3
6
7
3.4
ALIGNMENT:
3.4a
Exciter UHF Bandpass Filter:
1.
With the transmitter operating normally, place the OPERATE/STANDBY switch to
STANDBY. Carefully pull out the Exciter drawer and remove its top cover. Leave the
Power Adjust control as it would be for normal operation and place the Control/Interface
board OPERATE/SWEEP switch (A2PC4S1) to the SWEEP position. (The front panel
SWEEP MODE light will turn on.)
2.
Remove the modulator cable attached to the Exciter’s IF INput connector (J1). Also,
remove the RF cables attached to RF OUT connector (J3). Set up the test equipment as
shown in Figure 3
6
4. Set the VHF generator sweep width from 35 to 55MHz and use
45.75MHz and 41.25MHz markers if available.
3.
Remove the four screws securing the UHF Bandpass Filter (A2FL1) to the Exciter chassis
and place the OPERATE/STANDBY switch to OPERATE. Adjust variable capacitors C1
through C6, accessible through the holes on the front side of filter assembly, to obtain the
frequency response shown in Figure 3
6
5. A small tuning tool is required for this procedure.
4.
If the transmitter's channel is being changed to one that is more than 40MHz from the
factory preset, connect a spectrum analyzer to the 20dB attenuator in Figure 3
6
4 and find
the transmitter’s synthesizer/local oscillator (LO) frequency and the sweep signal appearing
on the low side of the LO. Tune the spectrum analyzer to the UHF Synthesizer's new
frequency as shown in Table 3
6
1, UHF Synthesizer Programming Chart. Program the
synthesizer for the new channel and look for the LO carrier on the analyzer. Tune the
analyzer 44MHz (36MHz for PAL B/G) below the LO frequency and tune capacitors C1
through C6 of the Bandpass Filter for maximum amplitude and bandwidth of the sweep
generator signal on the spectrum analyzer. Replace the analyzer with the diode detector
and oscilloscope, and adjust the Bandpass Filter for the response of Figure 3
6
5. In order
to obtain the appropriate bandwidth and loss characteristics (10dB minimum gain), it may
be necessary to open the filter and adjust the position of wire loading probes W1 through
W4. If possible, this procedure should be done using a network analyzer (or reasonable
substitute) to view input and output return loss, both of which should be less than 18dB.
IMPORTANT: If the transmitter’s output channel has been changed, the UHF
Bandpass Filter (A7FL1) at the unit’s output must be removed or replaced.
5.
Once the filter is appropriately tuned, reattach the filter to the Exciter chassis, move the
Control/Interface OPERATE/SWEEP switch to the OPERATE position (SWEEP MODE LED
off), replace the cover on the Exciter drawer and remove the test equipment.
6.
Recable the Exciter exactly as it was cabled when delivered and place the transmitter back
in operation according to Section 1.4 of this manual.
3.5
OUTPUT POWER CALIBRATION:
To ensure proper operation, the transmitter’s output power level and RF POWER meter calibration
should be checked once every year. With the meter switch in the FWD position, the RF POWER