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Replacement of PA output and driver transistors
Under normal operating conditions PA and Driver FETs will last indefinitely. The ALC circuit will fully protect
these transistors from incorrect antenna loads.
In data mode, if the cooling fan fails for any reason, a high temperature power reducing circuit will protect
these devices from overheating.
Damage to power transistors is rare and the reasons for failure should be properly investigated if a failed
transistor is found. Reasons for failure include lightning damage, power supply surges, and incorrect
adjustment of bias and power control circuits by users.
The driver FETs are ST Microelectronics type PD55008.
The PA transistors are Motorola type MRF455, or ST Microelectronics SD1446.
The procedure for replacement of driver and PA transistors will be described in detail:
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Disconnect 12V power feed at rear of transceiver,
- Prior to removing the PA transistors, the base emitter and base collector junctions should be checked
for forward conduction and reverse leakage using a test meter. In order to do this, the base leads must be
de-soldered using a large tipped iron preferably with a No. 8 (800 F) tip. The tab is bent up to facilitate
removal. The tab should be prized up clear of the PCB taking care not to damage the transistor package.
Forward voltage can then be checked BE 0.92 - 0.98V, BC 0.92 - 09.98V and reverse leakage BE>500K
BC>200K. In the event of transistor damage, continue as described.
- Carefully remove all four M3 crosshead screws which hold the PA transistors in place. Store them
safely.
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De-solder the large 100R resistors the MRF455's.
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Prize up the collector leads carefully until they are clear of the PCB. The collectors have a bevelled tab.
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Prize up both opposed emitter tabs carefully. Extra heat is required here as they are connected to the
ground plane. Remove the transistors through the PCB access hole.
- Remove excess solder from the four PCB pads and clean them with isopropyl alcohol or methylated
spirits.
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Clean all traces of heatsink compound from chassis heatsink area under the transistor being changed.
Ensure that no dirt, swarf or other foreign matter is located where the transistors will sit.
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Clean the thermal contact surface of the replacement device. Smear a thin layer of heatsink compound
evenly onto the transistor seat. Push the transistor into place and "wriggle" it in-situ. Remove the
transistor and inspect the heatsink area to check that heatsink compound has coated the seating area
of the transistor. A thin but complete smear of compound should be present. Do not apply too much or
two little as poor thermal conductivity will result.
- Seat replacement PA transistors carefully in place having bent up collector and base tabs 3mm from
their ends. The bevelled collector tab should face the large PA transformer.
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Reinsert the two bolts on each transistor. Go around all four screws in turn, gradually increasing torque
until the transistors are firmly seated.
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Readjust bias before use.