CIRCUIT
DESCRIPTION
29
Firing the Trigger Tube
As
soon
as
the card brush makes contact with the
contact roll through a hole in the card,
+150
volts
is
applied to the starting anode of the OA4G and more
than cancels the
-40
volts applied from the
bias
network. This causes the tube
to
fire.
The 47,000 ohm
resistor limits
the starting anode current when the
tube
fires.
Firing of the OA4G tube allows current
to
flow
from the negative
side
of the line, through contact
roll cover switch
2,
octal plug pin 6, the lower
section of the oscillator coil,
12SN7, 10,000 ohm
re-
sistor, cathode to main anode of the OA4G, octal plug
pin 4, center to outer commutator brushes, R2, card
lever contacts, contact roll cover switch 1, to the
posi-
tive side of the line. Current
flow
through this circuit
causes
a voltage drop of approximately 60 volts
across
the 10,000 ohm resistor with the polarity
as
shown in
Figure
25
(principle
2),
This polarity opposes the
polarity of the
40 volt negative bias on the grids of
the 25L6 power tubes and tends
to
swing these grids
to
a value of approximately 20 volts positive with
respect
to
their cathodes which are at zero potential.
However,
as
soon
as
the grids become positive, current
flows
in the grid circuit of each tube and
causes
a
voltage drop
across
the 47,000 ohm grid resistors with
the polarity
is
shown in Figure 25. This lowers the
positive potential on the grids
to
a value only slightly
greater than the cathodes.
The 4 mfd. capacitor partially discharges through
the
OA4G tube each half
cycle
that the alternating
voltage
across
the lower section of the oscillator coil
opposes the main
DC
voltage. The capacitor tends to
charge when the alternating voltage
across
the lower
section of the oscillator coil
aids
the main DC voltage.
In
this respect, the capacitor acts
as
a half wave filter
and tends
to
keep the bias supply steady when the
OA4G tube
is
conducting. Although the 4 mfd. capa-
citor steadies the bias supply when the
OA4G
is
con-
ducting, the bias supply decreases in value at this time
from
-40
volts to
-30
volts or
less.
When
the
OA4G
ceases
to
conduct at the time the center
com-
mutator brush opens the circuit, the 4 mfd. capacitor
fully charges
as
before, and
-40
to
-45
volts bias
is
again placed on the power tubes and the cathode
and starting anode of the
OA4Gtube.
Energizing the Sort
Magnet
With
the negative bias removed from the grids of
the 25L6 power tubes, they immediately conduct (prin-
ciple
6)
from the negative side of the line, through
contact roll cover switch 2, octal plug pin 8, 25L6
cathodes, 25L6 anodes, octal plug
1,
sort magnet,
card lever contacts, contact roll cover switch
1,
to the
positive
side
of the line; thus energizing the sort
magnet. The initial surge
of
current through the 25L6
tubes
is
of a comparatively large value due
to
the
action of the 8 mfd. capacitor in the screen grid circuit
of the 25L6 power tubes. This capacitor charges up to
the line potential of
150 volts
as
soon
as
the main line
switch
is
turned on.
When
the grid bias
is
removed
from the control
grids
of the 25L6 tubes, causing them
to
go into conduction, the charge on the 8 mfd.
capacitor maintains a high positive potential on the
screen grids; thus producing a peak plate current flow
through the tubes (principle
8).
This initial heavy
current surge through the three 25L6 tubes lasts only
a short time.
As
the capacitor discharges through the
screen grid circuits, the positive potential on the screen
grids,
decreases, finally reaching a steady state value of
approximately
65
volts because of the voltage drop
across
the 4,700 ohm resistor. Lowering the screen
grid voltage in this manner causes the plate current
to
decrease
to
its
normal steady state value. The fore-
going action described gives a large initial impulse to
the sort magnet, causing fast attraction of the sort
magnet armature; after which the current through the
sort magnet tapers
off,
remaining in sufficient magni-
tude, however, to keep the
armature attracted.
Steady state current
flow
through the sort magnet
on a 115 volt
AC
machine should be approximately
220 MA, measured when a punched card
is
fed
through
the machine
by
hand.
On
a 115 volt
DC
machine,
current
flow
measured in this manner should be
approximately 165
MA.
If
a meter with the proper
current scale
is
not available, the voltage drop across
the sort magnet can be measured, and should be ap-
proximately
13
volts for
AC
machines and 10 volts
for 115 volt
DC
machines. Steady state current
flow
through the sort magnet should never fall below 140
MA
on either AC or DC operated machines. This value
of
140
MA
through the sort magnet
is
comparable to
a drop of 8.4 volts
across
it.
If
the current through
the sort magnet falls below
140
MA
when the line
voltage
is
correct, it may be an indication that one or
more of the 25L6 power tubes are weak.
The action of the 8 mfd. capacitor and the
4,700
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