Service manual

July 1, 2002 68P81076C25-C
4-66 Troubleshooting Procedures: Power Amplifier Procedures
4.5.3 800 MHz Band
4.5.3.1 15 Watt and 35 Watt Power Amplifiers
This information will help you troubleshoot the 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 800
MHz, 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 to extreme care does exist
in all measurements and tests at 800 MHz.
4.5.3.1.1 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 component(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.
If the harmonic filter is damaged and needs to be replaced, then removal and replacement requires
the use of a hot air source capable of reflowing the solder beneath the filter hybrid. When replacing it,
add small amounts of fresh solder paste to the silver regions beneath the ceramic to assure
adequate electrical ground contact. Save the original input and output connectors ('J' straps); these
are not included with the replacement kit. No turning is required. The harmonic filter may be ordered
separately, but if the PA kit is ordered, a filter kit comes with the PA kit.
The pass device may be ordered separately or may be received as part of the hardware kit-it is not
part of the PA kit. The PA kit comes with all surface-mount components, including the harmonic filter
hybrid, but the harmonic filter cover is not included. Neither does the PA kit include the Power
Module, nor, on 35-Watt models, the final device and associated matching capacitors.
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 (68P81076C20).
NOTE: Due to the high frequency of operation, it is imperative that you use specified Motorola parts
when component replacement is necessary. At these frequencies, second and third order
properties of the components are very important and are part of the circuit's design. 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.