3-22 Troubleshooting
3.3.4.2.3 Troubleshooting the Final Amplifier
1. Inspect all final amplifier RF devices for signs of failure: Q4300, Q4400, Q4500, and
Q4600. Signs of failure may include a pin hole on the top of the device package, charred
marks at the output lead, or black marks around or on the inside of the PA cover above
any of these devices. Also inspect the combiner isolation loads – R4911, R2912, R4921,
R4922, R4931, and R4932 – for signs of over dissipation, as this is also indicative of a
final amplifier device failure. If any of Q4300, Q4400, Q4500, or Q4600 is found to be
damaged, it cannot be repaired and the kit must be replaced.
2. Check for blown fuses at F4316, F4416, F4516, and F4616. If a fuse is blown, this
indicates a damaged device that has shorted to ground. Inspect for failures downstream.
If Q4300, Q4400, Q4500, or Q4600 has failed, it cannot be repaired and the kit must be
replaced.
3. Apply 28.6 V and 14.2 V main DC supplies. Measure drain voltages at final amplifier
devices Q4300, Q4400, Q4500, and Q4600, which should be ~28.6 V.
4. Enable the PA and measure gate voltage at pin 5 of U4337, U4437, U4537, and U4637,
which should be approximately 3 V each and approximately equal to each other.
5. Measure voltage drop across each R4315, R4415, R4515, and R4615, which are 10 mΩ
sense resistors for drain current. Top side probe pads are present adjacent to each of
these resistors (TP4301/TP4302, TP4401/TP4402, TP4501/TP4502, and TP4601/
TP4602 respectively) to accomodate probing across the resistors. Each resistor should
have approximately 2 mV of drop across it indicating 200 mA. Note that this value can
vary with temperature, but all devices should be approximately equal.
6. Key the PA to rated output power, and measure drain voltage at final amplifier devices
Q4300, Q4400, Q4500, and Q4600, which should be ~28 V.
7. Measure voltage drop across each R4315, R4415, R4515, and R4615, which are
10 mΩ sense resistors for drain current. Each resistor should have approximately
20 mV of drop across it indicating 2 A of current drain. Note that this value can vary with
temperature, but all devices should be approximately equal.
8. If no issues are found in the steps above, and 28 V current does indicate a final amplifier
related issue, perform the troubleshooting steps on the Circulator and Output Board.
Do not use a hot plate when working on the RF Board, and do
not use a heat gun at/or around the coined areas, as this will
cause damage to the PCB.
Do not attempt to replace RF amplifier devices Q2300,
Q4300, Q4400, Q4500, or Q4600 on the RF Board. If any of
these devices fail, the entire RF Board kit must be replaced.
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