented with moving the IF
can coils in their receivers for other bands. For instance, they
easily achieved 3 KHz selectivity with the 40 meter (6 - 9.1 MHz)
receiver with its 2830 KHz IFs by adjusting the IF coil spacing to the
critical distance. That receiver is notorious for having a very wide
IF passband, making it almost useless for ham use. In stock condition,
one can hear every signal in the entire 40 meter band at once. As I
remember it, it is at least 30 KHz wide at the 3 db points.

I have moved the coils in the 80 meter (3 - 6 Mhz) receiver with its
1415 Khz IF. It is fairly easily done, and makes the receiver's
selectivity very noticably better. I install a small nylon spacer
between each coil after removing the original ceramic one.

Ken W7EKB and /24