Excessive Loss Of Sensitivity; Check Power Supplies - Tait TM9100 Service Manual

10.2

Excessive Loss of Sensitivity

This section covers the case where the receiver has suffered an excessive loss
Introduction
of sensitivity. As measured in
which implies a sensitivity that is more than 40mdB too low. The fault-
diagnosis procedure for this case consists of five tasks:
If the fault does not lie with the power supplies, it is probably in the control,
LO, IF1 or IF2 circuitry.
First check the two power supplies 3
Task 2 —
Check Power
circuitry.
Supplies
1.
2.
3.
4.
5.
6.
230 Receiver Fault Finding
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
Remove the main-board assembly from the chassis.
Check for 3.0V DC (3
can (see Figure 10.2).
TP601 test point: 3.0V DC
If the voltage is correct, go to
IC603 is suspect; go to
page 158.
2
Remove the
LO BOT
Check for 3.0V DC (3
Q403 of LO2 (see Figure
Q402 or Q403 collector: 3.0V DC
Alternative measurement points are the collector feed to Q401 of the
RF LNA under the
FE TOP
amplifier under the
IF TOP
If the voltage is correct, go to
(based on Q604 and Q605) in the PSU module is suspect; go to
Task 3 of "Power Supply Fault Finding" on page
Task 1, the receiver gain will be less than 1500,
0 and 3 0
V AN V RX
0 601 test point near the
) at the
V AN TP
Step 4. If it is not, the 3V regulator
Task 3 of "Power Supply Fault Finding" on
can.
0
) around the collector feed to Q402 or
V RX
10.2).
can (see Figure 10.3) or Q404 of the IF
can (see Figure 10.1).
Task 3. If it is not, the 3V RX switch
© Tait Electronics Limited August 2005
for the receiver
2
LO BOT
158.
TM9100 Service Manual