Tait TM8235 Service Manual page 257

Task 18 —
Second IF and
Repair of Matching
Circuitry
TM8100/TM8200 Service Manual
© Tait Electronics Limited June 2006
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
(see Figure 10.1). The frequency is the second IF and should be:
frequency at QN test point: 64.000kHz
3. If the second IF is correct, go to
4. Recalibrate the TCXO.
5. Check if the distortion fault has been removed. If it has, go to
Tasks" on page
6. Remove the
IF TOP
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
Tasks" on page
replace the board and go to
Step
157. If it has not, go to
can.
157. If the repair failed or no fault could be found,
"Final Tasks" on page
can
IF TOP
6. If it is not, go to Step
Step 6.
157.
Receiver Fault Finding
QN
4.
"Final
"Final
257