6-12 Maintenance 8525B/8528 Technical Service Manual
Issue 6 Publication No: 15-02036
11. Before re-installing the board, clean all the components in contact with
the heatsink and the corresponding surfaces of the heat sink. Re-coat the
cleaned areas with Jermyn Thermaflow A30. To re-install the board
reverse the removal procedure taking particular care with the following:
• When remounting the board on the heatsink, ensure that R16 and
V11 are correctly located in their holes in the heat sink and that a
mica insulator is under V15.
• Ensure that the screws removed in disassembly are re-installed in
their correct locations.
• Screws should not be tightened until the reassembly is complete.
To avoid straining connections to the PA transistors the screws
securing these should be tightened last.
6.3.5 Replacement of PA Transistors
Since the PA output transistors are fitted in matched pairs they should be
replaced only in matched pairs. The gain groupings of the SRFH1008
(selected MRF455s) transistors are identified by a coloured dot. Transistors
of the same dot colour only should be fitted.
The PA transistors can be replaced without removing the PA and Filter PCB
from the heatsink. It is necessary to release only the screws securing the
transistors and to unsolder connections as follows:
1. Remove the transistor flange fixing screws.
2. Use a de-soldering tool or ‘Solder-wick’ to remove the bulk of the
solder from each connection. Gently pull the leads away from the PCB
while heating the joint.
3. Clear away any excess solder from the emitter, base and collector pads.
4. Thoroughly clean the transistor mating surface on the heatsink with a
cloth or tissue.
5. Form the leads of the replacement transistor using the discarded
transistor as a guide.
6. Coat the transistor flange with a thin film of thermal compound, e.g.
Jermyn Thermaflow A30.
7. Check the orientation and fit the new transistor. Tighten the flange
fixing screws evenly.
8. Carefully resolder the transistor connections. This should be carried out
quickly using a very hot tipped soldering iron.
9. Re-adjust the bias current. Refer to 7.3.13.
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