Axcera-430B FCC Type Acceptance Report Parts List/Tune-up Info
February 2004
7-4
Locate (A9) the IF phase corrector board (1227-1250) mounted in the VHF exciter. The
amplitude correction portion of the board is not utilized in this configuration. As a result,
jumper W3 on J10 should be in the Disable position, to +6.8 VDC, and R35 and R31 should
be fully counter-clockwise (CCW). R68 is the range adjustment and should be set in the
middle of the range. The phase correction Enable/Disable jumper W2 on J9 should be in
the Enable position, to ground.
Switch the input video test source to select an NTSC 3.58-MHz modulated staircase or
ramp test waveform. Set up the station demodulator and monitoring equipment to monitor
the differential phase or intermodulation products of the RF output signal. There are three
corrector stages on the IF phase corrector board, each with a magnitude and a threshold
adjustment that are adjusted, as needed, to correct for any differential phase or
intermodulation problems. Adjust the R3 threshold for the cut-in point of the correction
and the R7 magnitude for the amount of the correction that is needed.
Jumper W1 on J8 is set to give the desired polarity of the correction shaped by the
threshold R11 and the magnitude R15 adjustments. After setting the polarity, adjust the
R11 threshold for the cut-in point of the correction and the R15 magnitude for the
amount of the correction that is needed. Finally, adjust the R19 threshold for the cut-in
point of the correction and the R23 magnitude for the amount of the correction that is
needed.
Note
:
Adjusting these pots changes all visual parameters and should be done
cautiously.
7.2.5 Linearity Corrector Adjustment
The IF linearity correction function consists of three non-linear cascaded stages, each
having magnitude and threshold adjustments, or cut-in points, on the ALC board. The
threshold adjustment determines at what IF signal level the corresponding corrector
stage begins to increase gain. The magnitude adjustment determines the amount of gain
change for the part of the signal that exceeds the corresponding threshold point. Refer to
the VHF exciter tray control locations drawing, ALC board
(1265-1305),
to find the
adjustments for the first through third linearity corrector stages. Because the stages are
cascaded, the order of correction is important. The first stage should cut in near white
level, with the cut-in point of the next stage toward black, and with the last stage
primarily stretching sync.
To adjust the linearity correctors from scratch, ensure that the transmitter is operating
at full power with the desired A/V ratio. Check that jumper W1 on J4 of the ALC board is
enabled between pins 1 and 2. Make sure that the ALC voltage is set to +0.8 VDC as
monitored on the front panel meter in the ALC position.
Insert a modulated ramp video test signal into the transmitter. Demodulate the output
signal of the transmitter and observe the waveform on a waveform monitor while also
looking at the signal on a spectrum analyzer. On the IF ALC board (1265-1306), preset
pots R34, R37, and R40 (threshold) fully CCW and the magnitude adjustments R13, R18,
and R23 fully clockwise (CW). On the IF phase corrector board (1227-1250), preset pots
R7, R15, R23, and R35 fully CW and R3, R11, R19, and R31 fully CCW.
Set the waveform monitor to differential step filter and the volts/division scale to .1
volts. Center the display to the blanking level (approximately).
Gradually adjust pots R3, R11, and R19 CW on the IF phase corrector board, as needed,
to minimize the observed thickness of the intermodulation as seen on the display.
Summary of Contents for 430B
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