Watt Power Amplifiers - Motorola ASTRO Digital Spectra Plus Service Manual

4-38
Power-Leveling Circuitry Test
With the radio connected for power measurements, vary the line voltage from 12.5 to 16 V. The
power should not vary more than 2 Watts. At a line voltage of 13.8 V, vary the frequency using the
three test modes. If power varies more than 2 Watts, measure the detected voltage on P0853, pin 9.
If this voltage varies more than 0.2 V over line and frequency variations, the power control circuitry
(most of which is located on the command board) may be malfunctioning. If the detected voltage
varies less than 0.2 V, the problem is likely in diode CR3900, the harmonic filter, the antenna switch,
or the output coax. Check continuity through the 12-pin DC connector P0853 on the PA board; check
digital/analog circuitry, and check 5-V regulator operation. See
Chart, for typical values.
With the radio connected for power measurements and a disconnected TX injection coax, the
detected voltage at P0853, pin 9, should measure approximately 1.3 V.
NOTE: If any part of the power leveling circuitry is replaced, perform the power set procedure. See
the ASTRO Digital Spectra and Digital Spectra Plus Mobile Radios Basic Service Manual
(6881076C20) for details.

4.5.1.3 50 Watt Power Amplifiers

This information will help you troubleshoot the ASTRO Spectra radio. Use this information, along
with the Theory of Operation, to diagnose and isolate the cause of failures. The principle tools
needed to troubleshoot a circuit to the component level are the schematic and the Theory of
Operation.
In addition to the schematic and theory, this section includes troubleshooting information that will
help you test and check the circuits to localize and isolate problems.
Prior to troubleshooting, it is important to review the Theory of Operation, including specific
precautions and troubleshooting methods. Because much of the radio's circuitry operates at high
frequency, measurements must be taken very carefully. Notes and cautions are added to the text to
alert the reader to this need in areas of greatest sensitivity. However, the need for extreme care does
exist in all measurements and tests at high frequency.
4.5.1.3.1 General Troubleshooting and Repair Notes
Most of the common transmitter symptoms are caused by either failure of the power amplifier or a
failure in the control circuitry. The initial troubleshooting effort should be toward isolating the problem
to one of those two areas. If either the control voltage or keyed 9.4 V are zero, then the problem is
likely to be in the control circuit. If those voltages are present, then the problem is more likely in the
power amplifier circuit.
If, for diagnostic reasons, a chip component needs to be removed to facilitate testing, such as a
series capacitor removed to allow for signal insertion, then the components (s) returned to the circuit
should be new parts. The application of a soldering iron to many chip components will tend to cause
leaching which could lead to failure.
After a PA board is replaced, or if any power control circuitry components are replaced, readjust the
power according to instructions in the ASTRO Digital Spectra and Digital Spectra Plus Mobile Radios
Basic Service Manual (6881076C20).
NOTE: Due to high operating frequencies, you must use specified Motorola parts when component
replacement is necessary. Substitute components may not work. It is also critical that you use
great care when replacing parts. Excessive solder or flux, longer than original leads on coax
connectors, misorientation of parts, and other commonly benign imperfections may cause the
radio's performance to degrade.
October 28, 2002
Troubleshooting Procedures: Power Amplifier Procedures
Table 4-12 on page 4-34, DC Voltage
6881076C25-D