Tait TB7100 - Page 205

TB7100 Service Manual Receiver Fault Finding 207
© Tait Electronics Limited October 2005
Task 5 —
Check Biasing
of IF Amplifier
If the lock status is normal, check the biasing of the IF amplifier.
1. Remove the
IF TOP can.
2. Check all components around Q404 of the IF amplifier
(see Figure 12.1).
3. Check the 3V supply voltage at L419; use the measurement point
shown in Figure 12.1.
4. Also check the amplifier bias conditions. First measure V
c
between
the collector of Q404 and ground (see Figure 12.1).
5. Secondly, check I
c
. To do so, unsolder and raise one terminal of L419
(tombstone position) (see Figure 12.1), connect a multimeter
between this terminal and the pad for the terminal, and measure the
current.
6. If the checks in Step 2 to Step 5 reveal no fault, go to Task 6
. If there
is a fault, repair it and go to Step 7.
7. Recalibrate the receiver using the calibration application.
8. Confirm the removal of the fault, and go to “Final Tasks” on
page 123. If the repair failed go to Task 7
.
Task 6 —
Check Matching
Circuitry
Having excluded the IF amplifier, check the matching circuitry for the
crystal filters.
1. Check all remaining components between T401 and IC400 — these
form the matching circuitry for the crystal filters XF400 and XF401
(see Figure 12.1).
2. If the above check reveals no fault, go to Step 3. If there is a fault,
repair it and go to Step 6.
3. Remove the
PIN TOP and LPF TOP cans.
4. Make a visual check of the components in the receive path of the PIN
switch and LPF circuits.
5. If the visual check reveals an obvious fault, repair it and go to Step 6.
If there is no obvious fault, go to Task 7
.
6. Recalibrate the receiver using the calibration application.
7. Confirm the removal of the fault, and go to “Final Tasks” on
page 123. If the repair failed go to Task 7
.
V
c
: 2.0 ± 0.2V
I
c
: 1.8 ± 0.5mA