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TB7100 Service Manual Receiver Fault Finding 217

Task 18 —

Second IF and

Repair of Matching

Circuitry

If the TCXO is not faulty, check the second IF and, if necessary, repair the

matching circuitry.

1. Input a large unmodulated RF input signal exceeding –90dBm at the

RF connector.

2. Use a needle probe to measure the frequency of the signal at the

test point — access is through the hole in the

IF TOP can

(see Figure 12.1). The frequency is the second IF and should be:

3. If the second IF is correct, go to Step 6. If it is not, go to Step 4.

4. Recalibrate the TCXO.

5. Check if the distortion fault has been removed. If it has, go to “Final

Tasks” on page 123. If it has not, go to Step 6.

6. Remove the

IF TOP can.

7. Check the components between T401 and IC400 — these form the

matching circuitry for the crystal filters XF400 and XF401

(see Figure 12.1).

8. Repair any fault, confirm the removal of the fault, and go to “Final

Tasks” on page 123. If the repair failed or no fault could be found,

replace the main-board assembly and go to “Final Tasks” on

page 123.

frequency at QN test point: 64.000kHz

Tait TB7100 Service Manual

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