Motorola ASTRO Digital Spectra Service Manual page 159

Troubleshooting Procedures: Power Amplifier Procedures
Troubleshooting the Driver Stage (Q3805)
• Make sure A+ is at the collector.
• Check for shorts and/or opens in the matching circuitry. Also look for faulty components
(cracked parts or parts not properly soldered).
• Measure the DC resistance from base to emitter. It should be less than 1-ohm. If not, check
L3812 and L3809 for proper soldering, and replace if faulty.
• Check the current drain of Q3805. Remove L3811 and R3819 and solder wires to the pads.
With an ammeter connected to these wires, check the collector current drain during transmit. It
should be around 2.0 to 4.0 A. If current drain is low, go to next step.
• Desolder the base of Q3805 and bend its lead slightly so it does not contact the PC board.
Check the base-emitter and base-collector junction diode voltages using the diode check
function of a multimeter. Normal voltage drop should be near 0.6 V. If either junction is open or
short circuited replace the device.
Analysis of the Final Amplifier Stage (Q3870 and Q3871)
Extreme care must be taken when troubleshooting the final amplifier due to the high RF currents and
voltages present.
A visual inspection of the matching networks should be done first. Check for defective solder joints or
burned components. Good soldering of the transistor device leads is essential. Make sure A+
voltage is reaching the collector of each final device.
Check the base-emitter and base-collector junctions of the final devices by removing L3930, L3933,
R3859, and R4007. Using the diode check function of a multimeter, the junctions should have a
forward voltage drop close to 0.6 V. Replace a final device if it has an open or shorted junction.
Capacitors C3860, C3861, C3862, and C3863 are placed on the bottom side of the PA board
underneath the base leads of the final devices. Extreme care should be used when replacing these
parts. Exact positioning is critical. Inspect for solder shorts on these capacitors before installing the
PA board in the radio chassis.
Installation of the PA board into the radio chassis must be done carefully. The PC board's screws use
a T-15 Torx bit and should be torqued to 12 to 14 inch-pounds. The device screws use a T-8 Torx bit
and should be torqued to 12 to 14 inch-pounds. Always apply thermal compound to the area under
the device flanges before installing the PA board.
Current drain of the final amplifier may be checked by measuring the voltage across R3849 during
transmit. A voltage drop of 0.10 V to 0.15 V indicates the finals are drawing 10 to 15 A., which is
within the acceptable range.
Testing the Antenna Switch and Harmonic Filter
Use care when replacing the harmonic filter. Removal of the filter is best accomplished by heating
the filter/PC board assembly with a heat gun or heat blower until the solder joint reflows.
Verify that the receive path of the antenna switch and the harmonic filter are functioning by testing
the receiver insertion loss as follows:
• Apply a low-level signal source at the antenna connector.
• Verify the conditions indicated in
• Measure the power at the receive coax.
• If the difference between the input and output (insertion loss) is less than 1 db, then the circuitry
is functioning properly.
68P81076C25-C
Table 4-8 for RX tests.
4-27
July 1, 2002