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

Troubleshooting Procedures: Power Amplifier Procedures
• Unsolder either L3859, R3875, or L3851 to isolate the driver and final stages. Measure the
collector emitter DC resistances. If the resistance is below 5k ohms, then replace the driver
device.
NOTE: The position of capacitors C3853 and C3854 is critical to the performance of the circuit. If they
are removed for any reason, they must be re-installed as close to the cap of the final device
as possible.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
• 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 ohm meter for forward and reverse continuity.
• In the transmit mode, adjust the control voltage for 55 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.
• Check for proper DC current through the PIN diodes; correct current is indicated if
approximately 1.5 V is present at the junction of C3900 an L3900 during the transmit mode.
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W A R N I N G

4.5.1.3.5 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).
6881076C25-D
Table 4-14 on page 4-39
DO NOT measure bias directly at the PIN diodes while in
transmit mode unless TX injection is removed.
for RX tests.
October 28, 2002
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