Pa Transistor Replacement - GE Mastr II Maintenance Manual

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MAINTENANCE LBI-4732
To remove the PA board: Refer to Figure 7.
1.
2.
3.
4.
5.
Remove the PA top cover and unplug the
exciter/PA cable
~
Unsolder the two
feedthro~
coils
~
and the thermistor leads ~ .
For Continuous Duty stations only, re-
move all heatsink sections from the
heat dissapator plate.
Remove the PA transistor hold-down nuts
and spring washer on the bottom of the
PA assembly.
Remove the four PA board mounting
screws
fG) ,
th~ive screws in the
filter ~ting
, and the retaining
screw in Q210 , and lift the board
out.
PA TRANSISTOR REPLACEMENT
When replacing a power transistor where
more than one are in parallel, make sure all
the paralleled transistors are from the same
manufacturer for proper operation.
1.
2.
3.
WARNING
The stud mounted RF Power Transis-
tors used in the transmitter con-
tain Beryllium Oxide, a TOXIC sub-
stance. 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.
To replace the PA RF transistors:
Unsolder one lead at a time with a 50-
Watt soldering iron. Use a scribe to
hold the lead away from the printed
circuit board until the solder cools.
Turn the transmitter.
NOTE
If the transmitter has a continuous
Duty Power Amplifier a section of
Heat Sink may have to be removed to
get to the transistor hold-down nuts.
Apply a light coat of silicon grease
when replacing the removed section
of Heat Sink.
Hold the body of the transistor to pre-
vent it from turning. Remove the tran-
sistor hold-down nut and spring washer
through the hole in the heatsink with
an 11/32-inch nut-driver. Lift out
the old solder from the printed cir-
cuit board with a de-soldering tool
such as a SOLDA PULLT®. Specjal care
should be taken to prevent damage to
the printed circuit board runs.
4_._·
5.
Trim the new transistor leads (if re-
quired) to the lead length of the re-
moved transistor. Cut the collector
lead at a 45° angle for future identi-
fication (see Figure 7). The letter
"C" on the top of the transistor indi-
cates the collector.
Figure 7 - PA Board Removal
Applying a coating of silicon grease
around the transistor mounting surface,
and place the transistor in the mount-
ing hole. Align the leads as shown in
the Outline Diagram. Then hold the
body of the transistor and replace the
holding-down nut and spring-washer, us-
ing moderate torque (8 inch-pounds). A
torque wrench must be used for this ad-
justment since transistor damage can
result if too little or too much torque
is used.
6. Make sure that the transistor leads are
formed as shown in Figure 8 so that the
leads can be soldered to the printed
circuit pattern, starting from the
inner edge of the mounting hole.
7. Solder the leads to the printed circuit
pattern. Start at the inner edge of
mounting hole and solder the remaining
length of transistor lead to the board.
Use care not to use excessive heat that
causes the printed wire board runs to
lift up from the board. Check for shorts
and solder bridges before applying power.
CAUTION
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Failure to solder the transistor leads as
directed may result in the generation of
RF loops that could damage the transistor
or may cause low power output.
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