Power Control Circuit; Carrier Control Timer; Maintenance; Disassembly - GE MASTR II Maintenance Manual

LBI4898
POWER AMPLIFIER
reading is taken on the one-Volt scale with
the High Sensitivity button pressed, and
read as 30 amperes full scale.
The output of Q206 is coupled through
a matching network (C250, C251, L229, C258,
C259, C266 and L237), applied to a lumped-
constant combiner circuit (C280, L237 and
L241), and added t~he output of Q207.
The outputs of Q206 and Q207 are added to
the outputs of Q208 and Q209 through lumped-
constant power combiner circuit C284, L249,
C294, L250 and C285. The combined PA output
is applied to 50-ohm microstrip W205, and
then to an M-derived, constant K low-pass
filter. The filter output is applied to
the antenna through antenna switch K20l.
Capacitors C286 through C293, C217,
C228 and C233 provide ground isolation for
±
ground operation.
r---------------WARNING --------------,
The stud mounted RF Power Transis-
tors used in the transmitter contain
Beryllium Oxide, a TOXIC substance.
If the ceramic, or other encapsula-
tion is opened, crushed, broken or
abraded, the dust may be hazardous
if inhaled. Use care in replacing
transistors of this type.
POWER CONTROL CIRCUIT
When the transmitter is keyed, recti-
fied RF from CR201 is applied to the base
of switch Q211, turning it on. Turning on
Q211 turns on voltage regulator Q212 which
supplies a constant voltage to Power Adjust
potentiometer R261.
Q215, Q216 and Q217 operate as an am-
plifier chain to supply voltage to the col-
lector of Q204 (Ampl-1). The setting of
R261 determines the voltage applied to the
base of Q215. The higher the voltage at
the base of Q215, the harder the amplifiers
conduct, supplying more collector voltage
to Q204. The lower the voltage at the base
of Q215, the less collector voltage is sup-
plied to Q204. Reducing the supply voltage
to Q204 reduces the drive to Q205, thereby
reducing the power output of the PA. The
power output can be adjusted by R261 from
approximately 50 to 100 Watts (75 to 100
Watts at 25-30
MHz).
Temperature protection is provided by
Q213, Q214, and thermistor RT201 which is
mounted in the PA heatsink. Under normal
operating conditions, the circuit is in-
active (Q213 is on and Q214 is off). When
the heatsink temperature reaches approxi-
mately 100°C, the resistance of RT201 de-
creases. This increases the base voltage
applied to Q213, turning it off. Turning
off Q213 allows Q214 to turn on, decreasing
the voltage at Power Adjust potentiometer
R261. This reduces the base voltage to
Q215 which causes Q216 and Q217 to conduct
6
less,
~~ducing
the collector voltage to Q204
(Ampl-1). This reduces the transmitter out-
put power, keeping the heatsink at a maximum
of approximately 100°C. When the heatsink
temperature decreases below 100°C, the tem-
perature control circuit turns off, allowing
the normal transmitter power output.
, CARRIER CONTROL fiMER
The Carrier Control Timer option shuts
off the transmitter on each transmission
after a one-minute timing cycle, and alerts
the operator that the transmitter is off by
means of an alarm tone in the speaker. The
transmitter can be turned on again by re-
leasing and keying the push-to-talk switch
on the microphone.
The timing cycle (transmitter keyed
time) is normally set at the factory for a
duration of one minute. A potentiometer
permits the timing cycle to be adjusted from
approximately 15 seconds to 3 minutes.
MAINTENANCE
DISASSEMBLY
To service the transmitter from the top:
1. Pull the locking handle down, then
pry up the top cover at the front
notch and lift off the cover.
To service the transmitter from the bottom:
1. Pull the locking handle down and pull
the radio out of the mounting frame.
2. Remove the top cover, then loosen the
two bottom cover retaining screws and
remove the bottom cover (see Figure 5).
3. To gain access to the bottom of the
~ iter
board, remove the six screws
A holding the exciter board and its
ti
ttom cover to the module mounting
frame, and remove the bottom cover.
To remove the exciter board from the radio:
1. Unplug the exciter/PA cable
@ .
2.
Remove the six screws ~ holding the
exciter board and its bottom cover to
the module mounting frame (see Figure
6).
3. Press straight down on the plug-in ex-
citer from the top to avoid bending the
pins when unplugging the board from the
system board jack.
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