Power Amplifier Procedures; Vhf Band; High-Power Amplifier; General Troubleshooting And Repair Notes - Motorola ASTRO Digital Spectra Plus Service Manual

Troubleshooting Procedures: Power Amplifier Procedures
4.5

Power Amplifier Procedures

This section provides band-specific troubleshooting procedures for the power amplifier.
4.5.1

VHF Band

4.5.1.1 High-Power Amplifier

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. 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 VHF
frequencies, 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 VHF frequencies.

4.5.1.1.1 General Troubleshooting and Repair Notes

Most of the common transmitter symptoms are not necessarily caused by failure of circuits on the PA
board. Failure of command board or synthesizer circuits can disable the transmitter. The initial
troubleshooting effort should be toward isolating the problem to one of these areas. If either the
control voltage or keyed 9.4 V are zero, then the problem is likely to be in the control circuit or
synthesizer. 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 tuning is required. The harmonic filter may be ordered
separately, but if the PA kit is ordered a filter kit comes with the PA kit.
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).
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.
Bench testing the high-power Spectra PA is most easily accomplished if a Spectra control head,
control cable, and power cable are available on the test bench. This greatly simplifies the
troubleshooting as several supply voltages are provided by the command board. Proper operation of
the command board circuitry can be simultaneously verified.
6881076C25-D
4-21
October 28, 2002