[Boatanchors] Re: Collins R-388 BCB mods

[Bill Cotter]

DW & Amen John:

The filters Joe Carr designed would work as expected _if_ the 
R-388's input impedance was 50Z, which it is not. At the very 
least, one would need the 12:1 un-un as proposed earlier. Even if 
the R-388 WAS 50Z at the RF input connection, the Carr filter 
renders the receiver useless on 120M, 160M and the BC bands. Not a 
very good compromise if you use your receiver for those bands.

And, blocking out BCB signals does nothing for overload and 
intermod on all the higher bands caused by strong adjacent stations 
and the low-Q impaired RF stage. I suppose if we all used whip 
antennas (as the receiver was designed) we wouldn't have as much 
problem. However, many of us have fairly decent antenna systems 
with 50Z source impedance. Even with a 12:1 un-un, the Q of the RF 
stage is severely damped.

For those owners who use their receivers, and are willing to modify 
them (W3HM & other mods) for better performance, link coupling of 
the band coils alleviates much of the problem by raising the Q of 
the front-end significantly, and goes a long way towards extending 
the utility of the receiver. It requires a minor amount of work and 
a realignment.

For the purists: The modification proposed is totally reversible if 
a future owner prefers to have a 'collectors item.'

73 bill n4lg

At 04:55 PM 10/1/2007, DW Holtman wrote:
>Hello,
>
>In my opinion, a lot better way to deal with BCB interference is 
>to build a high pass filter with a cut off frequency of around 3 
>MHZ. An excellent article on this subject is written by Joseph 
>Carr. It can be found at
>
>http://www.dxing.com/tnotes/tnote06.pdf
>
>I have built several of these units both the one on page 6 and the 
>one on page 7. They are easy to build, parts are readily available 
>and they work very well. This method does not require modifying 
>your receiver, something I hate to do.
>
>Best,
>DW Holtman
>WB7SSN


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Bill Cotter, N4LG             E-mail: n4alg at qx.net
173 Carolyn Lane               Home: (859) 887-5563
Nicholasville, KY 40356-9340   Work: (859) 323-6474

ARRL   ARCI #9878   QRPl #1646   QCWA #29393   FISTS #4535
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