[Hallicrafters] 3 IF stage Receivers

[Chuck McGregor]

>I'm not sure about whether having two or three IF stages makes much
>difference in BC band performance but I do know most general coverage 
>communications
>receivers have their sensitivity on the BC band lowered on purpose 
>probably to
>limit interference. Perhaps someone who knows can share how this is
>accomplished? Do they have a resistor across the BC band antenna coil to 
>numb the
>sensitivity?

Greg -
On the SX-71, Hallicrafters uses section 5-R of the bandswitch to switch 
the cathode resistance of the 6SK7 second IF amp.
Total resistance between the cathode and the sensitivity control line is:
Band 1     1680 ohms    (BCB)
Band 2     1K                 (1.6-4.7Mc)
Band 3     3700              (4.7-13.4Mc)
Band 4     1680              (12.8-34 Mc)
Band 5     1680               (46-56Mc)
Why its the same, for the broadcast band and the top two bands,  I don't 
know.  Can anyone shed some light on this ?
(Another switch, SW-2, section 1, reduces the cathode resistance on the 
first IF when the FM demod is selected.)

Phil Atchley, KO6BB has suggested rebiasing the second IF stage for more 
gain. His note didn't mention whether he returns the 2nd IF cathode 
resistor to ground, as the 3rd IF does,  or to the manual sensitivity 
control line as the 1st IF stage does.

73,
Chuck N7RHU


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