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

4-64
5. 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 CR5920, CR5921, and CR5922 with an ohmmeter for forward and reverse
continuity.
- In the transmit mode, adjust control voltage for 44 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 CR5922 and associated circuitry. Receiver sensitivity can degrade if power at this
port exceeds 10 mW.
- Check for proper DC current through the PIN diodes; correct current is indicated if
approximately 1.5 V is present at the junction of C5920 and L5920 during transmit mode.
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W A R N I N G

4.5.2.2.5 Power Control and Protection Circuitry

1. 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).
2. Temperature Sense Circuit Test
Temporarily install a 6.8k 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 (25 Watts).
September 5, 2008
DO NOT measure bias directly at the PIN diodes
!
while in transmit mode unless TX injection is
removed.
Troubleshooting Procedures: Power Amplifier Procedures
Table 4-19 on page 4-58 for RX tests.
6881076C25-E