NAD 7155 - Detector Coarse Adjustment; Detector Alignment (Final); Autosearch Level; Stereo Decoder, MPX Filters

DETECTOR COARSE ADJUSTMENT (OPTIONAL, NEEDED ONLY IF DETECTOR WAS REPAIRED)
Reduce sweep modulation level to + 75 kHz and set input level to 300 uV.
Adjust FM generator frequency so that both legs of the inverted U-shaped curve are equally high on the
display. The curve should be almost perfectly symmetrical.
Disconnect
detectorprobe
from
tuner
and
oscilloscope.
Connect
either
of
the
tape
1
rec.
outputs
to
the
oscilloscope.
The
oscilloscope
should
now
display
a
diagonal
line.
Adjust
L
102
primary
(closest
to
IC
102)
to
maxi-
mum curveheight
and
L
102
secondary
to
minimum
curve
height
and
straightest
possible
line.
Go
back
and forth
between
primary
and secondary
till
both
are peaked.
NOTE: Both the cores should be within 1.5mm from the top of the form.
DETECTOR ALIGNMENT (FINAL)
Disconnect detectorprobe and connect tape 1 rec. output to VTVM’s, oscilloscope and distortion
analyzer.
Switch
stereo
generator
to
1
kHz
100%
(+
75
kHz)
mono
modulation
and
oscilloscope
to
normal
internal
sweep
0.2
mS
and 0.5
V/cm sensitivity.
Detector reference frequency.
Reduce FM generator output level while monitoring THD from left channel. When THD increases to 3%,
fine tune the FM generator frequency to minimum THD. Reduce FM generator output level and fine
tune till no reduction in the 3% THD can be achieved by fine tuning. Use this frequency for all the follow-
ing detector, MPX and DYN SEP adjustments.
NOTE: The typical input level for this 3% THD should be 1.6 pV to 2.3 KV. This is done only to “line
up” the frequency from the generator to the tuner’s frequency.
lf IHF usable sensitivity (-30 dB THD + N = 3.16%) is to be verified, a proper IHF bandpass-
filter must be used.
Connect DMM across TP 104 (negative) and TP 105 (positive). Set FM generator output to 1000 uV
Adjust L 102 primary (closest to IC 102) for 0 V on DMM.
TOLERANCE +50 mv
Adjust L 102 secondary for lowest THD.
SPECIFICATION less than 0.1%
Repeat steps 3, 5 and 6 till no further improvements. Record the DMM's final reading for use later in
the adjustments. (1 — 3)
AUTOSEARCH LEVEL
Connect DMM between ground and TP 107,
Increase FM generator level upwards from 0 and adjust R 107 “MUTE” so that DMM reading goes from
0 V to approximately 4.8 V at 10 uV input.
TOLERANCE £2uv
STEREO DECODER, MPX FILTERS.
vco.
Connect a frequency counter and a 200 k ohm resistor in parallel between ground and TP 108.
Set FM generator to 1000 uV output and no modulation.
Adjust R 164 “MPX VCO” for a 19000 Hz reading on the counter.
TOLERANCE +100 Hz
Disconnect frequency counter and resistor and depress DYN SEP defeat switch (in).
Stereo switch threshold.
Modulate FM generator 1 kHz 100% left only plus 19 kHz pilot 8 - 10%.