High power—Fault finding and adjustments
NGT Transceiver System Technical Service Manual 239
Adjusting the power amplifiers (3061/3062)
Replacing output transistors
Adjusting the amplifier bias
To adjust the amplifier bias:
1 Ensure there is no audio drive to the NGT and disconnect the RF drive between the
NGT and the amplifier.
1 Connect a DC millivoltmeter across one of the 2-pin headers associated with the
current sense resistors on the Power Supply PCB.
1 Adjust the bias preset potentiometer on each Power Amplifier PCB fully
counterclockwise.
1 Switch on the NGT, hold down PTT, then adjust the relevant amplifier’s bias preset
potentiometer until the meter reads 2 mV (representing a bias current of 400 mA).
1 Check that the voltage across the other 2-pin header of the same Power Supply PCB
is 2 mV.
1 Release PTT.
1 Check the other Power Supply PCB/Power Amplifier PCB pair (3062 only).
1 Switch off the NGT.
1 Reconnect the RF drive.
Drawing 08-05977 (sheet 1) and drawings 08-05978 (sheet 1), 08-05978 (sheet 2) and
08-05978 (sheet 3)
CAUTION
If any of the output transistors on the Power Amplifier PCBs need to be
replaced, all of the transistors must be replaced with a set of transistors of
the same gain. The gain is indicated by a letter or colour marking on the
device body.
Drawing 08-05977 (sheet 1)
NOTE
The transceiver does not have to be in Service mode to perform this
adjustment.
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