Power Amplifier; Rf Amplifiers - GE MASTR II Maintenance Manual

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CIRCUIT ANALYSIS
LBI-4896
Following Ql02 is another Class A amp-
lifier, Ql03. The output of Ql03 is applied
to the second modulator stage. The second
modulator consists of two cascaded modula-
tor circuits consisting of CV102, Ll02,
Ll03 and CV103. Following the second modu-
lator is a Class A amplifier Ql04. The
output of the second modulator stage is
metered through Rl33, Rl45, Cll7 and CR104,
and is applied to the base of buffer Ql05.
Diodes CR105 and CR106 remove any amplitude
modulation in the second modulator output.
Buffer Ql05 is saturated when no RF
signal is present. Applying an RF signal
to Ql05 provides a sawtooth waveform at its
collector to drive the class C tripler,
Cl06. The tripler stage is metered through
Rl46. The output of Ql06 is coupled through
tuned circuits TlOl, Tl02 and Tl03 to the
base of doubler Ql07. TlOl, Tl02 and Tl03
are tuned to one-fourth of the operating
frequency. The doubler stage is metered
through Rl47.
The output of Ql07 is coupled through
tuned circuits Tl04 and Tl05 to the base of
second doubler Ql08. Tl04 and Tl05 are
tuned to one-half the operating frequency.
Ql08 is metered through Rl48.
The output of QlOS is coupled through
three tuned circuits (Tl06, Tl07 and Tl08)
to the base of amplifier Ql09. The cir-
cuits are tuned to the transmitter operating
frequency •
Ql09 is a class C amplifier with a col-
lector feed network consisting of Cl39, Cl41,
Ll04, Ll08 and Rl43. The stage is metered
through Rl49. The amplifier collector cir-
cuit consists of Cl42, Cl43, Cl46 and Ll05,
and matches the amplifier output to the in-
put of the power amplifier assembly.
POWER AMPLIFIER
The PA assembly uses three RF power
transistors and seven transistors in the
Power Control circuitry to provide a power
output of 50 Watts. The broadband PA has
no adjustments other than Power Control
potentiometer R216.
Supply voltage for the PA is connected
through power leads from the system board
to feedthrough capacitors C297 and C298 on
the bottom of the PA assembly. C297, C298
and C299, L296 and L297 prevent RF from
getting on the Power leads. Diode CR295
will cause the main fuse in the fuse assem-
bly to blow if the polarity of the power
leads is reversed.
Centralized metering jack J205 is pro-
vided for use with GE Test Set Model
4EX3All or Test Kit 4EX8Kl2. The Test Set
meters the Ampl-1 drive (exciter output),
Ampl-1 power control, Driver and PA current.
RF AMPLIFIERS
The exciter output is coupled through
an RF cable to PA input jack J203. The
RF is coupled through DC blocking capacitor
C202 to the base of Class C amplifier Q201
through a matching network. The network
matches the 50-ohm input to the base of
Q201, and consists of C205, C206, C235,
L201, L202 and L203.
Part of the RF input is rectified by
CR201 and used to activate the Power Control
circuit. Another portion of the rectified
RF is applied to voltage dividers R223 and
R224 for metering the Ampl-1 drive at J205.
Collector voltage to Q201 (Ampl-1) is
controlled by the Power Control circuit,
and is applied through a collector stabil-
izing network consisting of L224 and R225
and collector feed network L204 and C207.
The collector voltage of Q201 is metered
through R235 at J205.
The output of Q201 is applied to the
base of Class C driver Q202 through a low-
pass filter matching network (C209, C210,
L205 and L206). Resistors R202, R203 and
R204 lower the gain of Q202. Collector
voltage to Q202 is coupled through a col-
lector stabilizing network consisting of
L225 and R233 and collector feed network
L208 and C213.
Collector current for Q202 is metered
across tapped manganin resistor R230 at
J205 (Driver Current). The reading is taken
on the one-Volt scale with the High Sensi-
tivity button pressed, and read as 10 am-
peres full scale.
Following Q202 is an interstage coup-
ling network (C214 through C221, L209
through L211, R206 and R207.) The output
is applied to the base of the class C PA
stage, Q203. Supply voltage is coupled
through a collector stabilizing network
consisting of L226 and R234 and collector
feed network C222 and L212.
Collector current for Q203 is metered
across tapped manganin resistor R231 at
J205. The reading is taken on the one-Volt
scale with the High Sensitivity button
pressed, and read as 10 amperes full scale.
The PA output is coupled through an
output matching network (C224, C225, C226,
L213 and L214,) to an M-derived, constant
K low-pass filter. C230 through C233 pro-
vides ground isolation for
±
ground opera-
tion. The filter output is applied to the
antenna through antenna switch K20l.
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