[AMRadio] Modulator feedback

John E. Coleman (ARS WA5BXO) wa5bxo2005 at pctechref.com
Fri Mar 17 17:58:19 EST 2006

Get a high inductance choke (modulation reactor) to shunt the DC out of the
secondary of the modulation XFMR.  It will kill about 75 % of the talk back.
What's left is all second harmonic talk back and it won't feed back.  Did
you ever use an old speaker that required DC to get the magnetism (no
permanent magnet)?  Not much sound comes out until the DC goes in to make
the magnetism for the voice coil to work against.


-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Rick Brashear
Sent: Thursday, March 16, 2006 9:43 PM
To: Discussion of AM Radio
Subject: [AMRadio] Modulator feedback

I've posed a couple of questions on some other lists and received some 
good advice, but I am still in need of more.  My BC-610-E has a bad 
feedback or "talk back" problem.  I've placed a capacitor across the 
overload relay (RY-5) and that did eliminate a little of the talk back.  
However, with much audio gain at all, not nearly enough to attain 100% 
modulation, I get feedback.  Not just talk back, but feedback.  I have 
to assume it's coming from the modulation transformer.  The modulator 
bias is set for 40 ma as specifications state.  The bolts on the 
modulation transformer are all tight.  What am I over looking here?  I'm 
using a BC-614-E speech amplifier with a D-104 non amplified mic.  I did 
a few modifications to the dynamic input circuitry of the BC-614 to 
better match the D-104, but that is the only modification in either the 
speech amp or the BC-610-E.


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