5.6.6.5
Forward/Reflected Power Adjustments C6 and C40
a. With the transmitter operating at 25 kW and no modula-
tion, read the Reflected power indication on the front panel
meter.
b. Adjust C40, Reflected Balance control, to null the meter
indication. Note that C30 is added by P2 at the low end of
the band to allow the meter indication to null.
c. Depress the OFF button.
d. Locate P1 and P3 on the Output Monitor. Move both
jumper plugs from position 1-2 to position 1-3.
e. Turn the transmitter back on at full power. Note that the
Reflected meter position now indicates forward power and
the Forward meter position now indicates reflected power.
f. Operate the Forward/Reflected meter switch to the For-
ward power position. Adjust C6 to null this indication.
g. Depress the Off button and move jumpers P1 and P2 to
position 1-2.
5.6.6.6
Modulation Monitor Sample Adjustments
Refer to the Initial Turn-On procedure in SECTION II, Installa-
tion/Initial Turn-On, for the procedure to set the Modulation
monitor sample adjustments.
5.6.7
LED Board (A32)
The LED Board contains five overloads which can be preset to
the correct reference voltage or resistance before the board is
replaced. The Factory Test Data sheets list the voltage setting for
each overload except the Power Supply overload.
If the Factory Test Data information is not available, measure the
test point voltages on the original board before removing it from
the transmitter. Use Table 5-5 to record the voltages for future
reference.
5.6.7.1
LED Board Replacement
To replace the LED Board:
a. Turn off the Low Voltage supply at CB1 and CB2.
b. Remove all cables from the LED Board.
c. Remove the pushbutton caps from the VSWR Self Test
switch S2 and the Reset switch S3 on the ColorStat™
panel.
d. Remove the screws holding the LED Board to the Center
Control Compartment door and remove the board.
e. Using an Ohmmeter, measure the resistance from the right
side of R92 to ground. Record this measurement in Table
5-6 for future reference. Adjust R86 on the replacement
LED Board for the same resistance reading.
f. Install the replacement LED Board.
g. Turn on the Low Voltage supply at CB1 and CB2.
h. Using a digital multimeter, set the following test point
voltages to match those recorded on the Factory Test Data
sheets:
1. TP6 (R42): Average Current Overload
2. TP5 (R41): Overdrive Overload
3. TP7 (R68): Peak Current Overload
4. TP8 (R67): Underdrive Overload
If it is not possible to preset the replacement board voltages or if
the correct overload operation needs to be verified, the following
procedure for setting each overload should be used.
5.6.7.2
Overload Adjustment Procedures
The following procedures are used to set individual overloads on
the LED Board.
5.6.7.2.1
Drive Overloads
These overloads protect the RF amplifiers from drive levels
below 20.0 Vp-p or above 27.0 Vp-p. The nominal drive level is
21 to 25.0 Vp-p, measured at the MOSFET gate on the RF
amplifiers. The first step in setting the drive overloads is to
remove the supply voltage to all the RF amplifiers so that no
damage will occur while the drive level is varied. Next the drive
level will be varied to the overload limits and the overloads will
be set.
WARNING
ENSURE ALL PRIMARY AC VOLTAGE HAS BEEN REMOVED
FROM TRANSMITTER AND A GROUNDING STICK IS USED TO
GROUND ALL POINTS WHERE AC OR RF POWER HAS BEEN
APPLIED BEFORE PROCEEDING WITH THE FOLLOWING PRO-
CEDURE.
a. Remove all AC primary power from the transmitter at the
AC main breaker.
b. Remove the RF Amp Compartment and Driver Compart-
ment rear access panels.
c. Remove the following fuses:
1. A25: F1-F8
2. A24: F1-F9
3. F20: (on top of T1)
d. Reinstall the access panels.
e. Locate Big Step RF amplifier RF33 behind the interlocked
RF Amp Compartment access door.
f. Connect 10x scope probe with an extended tip, Harris part
610-1131-000, through the door to the anode of CR3 in
front of the heatsink. Ground the probe to door.
g. Set up the scope to measure an RF waveform of approxi-
mately 23.0 Vp-p.
h. Restore AC primary power at the main breaker.
NOTE
When measuring RF amplifier drive amplitudes or phasing, the
RF amplifier to be measured must be turned “ON” to give a
correct drive measurement. The drive waveform of an “OFF” RF
amplifier will be below 0.0 VDC and the peaks may be clipped.
TEST POINT CIRCUIT FUNCTION MEASURED DC
VOLTAGE
TP5
OVERDRIVE
TP8
UNDERDRIVE
TP7
PEAK CURRENT
TP6
AVERAGE CURRENT
Table 5-5. LED Board Preset Voltages
Section V - Maintenance
Rev. Z1: 06-02-00 Additional info to ECN 46105
888-2297-002
5-11
WARNING: Disconnect primary power prior to servicing.
