[ARC5] ARC-5/SCR-274-N IF Coil Mod

[Kenneth G. Gordon]

On 6 Mar 2014 at 12:15, D. Platt wrote:

> Gentlemen,
> 
> I've just completed the preliminaries of modifying a set of BC-454-A IF 
> coils to try and enhance selectivity.

Ah. Very interesting!

>  With some very basic information, 
> I have successfully moved the two coils apart about 1/2".

Hmmm....I move the coils apart in one 1415 KHz IF can, and thought afterwards 
that it might be too far apart. The BC-453 only moves the coils about 1/4". 
Still, with the IF at 1415 KHz, it might be necessary to move them that far.

>  There was 
> some trial and error in doing the first one, but I have found a fairly 
> easy way to accomplish the deed.

OK. Fill us in. I can also describe what I did. (I never finished that project, 
BTW, and am still waiting for a roundtuit to get to it.)

>  Having now done so, and making a 
> comparison of the two sets of coils, I find that selectivity has 
> improved by reducing the measured bandwidth about 2 kHz at the 6db 
> points.

That doesn't seem to me to be very significant. What was the 6db selectivity 
before you did this? Did you do all three IF cans?

>  I do find, however, that some additional external AF gain was 
> required as the losses in loose-coupling the IF coils has reduced 
> overall throughput amplitude.

Yes. That was the main reason I was ambivalent about moving them much further 
than about 1/4" apart. As I see it, max selectivity should not effect max IF 
gain. Perhaps there is a "sweet spot" where selectivity is improved as much as 
possible, yet gain is not sacrificed.

>  Has anyone else done the above and what 
> were your results?

See above. I have not yet finished my experiments on this. However, ARC did it 
right after WWII in order to attempt to make their older receivers compatible 
with modern needs. According to the report I have, they achieved 3 KHz 
selectivity (at the 6 DB point) even on the 40 meter receiver with its 2830 KHz 
IF.

>  Also, the coils are marked with the "S" (stabilized) 
> mark, as were other components in this junker set (made available 
> courtesy of W3PWW, Ted).  What, in reality, did the "S" modification 
> intend to do?

Supposedly, it marked the receiver as having much-improved stability in the HFO 
department so that they could be "lock-tuned" with more reliability. I have no 
idea what ARC did to achieve this. Perhaps Gordon White can tell us.

>  I sure can't readily see any component differences??

For one thing, I think at least one capacitor in the HFO was changed to a 
tempco type....but I'm not sure about this either.

Gordon?

Ken W7EKB