[Milsurplus] Analysis of RAK/RAL Power Supply CND-20131

Thu Dec 17 09:42:42 EST 2020

Having recently obtained an RAL and CND-20131 Power Unit myself, I couldn't
find your post more timely.  Thanks, Richard for sharing your data.

My testing revealed much the same information except my ballast tube /
power resistor arppear shorted) so the output voltages when I first checked
were out the roof - the ballast circuit is made to yield 60 volts or so to
those transformer windings and they were getting the full 120V so output
was twice intended - thankfully I had ordered some power resistors to load
the disconnected power supply so I didn't put that on the receiver.

With the supply switched to not use the ballast circuit AND the ballast
tube and resistor removed, I still get 220 volts DC and higher rather than
180-185 which the manual would suggest is correct.  The screen voltage
output is correct at 90 volts so the regulator is working.  I tested with
4K, 8K, 12K, and 16K power resistors as dummy loads.

All the above with 125 volt AC input as 120vac is uncommon these days.

So, okay to attach my RAL at 220 -240 VDC?  She does appear to be a tough
old bird.

On Wed, Dec 16, 2020 at 8:02 PM Richard <brunneraa1p at comcast.net> wrote:

> Having a CND-20131 which had not been "hammified" I decided to restore it
> to original and see if it worked as intended, having doubts about the 876
> ballast resistor and 3 mfd filter capacitors.  I replaced two
> non-functioning toggle switches, aged carbon resistors, and not having a
> good 874 voltage regulator, made an octal to four pin adapter, and not
> having a good 874 used an OC3/VR105 which was close enough, not having an
> OB3/VR90.
>
> The power transformer has low voltage taps for the ballast tube labeled
> 110, 115, and 120 volts, and full voltage taps marked 100, 115 and 120
> volts for direct line voltage connection.  The ballast resistor is in
> series with an 80 Ohm 200 Watt resistor connected across the line, and the
> power transformer is connected across the 80 Ohm resistor.  Theory is, as
> line voltage varies, the ballast resistor resistance will vary, maintaining
> constant voltage to the transformer. The ballast resistor is rated 1.7
> Amps, 40-60 Volts.
>
> First, I ran a curve on the ballast resistor + 80 Ohm resistor with no
> load on the power transformer, (5Z3 rectifier removed) and found no
> regulation.
>
> *Vin*           *V80 Ohm resistor*
>
> 90                 79.5
>
> 100               87.3
>
> 110               95.0
>
> 120             101.9
>
> 130             107.5
>
> Next I ran a curve with the direct connection with the 120 volt tap, and
> found about a 200 cycle frequency shift in the RAL over 90 to 130 volts
> which I find perfectly acceptable.  This was due to a one volt shift in the
> OC3 regulator.
>
> *Vin*       *B+*      *Vv reg.*    *Iv reg. ma.*
>
> 90       153.6      105.4       15
>
> 100     174         105.6       22
>
> 110     189.5      105.7       27
>
> 120     207         106         32.5
>
> 130     224         106.4      37.9
>
> Next was with the 876 ballast tube resistor in service.  There was no
> frequency shift from 90 to 130 volts.  Note the regulation.
>
> *Vin *      *B+*        *Vtransformer pri.*   *Vreg *    *Ireg. ma.*
>
> 90       225                 69.5               106.3       38.2
>
> 100     238                 74.5               106.7       42.3
>
> 110     251                 78.4               106.7       46.5
>
> 120     257                 81.3               106.8       47.8
>
> 125     255                 80.9               106.8       47.8
>
> 130     254                 81.0               106.8       47.4
>
> This raises an interesting design point:  These VR currents are insane to
> regulate a one mill 90 volt load, but serve to heavily load the power
> supply to get within the regulating current range of the 876 ballast tube.
> Note that the ballast tube and resistor are always connected when the power
> supply is on whether you are using them or not. so if not, remove or
> unscrew the tube to break contact to avoid heating the room and running the
> power bill up.
>
> About the two section filter: Back in the 1930's electrolytic capacitors
> lasted about five years, so any equipment expected to be reliable didn't
> use them, hence the RAK, RAL, and power supply didn't use them.  The power
> transformer is rated 500 volts center tapped, so the capacitors could
> charge up to 350 volts which is a long way from 180, but with an input
> capacitor of 3 mfd regulation will be terrible, and is thus OK.  There is a
> curious tap on the input choke, which I think makes a 120 cycle series
> tuned trap, as ripple voltage is only 30 mv, which is undetectable in the
> receiver.
>
> Also note the high B+ voltages.  These sets were never operated at 180
> volts. (well, maybe sometimes on batteries?)  The trouble shooting data all
> show tube socket voltages above 180 volts.
>
> Why did they do it this way?  I suspect some large egos were involved, and
> some unqualified people made decisions.  To cover their tracks they said
> the RAK was OK with the direct connection, but the RAL should always use
> the ballast tube.
>
> Richard, AA1P
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-- 
Cliff Miller
cliff52 at gmail.com
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