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

LBI-4896
'POWER AMPLIFIER
~-------------
WARNING
--------------~
The stud mounted RF Power Tran-
sistors used in the transmitter
contain Beryllium Oxide, a TOXIC
substance. If the ceramic, or
other encapsulation 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 Q204, turning it on. Turning on
Q204 turns on voltage regulator Q206 which
supplies a constant voltage to Power Adjust
potentiometer R216.
Q208, Q209 and Q210 operate as an amp-
lifier chain to supply voltage to the col-
lector of Q201 (Ampl-1). The setting of
R216 determines the voltage applied to the
base of Q208, The higher the voltage at
the base of Q208, the harder the amplifiers
conduct, supplying more collector voltage
to Q201. The lower the voltage at the base
of Q208, the less collector voltage is
supplied to Q201. Reducing the supply volt-
age to Q201 reduces the drive to Q202 and
Q203, thereby reducing the power output of
the PA. The power output can be adjusted
by R216 from approximately 15 to 50 Watts.
Temperature protection is provided by
Q205, Q207 and thermistor RT201 which is
mounted in the PA heatsink. Under normal
operating conditions, the circuit is in-
active (Q205 is on and Q207 is off). When
the heatsink temperature reaches approxi-
mately 100°C, the resistance of RT201 de-
creases. This·increases the base voltage
applied to Q205, turning it off. Turning
off Q205 allows Q207 to turn on, decreas-
ing the voltage at Power Adjust potentiom-
eter R216. This reduces the base voltage
to Q208 which causes Q209 and Q210 to. con-
duct less, reducing the collector voltage
to Q201 (Ampl-1). This reduces the trans-
mitter output power, keeping the heatsink
at a maximum of approximately 100°C. When
the heatsink temperature decreases below
100°C, the temperature control circuit turns
off, allowing the normal transmitter power
output.
CARRIER CONTROL TIMER
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 rekeying the push-to-talk switch
on the microphone.
6
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
o
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.
To remove the PA assembly:
1.
2.
Remove the PA top~ver and unplug the
exciter/PA cable B , the antenna,
receiver and PTT c les
(S} .
Remove the four side-rail screws
~
,
and unsolder the power cables from
the bottom of the PA assembly if desired.
To remove the PA board:
1.
2.
3.
Remove the PA
top~ver
and unplug the
exciter/PA cable
~
•
Unsolder the two feedthro~ coils ~
and the thermistor leads
~
•
Remove the PA transistor hold-down nuts
and spring washers on the bottom of the
PA assembly.
•
•
4.
~ ave
the four PA board mounting screws •
G ,
th~ive
screws in the filter
c sting H , and the retaining screw
in Q210 , and lift the board out.