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

4-74
Additional antenna switch tests are:
- Check CR9922 with an ohmmeter for forward and reverse continuity.
- In the transmit mode, adjust control voltage for 38 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 CR9922 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 C9920 and L9920 during transmit mode.
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W A R N I N G

4.5.3.1.5 Power Control and Protection Circuitry

1. Localizing Problems to a Circuit
Power leveling and current limiting (35-Watt models only) 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, voltage control limit, and interstage drive limit (on 35-Watt models only)
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 RT9650. Key the transmitter and
monitor the output power. The power meter should read approximately 1/2 the rated power
(7.5 Watts or 17.5 Watts).
3. Control-Voltage-Limit Circuitry Test
Disconnect P9641 (Transmitter injection) from the internal transceiver chassis. This will
require removal of the power amplifier assembly. With all other connections in normal
condition, key the transmitter and monitor the control voltage on pin 2 of the power module. If
the voltage exceeds 12.5 V, troubleshoot the control voltage limit circuitry.
4. Interstage Drive Limiter Circuitry Test (35-Watt models)
Check this circuit only when the final device Q9880) has failed. With the radio off, check
CR9930 and associated components.
5. Current-Limiting Circuitry Test (35-Watt models)
When ready to adjust current limit, decrease the relative current limit value with the keyboard
per instructions. After several decrements, the current limit should reduce power from 0.1
Watt to 0.5 Watt. After this test, reset the current limit. If the circuitry does not perform as
indicated, troubleshoot the current limit circuitry.
October 28, 2002
DO NOT measure bias directly at the PIN diodes while in
transmit mode unless TX injection is removed.
Troubleshooting Procedures: Power Amplifier Procedures
6881076C25-D