TM8100 Mobile Radio Service Manual Diagnosis of Faults on Main Board 199
May 2004 © Tait Electronics Limited
Tas k 2 —
Receiver Sensitivity Is
More Than 40 dB Too
Low
If the receiver sensitivity is more than 40 dB too low, the fault is probably
in the power-supply, control, LO, IF1 or IF2 circuitry.
1. Remove the main-board assembly from the chassis. Check for
3.0 V DC (3
V0 AN) at the TP601 test point [at 6K5/6D8] near the
LO2 BOT can (see Figure 9.14). If the voltage is correct, go to Step
2. If it is not, go to Task 3 of Subsection 9.1 on the power-supply
circuitry.
2. Remove the
LO2 BOT can. Check for 3.0 V DC (3V0 RX) around
the collector feed to Q402 [at 4G3/4E6] or Q403 [at 4G2/4D6]
of LO2 (see Figure 9.14). If the voltage is correct, go to Step 3.
If it is not, go to Task 3 of Subsection 9.1 on the power-supply
circuitry.
Alternative measurement points are the collector feed to Q401 [at
4F6/4G5] of the RF LNA under the
FE TOP can (see Figure 9.15)
or Q404 [at 4R6/4G14] of the IF amplifier under the
IF TOP can
(see Figure 9.13).
3. Check the logic signal
DIG SYN EN at pin 8 [at 4D1/4C3] of IC403
(see Figure 9.14). The signal is active high. The required status is
active — about 3.0 V. If it is, go to Step 4. If it is not, check the
signal continuity from the digital board to the receiver. Repair
any fault and go to Step 7. If the digital board itself appears faulty,
return to Subsection 8.1 and replace the complete main-board
assembly.
An alternative measurement point to the above is pin 24 [at
4L2/4D10] of IC400 under the
IF TOP can (see Figure 9.13).
4. Enter the CCTM command 72 to determine the lock status. The
result should be 111 (LO1, FCL, LO2). If it is, go to Step 5. If
the LO1 is not in lock, go to Subsection 9.3 on the frequency
synthesizer. If the FCL is not in lock, go to Subsection 9.4 on the
FCL. If the LO2 is not in lock, check the components around
IC403, Q402 and Q403 (see Figure 9.14). Repair any fault and
go to Step 7.
5. Remove the
IF TOP can. Check all components around Q404 [at
4R6/4G14] of the IF amplifier (see Figure 9.13). Check the
supply voltage at L419 [at 4R8/4H14]. Also check the amplifier
bias conditions: V
c
should be 1.5 ± 0.2 V and I
c
should be
1.8 ± 0.5 mA. Measure V
c
between the collector of Q404 and
the point shown in Figure 9.13. To check I
c
, unsolder and raise
one terminal of L419 (tombstone position), connect a multimeter
between this terminal and the pad for the terminal, and measure
the current. If there is no fault, go to Step 6. If there is, repair the
fault and go to Step 7.
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