Tait TM8235, Service Manual

Page: 257 / 578

TM8100/TM8200 Service Manual Receiver Fault Finding 257
© Tait Electronics Limited June 2006
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 10.1 ). 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 157 . 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 10.1 ).
8. Repair any fault, confirm the removal of the fault, and go to “Final
Tasks” on page 157 . If the repair failed or no fault could be found,
replace the board and go to “Final Tasks” on page 157 .
frequency at
QN
test point: 64.000kHz