[Hallicrafters] SX 42 question

[Charlie T]

Think about what that resistor is ultimately doing.
The tuned plate circuit is relatively hi impedance, so any resistor below
about 10kΩ across it will significantly lower the Q, and there-by
drastically increase the bandwidth.
This resistor ''may" have been added to broaden the passband of the front
end.

I don't believe it is terribly important as obviously, earlier (or later ??)
designs eliminated it.

Personally, I'd remove it and see how it works.

After all, I'd think you'd want a fairly narrow tuned front end for 10 & 6
anyway.

73, Charlie k3ICH

 


-----Original Message-----
From: hallicrafters-bounces at mailman.qth.net
<hallicrafters-bounces at mailman.qth.net> On Behalf Of w5jo at brightok.net
Sent: Sunday, July 21, 2019 11:37 AM
To: hallicrafters at mailman.qth.net
Subject: [Hallicrafters] SX 42 question

Hi all,

My SX-42 has a bit of a puzzle that I have been unable to unravel, maybe
someone here can help.  The build date on the chassis has been obliterated
and the serial number plate is missing.  However, the original paperwork,
including the service bulletins, accompanied the set and is dated Jan 1949. 
The radio has the components and wiring that matches the diagram, save for
one I have found.

There is a 4.7 K resistor across the primary of T7 which is the plate coil
in the first RF stage for band 5 (28-55 megacycles).  My diagram does not
show that resistor and looking at other diagrams that I can find shows a 1 K
resistor across the primary.

Can anyone point to a service bulletin that outlines the change from the
early version with the 1 K resistor and change to a 4.7 K.  As I said, my
diagram does not show a resistor in that location at all.

Jim
W5JO


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