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

[Richard]

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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