[AMRadio] Modulator feedback


Rick Brashear rickbras at airmail.net
Fri Mar 17 18:09:45 EST 2006


Thanks John, that's a good idea.
Rick

John E. Coleman (ARS WA5BXO) wrote:

>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.
>
>John,
>WA5BXO
>    
>
>-----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.
>
>Rick/K5IZ
>
>
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