Check Overall Receiver Operation; Voltage Measurement And Signal Tracing; Troubleshooting Charts - Motorola Handie-Talkie MTX Series Theory/Troubleshooting Manual

D. Check Overall Receiver Operation
1. 20dB Quieting Sensitivity Test
A good overall check of receiver operation is the
20dB quieting sensitivity measurement. This check
will indicate that the receiver has sufficient gain and
that all of the included circuitry is working properly.
The quieting signal is that rf signal input necessary
to reduce the audio output at the speaker by 20dB.
This measurement should be made with no modula-
tion. It will be necessary to hold the monitor button
during this test, or the radio's squelch circuitry will
remove the noise from the speaker.
Make the actual measurement (using an ac volt-
meter) by setting the noise voltage across the test
box speaker load (with no rf signal received at the
antenna) to one-half (1/2) of the rated audio power
output (1.85Vrms). Sufficient carrier signal from a
generator is then introduced via the antenna port to
reduce the noise output voltage to one-tenth (1/10)
of the previous reading. If all of the circuitry is oper-
ating correctly, this reading should be 0.5µV or less.
If the radio does not meet this specification, try to
retune the receiver using the procedure indicated in
the service manual. If this does not solve the prob-
lem, refer to the receiver troubleshooting chart.
2. 12dB SINAD
This procedure is a standard method for evaluating
the performance of an FM receiver, since it provides
a check of the rf, i-f, and audio stages. The method
consists of finding the lowest modulated signal nec-
essary to produce 50% of the radio's rated audio
output with a 12dB or better ratio of signal + noise +
distortion / noise + distortion. This is termed "usable
sensitivity."
To perform this measurement, connect the leads
from a SINAD meter to the audio output of the test
box. Set the Motorola service monitor or rf signal
generator to output a 1-millivolt signal. Modulate the
rf signal with a 1kHz tone at 3kHz deviation (VHF,
UHF, 800MHz) 1.5kHz deviation for 900MHz. Intro-
duce the signal to the radio at the exact channel
frequency through the antenna port. Set the volume
control for rated audio output (3.74Vrms). Decrease
the rf signal level until the SINAD meter reads 12dB.
The signal generator output (12dB SINAD measure-
ment) should be less than 0.35µV. If the radio does
not meet this specification, try to retune the receiver
using the procedure indicated in the service manual.
If this does not solve the problem, refer to the
receiver troubleshooting chart.
III. VOLTAGE MEASUREMENT AND
SIGNAL TRACING
To aid in troubleshooting, ac and dc voltage readings
are provided (in red) on the transceiver schematic dia-
gram in the service manual. When making these voltage
checks, pay particular attention to any notes that may
accompany the voltage reading of a particular stage.
If receiver sensitivity is high or if the rf power output
is lower than normal for a fully tuned transceiver, the dc
voltages on the printed circuit board should be checked.
These voltages should be referenced to ground.
CAUTION
When checking a transistor or module, either in
or out of the circuit, do not use an ohmmeter hav-
ing more than 1.5 volts dc appearing across the
test leads or an ohms scale of less than x 100.
It is recommended not to replace a transistor or
module before a thorough check is made. Read the volt-
ages around the suspected stage. If these voltages are
not reasonably close to those specified, the associated
components should be checked.
A low impedance meter should not be used for mea-
surement. If all dc voltages are correct, the signal should
be traced through the circuit to show any possibility of
breaks in the signal path.
CAUTION
The microcomputer is a static sensitive device
contained on the controller flex assembly. DO
NOT attempt to troubleshoot or disassemble the
microcomputer/controller flex assembly without
first referring to the "Safe Handling of CMOS
Devices" paragraph in the MAINTENANCE
section of the manual.
IV. TROUBLESHOOTING CHARTS
The troubleshooting charts on the following pages
will help isolate troubles in the different sections of the
radio. Start at the top of the appropriate chart and make
the checks as indicated. Most usual malfunctions will
respond to the systematic approach to troubleshooting.
Also, a flowchart is provided to aid in choosing the prop-
er troubleshooting chart.
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