Motorola ASTRO Digital Spectra Service Manual page 176

4-44
If the above DC bias conditions are correct, check to see if the LLA is providing drive power to the
pre-driver, Q3804. Do so by checking Q3804's collector current under normal drive conditions, as
follows:
• Remove R3810 and L3806 (Be sure to reinstall after testing).
• Solder wires to the remaining pads.
• Place an ammeter in series with the collector of Q3804. Check for 0.1 to 0.5 A. depending the
control voltage.
NOTE: With no RF drive to the input of the PA, Q3804's collector current should be zero.
If Q3804 draws no current under normal conditions, then check for a shorted or open input cable, or
for defective parts in the input network or matching circuitry between Q3801 and Q3804. If all the
above check out OK, then replace Q3801.
Testing Pre-Driver Circuitry
The pre-driver is a typical Class C stage, except the base is biased with resistors R3809 and R3806.
The necessary conditions for proper operation of this stage are input drive power, and bias
conditions as shown in
NOTE: If it is necessary to replace Q3804, use a hot-air blower to remove and replace the part. It is
important that the replacement device's case be properly soldered to its heatsink. Do so by
flowing a small bead of solder around the rim of the device while it is clamped in the hot-air
soldering device. The base and collector leads must be hand-soldered on the bottom of the
board.
Troubleshooting the Driver Stage
• Make sure A+ is at the collector.
• Check for shorts and/or opens in the matching circuitry. Also look for faulty components.
• Measure the DC resistance from base to emitter. It should be less than 1 ohm. If not, check
L3810 for proper soldering and replace if faulty.
• Check the current drain of the driver. It should be around 0.5 to 2.5 A. for 50-Watt operation. If
current drain is low, go to next step.
• Unsolder the base lead. Making sure the lead is not touching the PC board, check the
base-emitter and base-collector junction diode drops. Normal voltage drop should be between
0.4 and 1.0 V. If either junction reads outside this range, replace the driver device.
• Unsolder either L3854, R3875, or L3851 to isolate the driver and final stages. Measure the
collector emitter DC resistance. If the resistance is below 5k ohms, then replace the driver
device.
Troubleshooting the Final Device
• Make sure A+ is at the final's collector; if not, check for shorts and/or opens.
• Check the matching circuitry for shorts and/or opens. Also, check for faulty components.
• Measure the resistance from base to emitter; it should be less than 1 ohm. If not, check for
proper soldering on L3852 and L3853; replace faulty component (s).
• Current drain on the final device should be >6 A. for 50-Watt operation. If low current, go on to
the next step.
• Remove L3853 from the board and check the base-emitter and base-collector junction diode
drops. Normal voltage drop should be between 0.4 and 1.0 V. If either junction is outside the
range, replace the final device.
July 1, 2002
Troubleshooting Procedures: Power Amplifier Procedures
Table 4-16 above.
68P81076C25-C