Tait TM8235, Service Manual

Page: 313 / 578

TM8100/TM8200 Service Manual Transmitter Fault Finding (40W/50W) 313
© Tait Electronics Limited June 2006
Task 31 —
Check Power at
Directional Coupler
If, as determined in Task 26 to Task 30 , there is no fault in the initial RF
signal path, investigate the final signal path. This part of the circuitry may
also require investigation following certain checks in
“Transmitter RF
Power” . Begin by checking the directional coupler as follows:
1. If not already done, remove the
DIRC TOP
can.
2. Remove the coupling capacitors C348 , C349 , C350
(see Figure 11.20 ).
3. Solder one terminal of a 80pF (VHF) or 82pF (UHF) test capacitor
to the PCB at the point shown in
Figure 11.20 . Mount the capacitor
vertically. Use a test capacitor of the type GRM111, DLI C17,
Murata 1210, or the equivalent.
4. Solder a 50
Ω
test lead to the PCB: Solder the outer sheath to the test
pad shown in Figure 11.20 , and solder the central wire to the other
terminal of the test capacitor.
5. Connect the test lead to the test set.
6. Enter the CCTM command
326 5
to set the transmitter power level
to the maximum.
7. Enter the CCTM command
101 x x 0
, where
x
is the lowest
frequency (in hertz) for maximum power, as given in Table 11.8
(B1), Table 11.9 (H5) or Table 11.10 (H7).
8. Enter the CCTM command 33 to place the radio in transmit mode.
9. Measure the RF output power. This should be:
10. Enter the CCTM command 32 to place the radio in receive mode.
11. Enter the CCTM command
101 x x 0
, where
x
is the highest
frequency (in hertz) for maximum power, as given in Table 11.8
(B1), Table 11.9 (H5) or Table 11.10 (H7).
12. Repeat Step 8 to Step 10 .
13. If the power measured in both the above cases exceeds 70W (VHF)
or 60W (UHF), go to
Step 14 . If it does not, go to Task 32 .
14. Remove the test lead and test capacitor, resolder the coupling
capacitors in position, and go to
Task 33 .
RF output power: more than 70W (VHF)
more than 60W (UHF)