Motorola VRS750, Service Manual

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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.
PA Bias Tuning for UHF
Low Power VRS750s
(Procedure 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.
STEP 1 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 ).
Table 13. PA Bias High Power Specifications UHF/VHF VRS750s
RF Band PA Current (mA) Bias Current (mA)
VHF 90-110 90-110
UHF Band 1 30-45 600-700
UHF Band 2 30-55 400-480