184 Power Amplifier Board Replacement TB8100 Service Manual© Tait Electronics Limited September 200610.4 Replacing the 60W RF Power TransistorImportant If you do not follow these procedures correctly, the transis-tor may fail because of poor heatsinking.Important Do not apply too much heat or pressure to the PCB padsand tracks as you may damage them or lift them from thePCB. This will permanently damage the PA.Important Insulated gate FET transistors are susceptible to damagefrom static charges, due to their extremely high input resist-ance. To avoid possible damage to the device during han-dling, testing or actual operation, we recommend you fol-low these procedures: avoid unnecessary handling; whenhandling the device, pick it up by the cap, not the leads; donot insert or remove the device while the power is on;avoid contact with non-conductive plastic or non-conduc-tive styrofoam.Important If you replace an RF power transistor, you must recalibratethe PA bias using the Calibration Kit. Refer to the Calibra-tion Kit documentation 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 10.4 onpage 185.Removal 1. Remove the cover.2. Remove the two 4–40 UNC Allen recess head screws b securing thetransistor to the heatsink.3. Desolder and remove the two ground tags c.4. Desolder the transistor tabs d by heating them with a soldering ironand then carefully lifting them away from the PCB with a screwdriveror thin stainless steel spike. When the tabs are completely free of thePCB, remove the transistor.5. Remove any excess solder from the PCB pads with solder wick.6. Remove any old heatsink compound from the heatsink.Refitting 1. Lightly tin the underside of the transistor tabs. Remove any excesssolder to leave a thin, even layer of solder on the tabs.2. Apply a small amount of heatsink compound (Dow-Corning 340 orequivalent) to the transistor mounting surface. Use as little as possible
