Service Manual

63
8. Exit the VCO Attn. 20 kHz function.
VCO Attn. 12.5 kHz NOTE: Tuning of VCO Attn. for 25 kHz channel spacing must
be done first.
1. Under the Tx Align menu, select VCO Attn. 12.5 kHz.
2. Press PTT Toggle to key up the radio at the last non 0 transmit
frequency point (normally F7).
3. Inject a 1 kHz at 80 mVrms signal into the external mic connector
on the test box (VRS750 input).
4. Note down the deviation obtained.
5. Adjust the slider, adjust the spin control or key in softpot values
(press ENTER to confirm) for the frequency point until the
deviation is within range as defined in Table 12.
6. Press PTT Toggle to dekey the radio.
7. Program the softpot value by pressing the Program button.
8. Exit the VCO Attn. 12.5 kHz function.
PA Bias Tuning UHF/VHF
VRS750s (Procedure 1)
IMPORTANT NOTE: Tuning must be performed for both High
and Low power for UHF and VHF radios.
Take note that Procedure 1 IS NOT TO BE
PERFORMED for UHF low power level
tuning. Please refer to Procedure 2.
1. Ensure that the radio’s antenna output is terminated to a 50 Ohm
load. This is to prevent self destruction of the PA.
2. Under Tx Align, select PA Bias, select High Power.
3. 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.
4. Press Toggle Voltage to enable the PA control voltage edit box/
spin control.
5. Increase the Voltage softpot value and measure/read from current
meter. Call the new value Y.
6. Increase Y until Y-X is within the range given for PA current in
Table 13.
7. Press Toggle Voltage again to disable the PA control voltage edit
box/spin control.
Table 12. Deviation Specifications for VRS750 UHF/VHF Radios
Band Channel Spacing (kHz) Deviation (kHz)
UHF/VHF 12.5 2.20-2.30
UHF/VHF 20 3.40-3.60
UHF/VHF 25 4.40-4.60