[Milsurplus] Stablizing a BC-221 that has an RA-133 installed

[WA5CAB at cs.com]

The post-war RA-133 (and PP-79/UR in BC-221-B and -Q) is an AC operated 
rectifier supply built to replace the dry batteries in the BC-221.  B+ is regulated 
by an 0D3 (VR-150).  The +150 is dropped to the +135 volts that the BC-221 
was designed to operate from with a 2500 ohm 1 watt resistor.  There are two 
things wrong with this.  First, you can easily hear the oscillator frequency 
shift as you switch modes and the current requirement changes (it's worse than 
when operating off of dry batteries which I did once just to see).  Second most 
of the 2500 ohm resistors today have drifted up to 3500 ohms or higher.  The 
simple solution is to bridge the resistor with a 15 volt (or 14 or 16) 1 watt 
zener diode.  This regulates the B+ to the set at a solid 135 (or 136 or 134) 
VDC.

In the unlikely event that the resistor is still under 3K you may need to 
replace it with a 3.9K or 4.7K.  Actually you could just clip one end of the 
resistor I guess although I never had to consider that.  And it would put all of 
the current through the zener.  I have considered the possibility that maybe 
this resistor was built by the same people who built Black Beauties.  :-)  

Robert Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480   
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