TM8100 Mobile Radio Service Manual Diagnosis of Faults on Main Board 191
May 2004 © Tait Electronics Limited
9.4 Frequency Control Loop
Lock Status Fault diagnosis of the FCL is divided into four tasks. First discussed,
however, is the lock status of the radio, which will indicate whether or
not the FCL is suspect. The lock status might already have been
determined as part of the preliminary fault diagnosis. The following is a
summary:
1. Enter the CCTM command 33 to place the radio in transmit
mode.
2. Enter the CCTM command 72 to determine the lock status in
transmit mode. Note the response. The normal status is 110.
3. Enter the CCTM command 32 to place the radio in receive mode.
4. Enter the CCTM command 72 to determine the lock status in
receive mode. Note the response. The normal status is 111.
5. If the lock status in either or both receive and transmit mode is
a0a, where a is 0 or 1, the FCL is suspect; proceed to Task 1.
Tas k 1 —
Check VCXO and
TCXO Outputs
First check the VCXO output and the TCXO output as follows:
1. Use an oscilloscope probe to check the VCXO output at C536 [at
5G8/5H6] — probe the via situated between C536 and R537 (see
Figure 9.11). The signal should be a sine wave of about 1.1 V
pp
on 1.45 V DC. If it is, go to Step 3. If it is not, go to Step 2.
2. The VCXO circuitry (Q501, Q503, XL501 and associated
components) under the
VCXO BOT can is faulty. If not already done,
remove the main-board assembly from the chassis, and remove the
VCXO BOT can. Locate and repair the fault in the VCXO (see
Figure 9.12). Confirm the removal of the fault and go to Step 3.
If the repair failed, return to Subsection 8.1 and replace the
complete main-board assembly.
3. Use the oscilloscope probe to check the TCXO output at the
TP504 test point [at 5B8/5J2] (see Figure 9.11). (The test point is
at the junction of C510 and R513.) The signal is
SYN RX OSC and
should be a clipped sine wave of about 1.0 V
pp
. If it is, go to Task
2. If it is not, go to Step 4.
4. The TCXO circuitry (XL500 [at 5B8/5J2] and associated
components) under the
CDC TOP can is faulty. Remove the CDC TOP
can. Locate and repair the fault in the TCXO (see Figure 9.16 in
Subsection 9.5). Confirm the removal of the fault and go to Task
2. If the repair failed, return to Subsection 8.1 and replace the
complete main-board assembly.
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