Audio Developments AD144 - Manual (Page 25)

24
ADJUSTMENTS AND CALIBRATIONS
Refer to component location diagrams in TECHNICAL LIBRARY.
Line-up-tone oscillator - the preset (VR1) that adjusts its level at the outputs of the
mixer is mounted on the right-hand PCB of the output module and can be adjusted
from the copper foil side of the board.
Meters - either a VU or a PPM may be selected. A choice of 3 scales is available for
the PPM: BBC, N10, SDR.
VU meter - there is one preset, on the VU meter PCB, for meter adjustment. Set an
input module for a line input and introduce a 1kHz tone from an audio-signal
generator. Adjust levels to achieve a reading of +4dBu, at a main output, on an AC
millivoltmeter - measured across pins 2 & 3 on the output XLR. Switch the appropriate
meter to the selected output and adjust the preset to give a reading of 0VU.
PPM - driver PCB has three calibration controls (presets). VR1 adjusts the reference
level, VR2 and VR3 adjust the 'law' of the meter. VR2 adjusts the upper sector of the
scale and VR3 adjusts the lower sector. To initiate calibration, set each preset to its
mid position.
BBC Scale - using the tone generator and millivoltmeter as described above, adjust
mixer levels for a reading of 0dBu, on the millivoltmeter, from a main output. Switch
the appropriate meter to the selected output and adjust VR1 for a reading of 4.
Increase the output signal to +8dBu and adjust VR2 for a reading of 6. Decrease the
output signal to -8dBu and adjust VR3 for a reading of 2.
Repeat this procedure until an accurate set of readings is obtained without further
adjustment to any of the presets being required. Now check all points 1 to 7 on the
scale to determine whether they are within specification.
N10 Scale - for a meter calibration of TEST = 0dBu. With a main-output signal
of -6dBu, adjust VR1 to give a meter reading at -6. Increase the signal to +6dBu and
adjust VR2 to obtain a meter reading at +6. Lower the output signal to -18dBu and
adjust VR3 to obtain a meter reading at -18. Repeat this procedure to obtain an
accurate reading at each of these three points. Now check the calibration of all meter
points.
SDR Scale - with a main-output signal level of -6dBu, adjust VR1 to obtain a reading
of -12. Increase the output signal to +6dBu and use VR2 to obtain a reading of 0 on
the meter. Lower the output signal to -18dBu and adjust VR3 to give a reading of -24.
Repeat the procedure to obtain an accurate reading at each of these three points.
Now check the calibration of all meter points.