Motorola ASTRO Digital Spectra Plus Detailed Service Manual page 215

Troubleshooting Procedures: Power Amplifier Procedures
When testing is complete, replace any capacitors or resistors that were removed for testing
with new parts.
3. Testing the Final Stage (35-Watt Models Only)
The final stage is capable of producing over 50 Watts. Be sure to protect power measuring
equipment with series attenuation. 30 dB is usually adequate.
15 Watts are needed to drive the final stage. Because this may exceed the power available at
800 MHz in many repair facilities, these tests consider the module stage as the drive source
for the final stage. Therefore, check out the module first to ensure that it operates properly.
In the course of testing the final stage with the module as the power source, begin with
control voltage at zero and increase control voltage smoothly until output of the final stage
reaches 40 Watts. If control voltage reaches 10 V, but the power out does not reach 40
Watts, the final stage is defective. Under normal conditions, the protection circuitry limits the
power to the final stage to approximately 17 Watts maximum, protecting it from overdrive and
damage. Under test conditions, however, the protection circuitry is disabled. Observe the
above caution; the power module can produce in excess of 25 Watts.
Measure the output power by lifting the output side of C9856 and connecting to the center
conductor of a small-diameter coax which has its shield grounded. If the output stage does
not produce 40 Watts (at 10-V control voltage), then remove the RF drive and perform the
following tests:
- Check continuity from the collector lead to the A+ connector on the back of the radio.
- Examine the solder connections on all leads of the device (Q9880) and the clamped mica
capacitors.
NOTE: The position of the clamped capacitors adjacent to the device is critical to the
4. 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 2 dB, then the
circuitry is functioning properly.
6881076C25-E
performance of the circuit. If they are removed for any reason, they must be
re-installed with their leads approximately 70 mil s (0.070 inches) from the final
device cap.
Table 4-23 on page 4-67 for RX tests.
September 5, 2008
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