Tait TM8000 Series, Руководство по обслуживанию

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TM8100 Mobile Radio Service Manual Diagnosis of Faults on Main Board 197
May 2004 © Tait Electronics Limited
9.5 Receiver
Fault Conditions
Fault diagnosis of the receiver is divided into seven tasks. The symptoms
of the fault in the receiver circuitry determine which tasks need to be
carried out. There are four cases:
■
receiver sensitivity is too low
■
RSSI readings are incorrect
■
radio mute is faulty
■
receiver distortion is too high
For the case of low sensitivity proceed to Task 1. Depending on the
extent of the sensitivity loss, Task 2, Task 3 or Task 4 will also need to be
carried out. For the remaining three cases proceed to Task 5, Task 6 and
Task 7 respectively.
Task 1 —
Low Receiver
Sensitivity
Depending on the nature of the fault, a reduction in receiver sensitivity
of 1 dB is often due to a reduction in receiver gain of many decibels. It
is therefore easier to measure gain loss rather than sensitivity loss.
Consequently, if the receiver sensitivity is too low, first check the receiver
gain as follows:
1. Input an RF signal (not necessarily modulated) of –90 dBm (or
–84 dBm with a trigger-base radio) at the RF connector.
2. Enter the CCTM command 378 to measure the receiver output
level. Note the value
x
returned. (A change in the input level of
10 dB should result in a tenfold change in x .)
3. Normally
x
should be between 500 000 and 6000 000, depending
on the frequency band in which the radio operates. The
corresponding RF voltages at the
QN
test point [at 4P1/4C12] (see
Figure 9.13 ) are 12 mV
pp
and 120 mV
pp
respectively. (These can
be measured through a hole in the
IF TOP
can.) With an
unmodulated RF signal the frequency should be 64.000 kHz,
provided that the LO1, FCL and LO2 are locked and on the
correct frequency.
4. Given the value of
x
, go to the relevant task as follows:
■
x < 500, go to Task 2 (sensitivity is more than 40 dB too low)
■
x
< 40 000, go to Task 3 (sensitivity is about 15 dB too low)
■
x < 500 000, go to Task 4 (sensitivity is a few decibels too low)