TM 11-A62525514/TO 33A1131701
current for V9. The increased plate current flows
the amount of feedback voltage.
through R48, resulting in a decrease in plate volt-
e. For TD-723A/U, TD-723B/U, and
age in V9. The plate voltage of V9 determines the
TD-723C/U resistor R72 is inserted in the fila-
grid bias voltage of V8. When the plate voltage of
ment circuit of VI4 to limit the thermal emission
V9 becomes less positive, the control grid of V8
of the tube. R72 also stabilizes V14 with respect
r e c e i v e s a more negative bias. This action in-
to line voltage variations and protects the meter
creases the effective dc resistance of V8 and low-
from overload currents. For the TD-723D/U,
ers the output B+ voltage; therefore, the voltage
resistor R99 is inserted in the rectifier circuit of
change responsible for this sequence of events is
CR1 and CR2 stabilizing this circuit with respect
counteracted.
to variations of line voltage, protecting the meter
d. When the output voltage of V7 decreases,
from overload currents.
the grid bias voltage applied to V9 from V8 be-
6-9. Power Supply Circuit
comes more negative to decrease the plate current
of V9. The decreased plate current flows through
R48 and results in an increase in plate voltage in
the power supply circuit which operates as
V9. Grid voltage supplied to V8 from V9 becomes
follows :
l e s s negative, which decreases the effective dc
a. The ac line voltage is stepped up by the
resistance of V8 and increases the output of B+
high-voltage secondary winding of T1. Tube V7
voltage.
provides full-wave rectification of the stepped up
e. Network R45 and C19 applies residual ripple
voltage. The pulsating dc output of V7 is passed
voltage directly to the grid of V9. As a result, the
t h r o u g h filter C37, C17A, and C17B, and L1.
regulating characteristic of V8, V9, and V10 pro-
Capacitors C37, C17A, and C17B are connected in
duces cancellation of ripple voltage and makes the
a series-parallel combination to obtain operation
regulator circuit insensitive to transients.
with a safe dc working voltage. Resistors R84 and
Capacitor C18C (C38A) provides additional
R85 equalize the voltage distribution across these
filtering for the regulated output of V8.
capacitors and, at the same time, act as a bleeder
f. A negative bias supply is obtained by inser-
circuit. The filtered dc voltage is passed through
tion of R40 between the center tap of the high-
an additional filtering network which consists of
v o l t a g e secondary winding of T1 and ground.
R49, R50, R51, C17C, C17D (C18C), C18A, and
This action places the transformer center tap at a
C18B. It is then supplied to the voltage divider
n e g a t i v e potential with respect to ground. The
stage of the vtvm section at +300 volts dc.
bias voltage is filtered by R60 in conjunction with
b. Filtered dc voltage from V7 is also applied to
C27. It is then applied as grid bias to V12 and
the plate of series regulator V8. The output of V8
V13.
is obtained at the cathode. This output is gov-
g. HUM BAL variable resistor, is used to ad-
erned by the control voltage applied to the control
grid of the tube. The control grid voltage is, in
j u s t the effective ac potential point of the fre-
quency selective amplifier and vtvm filament cir-
turn, controlled by the conductive state of dc
cuit. It is used primarily to minimize hum
error amplifier V9. Tube V8 supplies the regulated
+225 volts for the plate circuits of the frequency
produced in these filament circuits.
h. R e s i s t o r R 6 7 i s i n s e r t e d i n t h e o u t p u t
selector circuit and V 12. The output of V8 is fed
u n r e g u l a t e d supply to reduce the unregulated
through voltage divider network R44, R42, and
voltage to +230 volts for application to the plate
R43 to the grid of V9. Voltage adjust control R43
circuit of V13, Capacitor C18D (C38C) provides
is used to vary the amount of voltage applied to
filtering for the +230-volt dc supply.
the grid of V9 from this source. Voltage regulator
i. When the ac source is 230 volts, the primary
V10 is connected in the cathode circuit of V9. The
of T1 has to be changed from two windings in
stabilized voltage drop across V10 provides V9
parallel for 115-volt operation to one winding in
with a reference bias. Screen grid voltage for V9 is
provided by voltage divider R86 and R87. Resis-
through each winding is 0.8 ampere, fuse F1 must
tor R48 is the plate load for V9 and the grid return
be rated for twice that value, or 1.6 amperes for
for V8. Resistor R46 limits the ionization current
115-volt operation and 0,8 ampere for 230-volt op-
of V10.
e r a t i o n . Refer to paragraph 2-4 for instructions
c. W h e n the dc output voltage from V8 in-
on how to convert T1 for either type of operation.
creases, the grid bias voltage applied to V9 from
V8 becomes more positive, causing higher plate
6-11
