[Milsurplus] TCS Audio

[W7QHO at aol.com]

In a message dated 7/3/06 8:58:03 PM, etgreeley at earthlink.net writes:


> Er...excuse my ignorance, pls, but what is the "neutralizing trick" to
> which you refer? Is it something different from normal neutralization
> "by the book"? With particular reference to the BC-223...
> 
> 

Ed,

Don't know who first discovered this, but there's a particular adjustment of 
the BC-375 PA neutralization cap which will virtually eliminate the conside
rable FMing these rigs exhibit under modulation.  In my experience this occurs 
with slightly more capacity cut in than the adjustment required for conventional 
neutralization, i.e., oscillator running, PA filaments off and minimum 
feed-through to the PA tank.  To find this "sweet spot" I run the set into a dummy 
load, adjust for 50W output, talk into the mic, listen on my Kenwood TS-50 
transceiver set to the FM mode and adjust the neutralizing cap for minimum audio 
out.  The correct setting is quite narrow and easy to miss. 

Note that if you operate the rig into a load different from the one used for 
the adjustment the rig may FM again.  I use a 50 ohm purely resistive dummy 
load for the adjustment and then operate on the air through an antenna tuner 
which presents the same 50 ohm load to the 375. 

Technique also works FB on other MOPA rigs with triode PAs like the BC-223 
(and, of course, the BC-191).  To load into 50 ohms requires a 100pf (or so) cap 
in series with the 223's antenna terminal, BTW.

Hope this is useful.

Dennis D. W7QHO
Glendale, CA