Motorola ASTRO Digital Spectra Plus Detailed Service Manual page 195

Troubleshooting Procedures: Power Amplifier Procedures
NOTE: If it is necessary to replace Q5803, 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 side of
the board.
Troubleshooting the Third Stage Q5850
• 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
L5851 and L5852 for proper soldering, and replace if faulty.
• Check the current drain of Q5850. Remove L5854 and R5850 and solder wires to the pads.
With an ammeter connected to these wires, check the collector current drain during transmit. It
should be around 1.5 to 2.0 A. If current drain is low, go to next step.
• Remove L5851 from the board and 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.
Troubleshooting the Driver Stage Q5851
• Make sure A+ is at the driver's collector. Check for shorts and or opens.
• Check the matching circuitry for shorts and/or opens. Also, check for faulty components.
(Cracked parts or parts not properly soldered.)
• Measure the resistance from base to emitter; it should be less than 1 ohm. If not, check for
proper soldering on L5855 and L5857. Replace faulty component(s).
• Current drain for this stage should be close to 5 A. If low current, go to the next step.
• Remove L5857 from the board and check the base-emitter and base-collector junction diode
drops. Normal voltage drops should be near 0.6 V. If either junction is open or shorted, replace
the device.
NOTE: The position of capacitors C5861, C5862, C5863 and C5864 is critical to the performance of
the circuit. If they are removed for any reason, they must be re-installed in the exact same
physical location from which they were removed.
Analysis of the Final Amplifier Stage (Q5875 and Q5876)
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 L5877, L5876,
and R5878. 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 C5885, C5886, C5887, and C5888 are placed on the bottom side of the PA board
underneath the 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.
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September 5, 2008