Tait TM8100 mobiles, Руководство по обслуживанию

Страница: 250 / 620

250 Receiver Fault Finding TM8100/TM8200 Service Manual
© Tait Electronics Limited November 2007
10.2 Excessive Loss of Sensitivity
Introduction
This section covers the case where the receiver has suffered an excessive loss
of sensitivity. As measured in
Task 1 , the receiver gain will be less than 1500,
which implies a sensitivity that is more than 40dBm too low. The fault-
diagnosis procedure for this case consists of five tasks:
■
Task 2 : check power supplies
■
Task 3 : check logic signal
■
Task 4 : check lock status
■
Task 5 : check biasing of IF amplifier
■
Task 6 : check matching circuitry
If the fault does not lie with the power supplies, it is probably in the control,
LO, IF1 or IF2 circuitry.
Task 2 —
Check Power
Supplies
First check the two power supplies
3
V
0
AN
and
3
V
0
RX
for the receiver
circuitry.
1. Remove the board from the chassis.
2. Check for 3.0V DC (
3
V
0
AN
) at the
TP
601
test point near the
LO
2
BOT
can (see Figure 10.2 ).
3. If the voltage is correct, go to Step 4 . If it is not, the 3V regulator
IC603 is suspect; go to Task 3 of “Power Supply Fault Finding” on
page 174 .
4. Remove the
LO
2
BOT
can.
5. Check for 3.0V DC (
3
V
0
RX
) around the collector feed to Q402 or
Q403 of LO2 (see Figure 10.2 ).
Alternative measurement points are:
■
K5 band: the collector feed to IC404 or IC405 of the RF LNA
under the
FE TOP
can (see Figure 10.4 )
■
All other bands: the collector feed to Q401 of the RF LNA under
the
FE TOP
can (see Figure 10.3 )
■
All bands: Q404 of the IF amplifier under the
IF TOP
can
(see Figure 10.1 ).
6. If the voltage is correct, go to Task 3 . If it is not, the 3V RX switch
(based on Q604 and Q605 ) in the PSU module is suspect; go to
Task 3 of “Power Supply Fault Finding” on page 174 .
TP601 test point: 3.0V DC
Q402 or Q403 collector: 3.0V DC