NAD 4225 - IF ADJUSTMENT; DETECTOR COARSE ADJUSTMENT; DETECTOR ALIGNMENTS (FINAL); AUTOSEARCH LEVEL

6.Repeat
step
2+3+4+5
if
curve
height
is
outside
of
tolerances,
if
necessary
distribute
the
error
between
90
and
105
MHz.
Check
tuning
voltage
again
if
tolerances
not
possible
to
schieve.
C IF ADJUSTMENT
1.Set
tuner
to
approximately
98
MHz
(the
tuner
must
be
tuned
to
an
unoccupied
frequency)
enter
into
preset
3,
and
tume
FM
generator
to
display
a
curve
on
the
oscilloscope.
2,Adjust
I
1
(IFT
tuner
frontend)
to
maximun
and
symmetrical
curve
on
the
display,
using
as
little
input
signal
as
possible.
D
DETECTOR
COARSE
ADJUSSTMENT
(OPTIONAL,
NEEDED
ONLY
IF
DETECTOR
WAS
REPAIRED).
1.Reduce
sweep
modulation
level
‘to
+
75
KHz
and
set
input
level
to
300
uV.
2.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.
3.Disconnect
detector
probe
from
tuner
and
oscilloscope.
Connect
either
of
the
tape
rec.
outputs
to
the
oscilloscope.
4.Adjust.
1102,
FM
DET
PRIMARY
to
maximum
curveheight
and
1103,
FM
DET
the
front
of
unit
to
minimum
curveheight
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.
E DETECTOR ALLIGNMENTS (FINAL)
1.Disconnect
detector
probe
and
connect
tape
rec.
outputs
to
VIVM’s
oscilloscope and distortion analyzer.
2.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.
3.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 following detector, MPX adjustments.
NOTE: The typical input level for this 3% THD should be 1.6 uV to 2.3 uvV.
This is done only to "lineup" the frequency from the generator to the
tuner’s frequency. IF IHF usable sensitivity (-30 dB THD+N=3.16% THD+N)
is to be verified, a proper JHF bandpassfilter must be used.
4.Cornmect DMM across P112 (negative) and P113 (positive). Set FM generator
output level to 1000 uV.
5.Adjust I102, FM DET PRIMARY for OV on DMM
TOLERANCE + 50 mV
6.Adjust I103, FM DET SECONDARY for lowest THD
SPECIFICATION less than 0.1%.
7.Adjust C917 for lowest THD.
8.Repeat steps 5-7 till no further improvements. Record the DMM’s final
reading for use later in the adjustment.
F AUTOSEARCH LEVEL
1.Connect. DMM between P120 and ground.
2,Set FM generator level to 10 uV and adjust R117 so that DMM reading goes
from 8.5V to approximately 0.55V at 10 uV.
TOLERANCE + 0.1V.