[MRCA] Fwd: Re: [ARC5] ARC-5/SCR-274-N IF Coil Mod

[D. Platt]

-------- Original Message --------
Subject: 	Re: [ARC5] ARC-5/SCR-274-N IF Coil Mod
Date: 	Thu, 06 Mar 2014 16:35:58 -0500
From: 	D. Platt <jeepp at comcast.net>
Reply-To: 	jeepp at comcast.net
To: 	ARC5 at mailman.QTH.net



After a learning curve with the first coil, I found that the best way 
(sic) was to remove the long screw that holds the coil group together.  
Then, I carefully jigger the lower ceramic spacer out.  At this point, I 
carefully heat and slide the wires from the _lower_ coil bobbin down a 
bit to provide some slack.  I then move the lower bobbin downward about 
1/2".  I then replace the long screw up through the lower bobbin and 
then up through the sleeved upper portion and replace the tiny nut.  To 
hold everything in place, I used a drop of epoxy to the upper and lower 
bobbins and the long screw.  Its solid enough and holds alignment.  I 
believe that once you do one of these, the others will be a lot easier.  
I've been talking with Breck, K4CHE and his experience with this mod.  
He got some very good selectivity curves on his.  Now, what I will do is 
look up the audio mod in ER.  Also, I recall Norm (N3RZU) talking about 
this method a number of years ago, back when we used to work 
together.    Oh... before I forget. I mentioned this to Breck, but if I 
reduce the B+ on this receiver to about 150v (down from 220v), the thing 
seems to provide a lot more output?  I can't figure that, yet.  This one 
is also re-capped and, yes, I did all three coils.

Jeep - K3HVG

On 3/6/2014 3:18 PM, Kenneth G. Gordon wrote:
> 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
>
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