Power Amplifier; Rf Amplifiers - GE MASTR II Maintenance Manual

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CIRCUIT ANALYSIS
LBI4898
Following Ql02 is another Class A Ampli-
fier, Ql03. The output of Ql03 is applied
to the second modulator stage. The second
modulator consists of two cascaded modulator
circuits consisting of CV102, Ll02, Ll03 and
CV103. Following the second modulator 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 sig-
nal is present. Applying an RF signal to
Ql05 provides a sawtooth waveform at its
collector to drive the class C tripler, Ql06.
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 QlOS. Tl04 and Tl05 are tuned
to one-half the operating frequency. QlOS
is metered through Rl48.
The output of Ql08 is coupled through
three tuned circuits (Tl06, Tl07 and TlOS)
to the base of amplifier Ql09. The circuits
are tuned to the transmitter operating fre-
quency •
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 six RF power tran-
sistors and seven transistors in the Power
Control circuitry to provide a power output
of 100 Watts. The broadband PA has no ad-
justments other than Power Control potentio-
meter R261.
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, L297 and L298 prevent RF from getting
on the Power leads. Diode CR295 will cause
the main fuse in the fuse assembly to blow
if the polarity of the power leads is re-
versed.
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. L251
through L257 in conjunction with bypass cap-
acitors C4210 through C4216 keep RF off of
the metering leads.
RF AMPLIFIERS
The exciter output is coupled through
an RF cable to PA input jack J201. RF from
the exciter is coupled through DC blocking
capacitor C201 to the base of Class C ampli-
fier Q204 through a matching network. The
network matches 50-ohm input to the base of
Q204, and consists of C205, C206, C207, L201
and L202. R203 and R204 lower the gain of
the amplifier stage.
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 R201 and
R202 for metering the Ampl-1 drive at J205.
Collector voltage to Q204 (Ampl-1) is
controlled by the Power Control Circuit, and
is applied through a collector stabilizing
network consisting of L258 and R272 and
collector feed network L205 and C213. The
collector voltage of Q204 is metered through
R271 at J205.
Following Q204 is a matching network
(C208 through C212, L204 and L206} to a
resistive pad (R207, R208 and R209). The
output of the resistor network is applied
to the base of the Class C driver (Q205)
through a matching network consisting of
C218, C219, C220, L207 and L208. Resistors
R207 through R215 lower the gain of driver
Q205.
Collector voltage to Q205 is coupled
through a collector stabilizing network
consisting of L259 and R273 and collector
feed network L211 and C226. Collector cur-
rent for Q205 is metered across tapped man-
ganin resistor R249 at J205 (DRIVER CURRENT).
The reading is taken on the one-Volt scale
with the High Sensitivity button pressed,
and read as 10 amperes full scale.
Following Q205 is a matching network
(C221 through C225, L210 and L214) that
matches the driver output to the input of
the first power divider circuit (C230, C231,
L214, L215 and L216).
The power amplifier stages consist of
four identical paralleled Class C amplifiers
(Q206 through Q209). The output of the first
power divider circuit is applied to four
additional power dividers. C234-L217 and
C235-L218 provide drive for Q206 and Q207,
while C236-L219 and C237-L220 provide drive
for Q208 and Q209.
The output of C234-L217 is applied to
the base of Q206 an impedance-matching net-
work "(U217, L221, C238, C242 and C243). Re-
sistors R220 through R223, R236 and R237
lower the gain of Q206. Supply voltage for
Q206 is coupled through a collector-stabili-
zing network consisting of L260 and R274 and
collector feed network L223 and C270 •
Collector current for Q206 through Q209
is metered across tapped maganin resistors
R250 and R251 at J205 (PA CURRENT). The
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