September 5, 2008 6881076C25-E
4-56 Troubleshooting Procedures: Power Amplifier Procedures
Low-Voltage Current Drain Cutback
An additional circuit associated with the over-voltage protection circuit is the low-voltage current
drain circuit. This circuit acts to reduce the transmitter current drain under conditions of low supply
voltage. This action extends the available transmit time when, for example, the transmitter in a
vehicular installation must be used when the engine is not running. Operation of this circuit is similar
to the over-voltage circuit. R5819 and R5820 form a voltage divider which is connected to the base
of transistor Q5804. If the A+ voltage drops below approximately 12 V, Q5804 will begin to conduct.
This turns on Q5802, which supplies a DC bias voltage to the forward power detector as explained in
the Theory of Operation for the over-voltage protection circuit. The transmitter output power is
reduced by the power control, which results in reduced current drain and extended battery life.
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 CR5875 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).
Figure 4-8. Block Diagram for Spectra High-Power Power Amplifier
4.5.2.2 40 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 UHF
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 UHF frequencies.
4.5.2.2.1 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.
FINAL AMPLIFIER
Q5875
25C29
Q5876
25C29
Q5851
25C30
Q5850
25C27
Q5803
25C09
Q5801
82D50
LLA 2ND STAGE 3RD STAGE DRIVER
250mW 2W 15W 50W
30mW
J5901
INJECTION
CONTROL
VOLTAGE
K9.4
9.6V
FILTERED
A+
FILTERED
A+
FILTERED
A+
PIN
ANTENNA
SWITCH
HARMONIC
FILTER
DIRECTIONAL
COUPLER AND
DETECTOR
K9.4
TO
RECEIVER
E5802
FORWARD
POWER
DETECT
J3853
ANTENNA
CONNECTOR
MINI UHF
110W
125W
MAEPF-22045-O
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