Motorola VRS750 Service Manual page 76

PA Bias Tuning for UHF
Low Power VRS750s
(Procedure 2)
STEP 1
64
8. Press Toggle Bias 1 in order to enable the PA Bias softpot.
9. Increase the softpot value to obtain Z until Z-Y is within the
range for Bias Current as indicated in Table 13.
10. Press Toggle Bias 1 again to disable the PA Bias softpot.
11. Program the Bias softpot into the radio's codeplug via pressing
Program.
12. Exit the PA Bias High Power function.
13. Under Tx Align, select PA Bias, Low Power. (If UHF, skip to step
15 and continue to Procedure 2.)
14. Repeat steps 3-11.
15. Exit the PA Bias Low Power function.
Table 13. PA Bias High Power Specifications UHF/VHF VRS750s
RF Band
VHF
UHF Band 1
UHF Band 2
NOTE: Failure in the instructions below means that you
cannot get a reading which is within the range of the
current window as specified in Table 14 for each
individual step.
Ensure that the radio's antenna output is terminated to a 50 Ohm
load. This is to prevent self destruction of the PA.
1. Under Tx Align, select PA Bias, select Low Power.
2. Measure/read the dc current drawn from radio (specifications will
vary from radio to radio). Call this X. This value should typically
range from 70-100 mA or 0.07-0.1 A.
3. Press Toggle Voltage to enable the PA control voltage edit box/
spin control.
4. Increase the Voltage softpot value and measure/read from current
meter. Call the new value Y.
5. Increase Y until Y-X is within the range given for PA current in
Table 14 (STEP 1) below.
****** If a failure occurs, go to STEP 2 otherwise proceed
6. Press Toggle Voltage again to disable the PA control voltage edit
box/spin control.
7. Press Toggle Bias in order to enable the PA Bias softpot.
8. Increase the softpot value to obtain Z until Z-Y is within the
range for Bias Current as indicated in Table 14 (STEP 1).
PA Current (mA) Bias Current (mA)
90-110
30-45
30-55
90-110
600-700
400-480