[ARC5] Easy selectivity improvement....isn't... (long)

[Kenneth G. Gordon]

The article in ER magazine about moving the ceramic spacers by Norm 
Chipps discusses how ARC did it, not how "normal" hams did it. It seems 
clear to me by now that no ham I have ever heard of tried this procedure, 
and there is no wonder that it hasn't been tried....at least to my knowledge.

For us, the process is most certainly NOT easy, by any stretch of the 
imagination.

I've spent an hour or several dismantling a 1415 KHz IF transformer and 
examining it closely, shopping for and buying suitable parts, then 
reassembling it with increased spacing between the coils.

The longer ceramic spacer at the bottom is threaded: 3-48 on the top and 
4-40 on the bottom. This makes it impossible to simply move from the 
bottom to the center. That threaded "spacer" holds the entire assembly 
together, in fact.

Furthermore, finding 3-48 screws longer than 1" is almost impossbile, 
although McMaster-Carr does have a 3' (foot) long piece of 3-48 all-thread 
for $1.97.

The two smaller spacers between the two coils are not threaded, and the 
hole through the center of each one is larger in diameter than the long 3-48 
screw that goes down through the center of the assembly to hold it together. 
The reason for this size difference will be clear a bit further down this e-mail.

There is also a metal disk with a hole through the center that is spaced 
midway between the two coils. The small ceramic spacers rest against either 
side of this metal disc, and the IF coils rest against the small ceramic 
spacers.. 

Although I haven't yet checked to be certain, that metal disk appears to be 
grounded. At least it is soldered to one of the wires that connects to one of 
the pins on the bottom connector, and what appears to be the center-tap of 
the top coil is also soldered to this same wire.

I am not sure what is the purpose of this metal disk. Perhaps someone here 
can tell us what it is for. I suspect it has something to do with "adjusting" the 
passband curve since it probably constitutes a shorted turn. Or maybe it is 
simply a shield between the two coils.

Lastly, there is a fiber insulated "pipe" which surrounds the long 3-48 screw, 
insulating it from the metal disk and from both of the ferrite cores of the coils

So, I went to the local Michaels craft supply store today, and found a fairly 
large tin of plastic beads that are about the same diameter, and have about 
the same sized holes through their centers as the small ceramic spacers and 
bought this. It was $6.00 and has enough "beads" in it to "do" several 
hundred IF cans, if I ever felt that ambitious.

I also stopped at our local hardware store and bought 6 ea pieces of 3" long 
4-40 "all thread". Those were more expensive than the entire large can of 
beads.

The IF can has a piece of fiber "square stock" about 1" long and about 3/16" 
square at the top of each can which holds the assembly centered in the can, 
and through which the long 3-48 screw goes to hold the assembly together.

I carefully wrapped one layer of electrical tape around about 1 3/4" of the 
4-40 all-thread, leaving about 1/4" un-covered at the "top" end and an inch or 
so at the bottom end.

I then cleaned out the centers of the two coils' ferrite cores with a 1/8" drill bit 
as there was some rust and bits of glue in each one, then starting at the 
bottom and after enlarging the hole in the bottom piece of mica with my 
Dremel tool and a grinding bit, passed the insulated 4-40 all-thread screw up 
through the bottom coil, one of the plastic beads, one of the small ceramic 
spacers, through the metal disk, another of the small spacers, another plastic 
bead, through the top coil, and then carefully "screwed" the end of the 4-40 
all-thread into the fibre spacer at the top while making sure that the coils 
didn't rotate..

This assembly JUST fits in the necessary space, and when I cut the end of 
the all-thread off, and add a 4-40 nut, flat washer, and lock washer, the 
assembly will be as solid as it was originally, and will plug right back into the 
receiver.

I did this mechanical "exercise" before I had taken the trouble to measure a 
properly aligned receiver's IF passband and its shape since after dismantling 
the IF transformer I was not at all certain that I could get it back together with 
the increased spacing between the coils that would be needed to achieve 
increased selectivity.

It can be done, but it certainly isn't easy.

Also, I am not at all sure that my spacing might not be too much, and will 
badly effect gain through that stage. The spacing between the coils is about 
1/16" greater than the length of the long ceramic spacer which was originally 
suggested to be used to increase the spacing between the coils.

So, the next exercise will be to carefully align a receiver with a standard set 
of IF cans in it, then do the same with a set of "enhanced selectivity" IF cans 
in it, then measure passband shape and selectivity.

I'll "rebuild" the other two IF cans, then will see what I get when I measure 
the IF passband and gain.

Lastly, I am not at all sure, at this point, that the result is worth the effort.

Kenneth G. Gordon W7EKB

"Courage is being scared to death but saddling up anyway."--- John   Wayne