Yes, I must agree with Richard's well written
explanation of
the effects of overcoupling (more than critical coupling). However
I do not believe that in the instance of the 453's 85kc IF cans that
with the rods pushed all the way in that they became overcoupled.
With resultant double peaked bandpass.
This is of course is just my observation with a bit of practical
measurment
using relatively crude methods. It would seem then,to me, that the double
peaked
curve encountered with overcoupling (more than critical coupling)
is, in this instance, not the condition that results in a narrower
bandpass
when the coils are moved further apart.
During the glory days of TV service, IF alignment with sweep
generators and
oscilloscopes, it was quite fascinating to see theoretical bandpass
tuning in the
real world. Overcoupling was often (always?) used to achieve the
necessary wide
bandwith for a TV channel. While transformer coupling was not normally
adjustable, the peaks and skirts of the quite wide bandpass could be
moved
around with the adjustments available, and measured with fairly simple
equipment.
But I digress (a lot, sorry)
Old Chief Lynn
A lot TV sets and other equipment with wide band IF's used stagger
tuning. This works fine but is a PITA to align. As you say a sweep
generator, markers, and a scope are the only way. Actually, I've found
sweeep alignment of standard plain vanilla IF's is often helpful
especially if you want to get the peak symmetrical.
I've tried to find a schematic or other info on the BC-453 but have not
so far been able to. Evidently the IF is different from other command
sets. There is plenty of info on the other receivers and transmitters but
somehow this one seems to have fallen through the cracks.

Richard Knoppow

Google found a rather poor schematic, but it doesn't provide
much enlightening info on the 85kc IF cans. I think I got the
schematic successfully posted on ABPR.
Even when the 453 was a hot item, I could never find much that
explained it's bandwidth adjustment. Just a few articles in 73, QST
and what was that other popular ham rag? They just said pull the
rod out. I, of course did pull one out. By the roots. But, no problem,
there were plenty more available without attempting a repair. Used
ones with a little mud, or having been stepped on were $5 or less.
Oh, and yes, I guess that "stagger" tune was the common way that
TV set IF's were (are?) broadbanded.
Lynn, W7LTQ