334 Transmitter Fault Finding (25W) TM8100/TM8200 Service Manual
© Tait Electronics Limited June 2006
Task 4 —
Check Forward and
Reverse Powers
First check the forward and reverse powers for an indication of which part
of the circuitry is suspect.
1. Enter the CCTM command 326 4 to set the transmitter power level
high.
2. Enter the CCTM command 33 to place the radio in transmit mode.
3. Enter the CCTM command 318 to check the forward power.
The value returned is the voltage in millivolts corresponding to the
power level, and should be as shown in Table 12.3.
4. Confirm the above result by checking the level at the
FWD PWR test
point (see Figure 12.4) using an oscilloscope.
5. Enter the CCTM command 319 to check the reverse power.
The value returned is the voltage in millivolts corresponding to the
power level, and should be as shown in Table 12.3.
6. Confirm the above result by checking the level at the
REV PWR test
point (see Figure 12.4) using an oscilloscope.
If the oscilloscope momentarily indicates a very high reverse power,
then the most likely scenar io is that the antenna VSWR threshold has
been exceeded and the PA has shut down to very low power.
7. Enter the CCTM command 32 to place the radio in receive mode.
8. If the values obtained in Step 3 and Step 5 are both correct, and there
is no indication of a momentary high reverse power, go to Ta sk 5
.
If one or both are incorrect, go to Step 9.
9. Check the connection from the RF connector on the radio to the test
set.
10. If there is no fault, go to Step 11. If there is, rectify the fault and
repeat the above measurements.
11. If the reverse power is momentarily too high, the directional coupler,
PIN switch or LPF is suspect; go to Tas k 2 9
. Otherwise go to Ta sk 5.
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