Tait TB7100, Руководство по обслуживанию

Страница: 215 / 370

TB7100 Service Manual Receiver Fault Finding 217
© Tait Electronics Limited October 2005
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
QN
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