182 Power Amplifier Board Replacement TB8100 Service Manual© Tait Electronics Limited September 200610.3 Replacing the 60W BoardImportant There is heatsink compound between the RF power tran-sistor and the heatsink. You may need to carefully prise thetransistor away from the heatsink with a small screwdriver.Any objects caught in the heatsink compound underneaththe transistor which prevent effective grounding and/orheatsinking may cause the transistor to fail.Important If you replace the 60W board, you must recalibrate the PAbias using the Calibration Kit. Refer to the Calibration Kitdocumentation for more details.Refer to “Power Amplifier Disassembly and Reassembly” on page 173 fordetails on removing and refitting the cover, and for screw torque settings.The circled numbers in the following instructions refer to Figure 10.3 onpage 183.Removal 1. Remove the cover.2. On a 12V 50W PA, remove the boost regulator board (as describedin “Replacing the Boost Regulator Board in a 12V PA” onpage 193).3. Remove the bridging links b connecting the board to the controlboard.4. Desolder the red +28VDC power feed wire c. On a 12V PA, alsodisconnect the black earth wire.5. Desolder the RF links d connecting the board to any adjacentboards.6. Remove the 4-40 UNC Allen screws e securing the RF powertransistor f to the heatsink.7. Remove the M3 Torx screws securing the board to the heatsink.8. Carefully lift the board directly upwards off the locating pins g andremove it from the heatsink.Refitting theOriginal Board andTransistorUse the following instructions if you are fitting the original 60W board andRF power transistor, and haven’t disturbed any of the solder joints aroundthe transistor.1. Ensure the heatsink and bottom of the transistor are clean. Removeany old heatsink compound.2. Apply a thin layer of fresh heatsink compound to the bottom of thetransistor. Use as little as possible, while still covering the whole ofthe mounting area of the transistor.12V28V12V
