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5.1
The linear power amplifier in your TL—922A is a grounded
grid class AB, amplifier consisting of two Eimac Hi-mu,
Zero bias triodes, 3-500Z, providing the following features:
5
Low IMD
(intermodulation distortion) through nega-
tive RF feedback.
Z.
The grounded grid circuit does not require neutraliza-
tion.
3.
Due to its low input impedance, no cathode-swamp-
ing resistors are required.
4.
High stability at any frequency.
5.
Excitation energy is added to the RF output and is
not lost.
Figure 5-1
shows a circuit diagram
of the Linear Power
Amplifier.
The
input
matching
circuit,
comprised
of LO
through
L14,
Cin-1,
and Cin-2, matches
the exciter im-
pedance with the input circuit impedance to efficiently feed
exciter energy into the power tubes.
The tubes are heated
by the center
tapped
filament
transformer
T2
requiring
current only half the usual, preventing voltage
drop due to
wire resistance.
The choke coils L20 and
L3 high frequency
isolate the
filaments from the filament transformer. The bias voltage,
which determines the operating point of the tubes, is prod-
uced across zener diode D2. This diode carries the full cur-
rent of the tubes excepting filaments. The grids are DC
grounded through L7 and L8 and high-frequency grounded
by C28 through C33.
The
pi network,
comprised
of VC1,
L4, L5 and VC2,
matches the plate impedance to the antenna. The tubes are
cut off to stand
by through a positive voltage of approxi-
mately 100 V applied to the filament circuit.
5.2
POWER SUPPLY
The plate high voltage is supplied from a voltage doubling
rectifier comprised of a large-capacity, small-sized trans-
former T1, fourteen high-voltage rectifier diodes rated 800
PIV and eight 200 uF electrolytic capacitors rated 550 V
surge. To improve reliability, the diodes are P.C.B. mounted
and the electrolytic capacitors are held by plastic spacers.
Filament voltage, relay energizing voltage (in common with
the tube cut-off bias voltage), and pilot lamp voltage are
supplied from transformer T2. Each primary of the power
transformers T 1 and T 2 has two windings, each of which is
fused.
Note
that fuses are not changed
when
the input
line voltage has been changed (see Fig. 5-2).
5.3.
FAN MOTOR
DELAY
STOP CIRCUIT
The cooling fan is held on for approximately 2 minutes to
cool the power tubes after the POWER switch is turned off.
This is accomplished
by the fan motor delay stop circuit
comprised of a bimetal thermal relay.
Figure 5-3 is a schematic diagram illustrating the circuit.
When the POWER
switch is at OFF, both the relay RL1 and
the thermal relay DL1 are not activated and the fan is off.
C34
La us
ANT
EXCITER
Figure 5-1 — Linear Power Amplifier Circuit
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13
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