TM8100 Mobile Radio Service Manual Diagnosis of Faults on Main Board 187
May 2004 © Tait Electronics Limited
Tas k 2 —
VCO Checks
If there is no fault with the digital board or power supplies, check the
VCO. The following procedure applies to the UHF board (bands H5
and H6); the procedure for the VHF board (bands B1 and D1) is similar.
1. Remove the
VCO TOP and SYN TOP cans.
2. Enter the CCTM command 32 to place the radio in receive mode.
Measure the voltage on the collector of Q504 [at 5K3] (see
Figure 9.9). It should be about 5 V. If it is, go to Step 3. If it is
not, check the circuit between Q504 and pin 3 of Q508 [at 5Q6].
If a fault is found, repair it and confirm that the voltage is now
correct; then enter the CCTM command 72 again and take the
action indicated; if the lock status is still 011 or 010, go to Step 3.
If the repair failed or the fault could not be found, return to
Subsection 8.1 and replace the complete main-board assembly.
3. Use the CCTM command 101 to program the radio with the
centre frequency of the frequency band. The format of the
command is 101 aa0, where a is the transmit frequency in hertz.
Enter the CCTM command 335 1 to switch on the transmit-
receive switch in the synthesizer. Carry out the checks in Step 4.
After these checks enter the CCTM command 335 0 to switch off
the transmit-receive switch.
4. Measure the voltage on the collector of Q510 [at 5Q3] (see
Figure 9.9). It should be about 5 V. If it is, go to Step 5. If it is
not, check the circuit between Q510 and pin 3 of Q508 [at 5Q6].
If a fault is found, repair it and confirm that the voltage is now
correct; then enter the CCTM command 72 again and take the
action indicated; if the lock status is still 010, go to Step 5. If the
repair failed or the fault could not be found, return to
Subsection 8.1 and replace the complete main-board assembly.
5. Use the CCTM command 101 to program the radio with the
lowest frequency of the frequency band. Enter the CCTM
command 33 (transmit mode) and then the command 32 (receive
mode), and in each case carry out the checks in Step 7. (If the
radio fails to enter transmit mode, go to Task 4.)
6. Repeat Step 5 with the radio programmed with the highest
frequency of the frequency band.
7. Measure the VCO tuning voltage at C565 [at 5M5] (see
Figure 9.9). The tuning voltage should be steady DC with no
modulation or noise. The DC values listed in Table 9.2 should be
obtained. (The corresponding values for VHF radios are also
given in the table.) If the voltages are correct, return to
Subsection 8.1 and replace the complete main-board assembly. If
there is unexpected modulation or noise, go to Task 3. If the
voltages are not correct, go to Task 4.
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