[ARC5] Easy selectivity increase for receivers.

[Kenneth G. Gordon]

On 3 May 2013 at 10:05, Richard Knoppow wrote:

>      The reason for adjustable coupling may have been to 
> make alignment easier.

Well, yes. But how and why it was done depends on which models of those receivers, and 
whether they had been modified by ARC for flatter selectivity, and more gain, or were 
original.

>     I have not looked at the instructions for this receiver 
> but suspect the variable coupling is used mostly for 
> alignment.

As I said, it depends on the particular receiver in question:

The earlier AN/ARC-5 maintenance manual, AN 16-30ARC5-2, we find the following in 
regard to the R-23A models:

"4-26. Flatter selectivity response near resonance was obtained by a redesign of the i-f 
coupling units. Coupling between the primary and secondary coils was altered and made the 
same in each of the units under operating conditions. In the modified receiver the i-f coupling 
units are aligned with the coupling rod up (loose coupling) but after alignment all of the rods 
are pushed in as far as they will go. (In the R-23/ARC-5 the rod in the first i-f coupling unit 
was pushed down but the second and third i-f rods were left up). Maintenance and repair 
shops particularly should note this difference and record it on their procedure charts."

In my original copy of the older SCR-274N manual, it specifically, for the BC-453, states 
clearly that the outer two transformers were to be set at the maximum coupling position 
(over-coupled), while the center one was to be set to the minimum coupling position, then 
were to be left permanently in that condition for operation.

All the manuals I have so far read mention moving all rods to the minimum coupling condition 
for alignment of the IFs.

In the much later NAVWEPS 16-30ARC5-501, the following statement is included:

"The receivers R-23, R-24 and R-23A normally operate with over-coupled I.F stages. For 
alignment, each I-F stage must be loosely coupled by pulling out the bakelite rod to the snap-
out position. After the entire alignment is completed, the I-F stage must be returned to the 
same over-coupled condition. For the R-23 and R-24, over-couple the first I-F stage; for the 
R-23A and R-148, over-couple all I-F stages."

This is a change from the earlier manuals.

Most hams who were aware of this variable coupling feature, when using those receivers as 
"Q-5ers", set all three transformers to their most selective, minimum coupled, position, then 
aligned the receivers, if they appeared to need it. Most didn't.

Ken W7EKB