230 Receiver Fault Finding TM9100 Service Manual© Tait Electronics Limited August 200510.2 Excessive Loss of SensitivityIntroduction This section covers the case where the receiver has suffered an excessive lossof sensitivity. As measured in Task 1, the receiver gain will be less than 1500,which implies a sensitivity that is more than 40mdB 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 circuitryIf the fault does not lie with the power supplies, it is probably in the control,LO, IF1 or IF2 circuitry.Task 2 —Check PowerSuppliesFirst check the two power supplies 3V 0 AN and 3 V 0 RX for the receivercircuitry.1. Remove the main-board assembly from the chassis.2. Check for 3.0V DC (3V 0 AN) at the TP 601 test point near the LO 2 BOTcan (see Figure 10.2).3. If the voltage is correct, go to Step 4. If it is not, the 3V regulatorIC603 is suspect; go to Task 3 of “Power Supply Fault Finding” onpage 158.4. Remove the LO 2 BOT can.5. Check for 3.0V DC (3V 0 RX) around the collector feed to Q402 orQ403 of LO2 (see Figure 10.2).Alternative measurement points are the collector feed to Q401 of theRF LNA under the FE TOP can (see Figure 10.3) or Q404 of the IFamplifier 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 toTask 3 of “Power Supply Fault Finding” on page 158.TP601 test point: 3.0V DCQ402 or Q403 collector: 3.0V DC
