[600MRG] [ARC5] MF and the BC-453

[Kenneth G. Gordon]

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