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21
Input Level Set
1. Preset the 708 front-panel switches as follows:
POWER — ON
MODE — MONO
PILOT — OFF
O’SHOOT COMP. — OUT
PRE-EMPHASIS — OUT
1. The input gain range should be jumpered for high-level inputs; this
is the
H
position on jumpers strips
JMP2
and
JMP3
. Apply the
output of the Audio Generator to both the right- and the left-
channel
PROGRAM LINE INPUTS
. Feed these two inputs in
parallel, and in phase, by connecting the test signal “hot” lead to
both
+
terminals, and the test ground lead to
G
. Both – terminals
also should be connected to the
G
ground terminal.
2. Set the Audio Generator frequency to 400Hz, output level to
+10dBu.
3. Clip the two ‘scope probes to the output of the filter buffer
amplifier, IC34. The most convenient monitor points are the right-
hand ends of R128 and R135, for the left and right channels,
respectively. The monitor points are identified in the circuit board
legend with
L
and
R
designations next to the right-hand end of the
two resistors.
4. Adjust both front-panel
L
and
R
INPUT GAIN
controls for a scope
display of approximately 9 volts peak-to-peak. Position the left
channel waveform slightly above the right to keep track of which is
which.
Low-Pass Filter
Matching
The low-pass filter for the right program channel is not adjustable;
rather, it employs close-tolerance components to meet design
requirements. Since close frequency and phase response matching
between stereo channels is essential to good Stereo Generator crosstalk
performance, trim controls have been included in the left-channel filter
so that it may be tuned to match right channel response.
1. Confirm that the
PRE-EMPHASIS
and
O’SHOOT COMP
switches
are in their
OUT
positions. Frequency and phase response
matching is optimized with these circuits inactive. The scope
should continue to monitor the output of the filter buffer stage.
2. Increase the Audio Generator frequency to the first filter null,
approximately 19.1kHz. Observing the lower (right channel) trace,
down-range both oscilloscope vertical attenuators and fine-tune the
Audio Generator frequency for the deepest null in the right channel
signal.
HINT:
When properly nulled, the signal waveform will be very close to
the noise floor. If you find the nulled signal difficult to resolve, try
triggering the ‘scope timebase directly from the Audio Generator. This
will make the waveform “hold still” and stand out from the (random)
noise.