204 Receiver Fault Finding TB7100 Service Manual
© Tait Electronics Limited October 2005
12.2 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 3V0 AN and 3V0 RX for the receiver
circuitry.
1. Remove the main-board assembly from the chassis.
2. Check for 3.0V DC (3
V0 AN) at the TP601 test point near the LO2 BOT
can (see Figure 12.2).
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 132.
4. Remove the
LO2 BOT can.
5. Check for 3.0V DC (3
V0 RX) around the collector feed to Q402 or
Q403 of LO2 (see Figure 12.2).
Alternative measurement points are the collector feed to Q401 of the
RF LNA under the
FE TOP can (see Figure 12.3) or Q404 of the IF
amplifier under the
IF TOP can (see Figure 12.1).
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 132.
TP601 test point: 3.0V DC
Q402 or Q403 collector: 3.0V DC
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