Motorola ASTRO Digital Spectra Plus Detailed Service Manual page 178

4-36
Testing Driver Circuitry
The driver is a typical Class-C stage, except the base is biased with resistors R3809 and R3810. The
necessary conditions for proper operation of this stage are input drive power, and bias conditions as
shown in Table 4-13 on page
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 side of
the board.
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 L3851; replace faulty component(s).
• Current drain on the final device should be >3.5 A. for 25-Watt operation. If low current, go on to
the next step.
• Remove L3851 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 this
range, replace the final device.
NOTE: When replacing either the driver or final device, apply thermal compound on the heatsink
surface. Torque the screws to the correct value; see the ASTRO Digital Spectra and Digital
Spectra Plus Mobile Radios Basic Service Manual (6881076C20).
Testing the Antenna Switch and Harmonic Filter
Verify that most of this circuit is functioning properly by testing the receiver insertion loss as follows:
• Apply a low-level signal source at the antenna connector.
• Apply 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.
• Additional antenna switch tests are:
- Check CR3920, CR3921, and CR3922 with an ohmmeter for forward and reverse
continuity.
- In the transmit mode, adjust control voltage for 28 Watts at the antenna connector. Check
for less than 10 mW at the end of the receive input cable. If power exceeds 10 mW, then
check CR3922 and associated circuitry. Receiver sensitivity can degrade if power at this
port exceeds 10 mW.
September 5, 2008
Troubleshooting Procedures: Power Amplifier Procedures
4-35.
Table 4-10 on page 4-30 for RX tests.
6881076C25-E