Tait TM8235, Руководство по обслуживанию

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242 Receiver Fault Finding TM8100/TM8200 Service Manual
© Tait Electronics Limited June 2006
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 168 .
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 the collector feed to Q401 of the
RF LNA under the
FE TOP
can (see Figure 10.3 ) or 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 168 .
TP601 test point: 3.0V DC
Q402 or Q403 collector: 3.0V DC