25/10 Watt Power Amplifier; General Troubleshooting And Repair Notes - Motorola ASTRO Digital Spectra Plus Detailed Service Manual

Troubleshooting Procedures: Power Amplifier Procedures
The directional coupler samples a small amount of forward power during transmit. This power is
rectified by a detector diode CR3904. This rectified DC voltage is fed back to the RPCIC where it is
compared to a reference voltage. An error voltage is generated which is ultimately translated into the
control voltage via RPCIC circuitry and amplifiers Q503 and Q504 on the command board. Control
voltage is routed to the LLA stage, thereby completing the feedback loop. In operation, the control
loop tends to maintain the forward detected voltage constant versus frequency and line voltage
variations. Proper operation can be observed by monitoring the forward detected voltage while
varying the supply voltage from 13.4 to 16.1 V. Forward detected voltage should not change more
than a few hundreths of a volt. Note that the forward power may not necessarily be level if one of the
other protection circuits such as temp-sense or current limit is engaged.
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.
Miscellaneous Circuits and Notes
Diode CR3840 acts as a reverse protect diode. This diode also protects from over-voltage
conditions, as it has a Zener breakdown voltage of approximately 28 V. When replacing this diode,
care must be taken to place the diode with the cathode marking ring down (towards the PC board)
NOTE: The control voltage drive and K9.4 supplies from the command board are not current limited.
A momentary short on either of these supplies will cause damage to transistors on the
command board. Use caution when troubleshooting circuits that use these.

4.5.1.2 25/10 Watt 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. 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.

4.5.1.2.1 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.
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).
6881076C25-E
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September 5, 2008