[Milsurplus] Help with BC-721

[David Stinson]

----- Original Message ----- 
From: "Bob Blecke" <cqm964 at comcast.net>
To: <milsurplus at mailman.qth.net>
Sent: Sunday, April 01, 2007 10:14 AM
Subject: [Milsurplus] Help with BC-721


>A friend and I obtained a pair of BC-721's in excellent condition, both set
to the >same frequency (verified by looking at the crystals, etc.).....
>am not getting clear audio.......

First, congrats on having $700-$900 worth of radios
(depending on which way the wind is blowing :).

First thing:  Check the switch in the bottom cover of
the radio- the one that's supposed to be activated by
a rod that projects from the radio when the antenna is folded.
Make sure those contacts are connecting you to the
use the internal microphone, not the external throat mike jack.

Second: there's something different about the circuit of
a 721 from a 611, that can make it inoperative, but I can't
remember right now what it is and I've lost my 721 manual
in the last move.

Robert Downs  is the expert, but here's a couple of tips
from my experiance reworking a half-dozen BC-611s:

The microphone elements are often bad.
Even some new ones are either low output or just sound bad.
IIRC, Robert has said the the magnets age
and don't give the proper field to the moving coil.
I've replaced dead elements with small 32-ohm speakers
(8- and 16-s don't have enough voltage output),
which I feed into the salvaged microphone transformer.
Those sound pretty good and give plenty of drive.

Transmitter/Antenna tuning on the BC-721/611 is very critical
and it does effect modulation.  You really must have the
correct tuning case and follow the procedure if you want the
radios to work correctly (or at all).
Perhaps someone who's retired and not trying to raise a
house full of kids would set up a BC-611/721 "tune-up" service
for a fee.

73 Dave S.