Data Sheet Chapter 5

500-Watt VHF Transmitter Chapter 5, Detailed Alignment Procedures
425A, Rev. 0 5-7
board (1142-1601). The output power
level should drop to 20% because of the
VSWR cutback and DS4 should be
illuminated. The front panel Module
Status LED should not be lit.
Reconnect J4 and adjust R59 to begin
cutting back on the output power level
when the reflected level increases above
20%.
In the Power Supply Voltage position, the
front panel meter is calibrated to +48
VDC using R86 on the AGC control board.
5.11.2 (A1-A1) Phase Shifter Board
(1198-1602; Appendix D)
There are no adjustments to (A1-A1) the
phase shifter board (1198-1602). The
front panel has adjustments for phase
that are made during the amplifier array
setup procedure.
5.11.3 (A2-A1) VHF Filter/Amplifier
Board (1218-1104; Appendix D)
The (A2-A1) VHF filter/amplifier board
(1218-1104) has approximately 15 dB of
gain. Tune the channel filter capacitors
C29 and C20 (loading), C26 and C23
(center frequency), and C24 (coupling) at
J6 on the board for the best response.
Set voltage adjust pot R19 for +24 VDC
at the anode of CR5.
The idling current, no RF drive applied, of
the device Q1 is set for 250 mA. To set
the current, remove the RF drive,
measure the voltage across R16 (a 1Ω
resistor on the filter/amplifier board) and
adjust R13 for .25 volts (using Ohms’
Law: [E=I x R] : [E=250 mA x 1 Ω] :
E=250 mV).
5.11.4 (A2-A2) High-Band VHF
Amplifier Board (1219-1103;
Appendix D)
The (A2-A2) VHF high-band amplifier
board (1219-1103) has 20 dB of gain and
is biased for .6 amps of idling current, no
RF drive applied. Adjust voltage adjust
pot R10 for +24 VDC at pin 0 of the
regulator IC U1. To set the bias, remove
the RF drive from the board, measure the
voltage across R6 and R7 (two 1Ω
resistors in parallel on the high-band
driver board), and adjust R4 for .30 volts
(using Ohms’ Law: [E=I x R] : [E=.6
amps x .5 Ω] : E=.30 volts).
Connect a spectrum analyzer to output
jack J2 on the board and adjust C15 for
peak output.
5.11.5 (A3-A1) Overdrive Protection
Board (1198-1601; Appendix D)
The level of the RF input and output of
the (A3-A1) overdrive protection board
(1198-1601) should be +16 dBm during
normal operation.
To set up the overdrive circuit, check that
the output power level of the transmitter
is at 100% and adjust R11 on the board
for a reading of .4 VDC at TP1. Increase
the output power level of the transmitter
to 110%, sync only, and adjust R12 until
the output power begins to drop off.
Return the output power level of the
transmitter to 100%.
5.11.6 (A3-A2) VHF High-Band
Amplifier Board (1219-1114;
Appendix D)
The (A3-A2) VHF high-band amplifier
board (1219-1114) has 20 dB of gain and
is biased for 3 amps of idling current, no
RF drive applied. To set the bias, remove
the RF drive from the board and preset
bias adjust pots R5 and R9 CCW.
Measure the voltage drop across R5 (the
.1Ω/10W resistor on the current metering
board) and slowly adjust bias adjust pot
R5 for .15 volts. Continue to measure the
voltage drop across R5 and slowly adjust
the other bias adjust pot R9 for .3 volts
(using Ohms’ Law: [E=I x R] : [E=3
amps x .1 Ω] : E=.30 volts).
Connect a spectrum analyzer to output
jack J2 on the board and adjust C9 and