Power Control And Protection Circuitry - Motorola ASTRO Digital Spectra Plus Detailed Service Manual

4-54
Installation of the PA board into the radio chassis must be done carefully. The PC board screws use
a T-15 Torx bit and should be torqued to 6 to 8 inch-pounds. The device screws use a T-8 Torx bit
and should be torqued to 6 to 8 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 R5875 during
transmit. A voltage drop of 0.10 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. Additional antenna switch tests are:
- Check CR5900, CR5902, CR5904, and CR5905 using the diode-check function of a
multimeter. Note that CR5904 and CR5905 are on the bottom side of the board. These two
diodes affect the receive path only and are unrelated to transmitter problems.
- Check for proper DC current through the PIN diodes; correct current is indicated if
approximately 1.5 V is present at the junction of R5900 and L5900 during transmit.
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W A R N I N G

4.5.2.1.3 Power Control and Protection Circuitry

Localizing Problems to a Circuit
Power leveling and current limiting are set to values detailed in the ASTRO Digital Spectra and
Digital Spectra Plus Mobile Radios Basic Service Manual (6881076C20). These values will vary from
unit to unit, depending on the unique variations of each unit. If symptoms indicate that either of these
circuits have failed, verify that the radio has been properly aligned before investigating the circuitry.
Temperature sense and control voltage limit are fixed by design and are not influenced by the
alignment of the radio. If symptoms indicate that these circuits have failed, then troubleshoot the
circuit.
The tests that follow are intended to provide a convenient means of verifying that a particular circuit
is functioning properly. These tests will isolate the failure to a minimum number of components.
Refer to the Theory of Operation and the schematic for information needed to identify the failed
component(s).
Temperature Sense Circuit Test
Temporarily install a 2.2k ohm resistor in parallel with RT5875. Key the transmitter and monitor the
output power. The power meter should read approximately one-half the rated power.
September 5, 2008
Table 4-17 on page 4-49
DO NOT measure bias directly at the PIN diodes while in
transmit mode unless TX injection is removed.
Troubleshooting Procedures: Power Amplifier Procedures
for RX tests.
6881076C25-E