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

[J Forster]

Hi Robert,

Bridging the resistor with a Zener is a really bad idea, UNLESS you pull
the VR-150. The circuit is a SHUNT regulator. The problem is that a
zener has a really low incremental impedance (a few ohms for a 15 V
device) so any increase in line voltage sufficient to make the Zener
conduct, effectively reduces the current limiting resistor from several
K Ohms, to a very few. This will fry the VR tube very quickly due to
excessive current. Not a good thing.

If you do pull the tube, the situation is still not good. Any voltage
variations in the unregulated supply, will be passed on to the
instrument proper, volt for volt. In fact the % regulation is made worse
by such a scheme.

If you really want to regulate it, I'd tack in a three terminal
regulator IC taking the input before the series resistor, and the output
to the + end of the VR tube, and the common to ground through the
appropriate resistive divider to set the desired voltage. And you pull
the VR tube.

Best,
-John

=====================

WA5CAB at cs.com wrote:

> 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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