[ARC5] Easy selectivity increase for receivers.

[john rose]

According to the ARC5 manual AN 16-30ARC5-2, the selectivity at 2 times the minimum signal input strength is 7.3 kc either side of the center frequency for the IF in the 3-6 kc receiver. In the 6-9.1 it's 13 kc either side of center frequency. Gee, doesn't that clear it all up? Given the convention used these days of quoting the total bandwidth, I would put my money (both nickels) on the author working with the 1415 kc cans. Was the surgery done on all three of the cans? I would expect so.

Bill, you're right that the IF coils are built differently. The coils in the R-23 and R-24 are not separated by any ceramic tube. The R-25, R-26 and R-27 are. The setup in the RAT 4200 kc is different again. At least this is what I can figure out from the drawings.


> From: wlfuqu00 at uky.edu
> To: kgordon2006 at frontier.com; Arc5 at mailman.qth.net
> Date: Fri, 3 May 2013 04:46:02 +0000
> Subject: Re: [ARC5] Easy selectivity increase for receivers.
>
> If I recall the low frequency receiver IF transformers are constructed differently than the higher frequency ones.
> I'll have to take a look. Reducing the coupling should increase the selectivity somewhat. Perhaps even
> putting a shorted copper strap turn between the two coils.
> Sounds likes there could be some experimentation here. However, I don't expect a lot of improvement at MHz
> IF frequencies. I liked the regeneration approach, Q multiplier more or less. Worked well for me
> in the 60's. Maybe after Dayton Hamvention and after I get some other projects finished, such as my completely restored1970
> hot rod VW Beetle. And my two Berkeley's. Never ending.
> 73
> Bill wa4lav
>
> ________________________________________
> From: arc5-bounces at mailman.qth.net [arc5-bounces at mailman.qth.net] on behalf of Kenneth G. Gordon [kgordon2006 at frontier.com]
> Sent: Thursday, May 02, 2013 6:35 PM
> To: Arc5 at mailman.qth.net
> Subject: [ARC5] Easy selectivity increase for receivers.
>
> I had run across the following method recently myself, then was told by Ray
> Osterwald of Electric Radio that Norm Chipps N3RZU had published the
> same info back in the June 1999 issue of ER magazine beginning on page
> 29.
>
> "Operating" on the IF cans of the 3-6 MHz and 6-9.1 MHz receivers,
> following the same method used to vary the selectivity in the LF receivers,
> results in greatly increased selectivity, and no apparent loss in gain.
>
> Remove IF cans' covers, remove the long screw down the center, and
> replace the shorter ceramic spacer in the center with the longer one that is
> on the bottom, then put the shorter one at the bottom, and replace the screw.
>
> Be careful of the wires that connect the coils to the connections: those may
> have to be heated up until the solder melts then "slid" down the connection
> wires to get the coils further apart.
>
> This reduces the bandwidth by 50%. In one case, this operation reduced the
> bandwidth from 15 Khz to 7 KHz, although the author did not make it clear to
> which receiver he was referring.
>
> Going further and moving ALL the ceramic spacers to the center, between
> the coils, reduces the bandwidth even further, but does reduce the overall IF
> gain a bit.
>
> Bandwidth in several cases was reduced to 4 KHz, but the author doesn't
> make it clear which receiver he is talking about here either.
>
> Apparently, according to the article, ARC did just this sort of thing with many
> of the receivers they bought back from the gummint after WWII, then sold.
>
> Kenneth G. Gordon W7EKB
>
> "Courage is being scared to death but saddling up anyway."--- John Wayne
>
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