[Milsurplus] BC-312 rec selectivity

D C Macdonald [email protected]
Wed, 28 Apr 2004 01:57:32 +0000


This resistor is referred to as a "swamping" resistor.
They are used to broaden the frequency response
of an LC circuit by lowering the Q of that circuit.

Many years ago, I had an NCX-5 that would lose
output fairly quickly after installing new final tubes.
One day while looking for another problem, I noticed
a resistor across one of the interstage coils in the
transmitter chain.  Remembering a lesson in Air Force
tech school, I removed it to see what would happen.
When I loaded up, power went  WAY  up, and the
coil smoked a bit before I could unkey.  I then put
the ohmeter on the resistor in question.  I was shown
as a 4.7 k in the manual.  It had dropped down to
just below 1 k.  After I replaced the resistor, I never
had anymore problem with low output.

Sorry this is so long, but I wanted to give another
example of this old-time radio technique.  If you
got TOO narrow in audio response by removing the
resistor you could replace it with a larger one to
get a smaller increase in Q and selectivity.

Mac, K2GKK/5
Oklahoma City



----Original Message Follows----
From: [email protected]
Reply-To: [email protected]
To: [email protected], [email protected]
Subject: [Milsurplus] BC-312 rec selectivity
Date: Tue, 27 Apr 2004 18:32:48 -0700

I'm using a different browser so i don't remember just where the
question concerning selectivity on this series resistor originated.  To
Quote Bill:


"The resistor is across the primary of the first IF transformer, L29.
It's
R38, 60K, and when removed increases the selectivity without hurting AM
audio quality.  Removing it makes my BC-312 more selective than my two
BC-348Q's.  About 6kc for 6db down".

Bill N6PY

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