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8. ADJUSTMENTS
8.1
FM TUNER
The tuning coil in the FM front end does not have
an adjusting core.
Consequently,
tracking adjust-
ments
at 90MHz
are performed
by regulating the
gap between rotor and stator of the tuning capaci-
tors (VC,, VC,, VC3). The expression adjust VC, ,
(VC,
, VC;) found in the text means that the two
outer rotor blades of each of these tuning capaci-
tors
are
to be extended
outwards
with
spatula
(Part No. GGK-066) as shown in Fig. 5.
c
Q
AX Se
CAN
&
Spatul
RC
Dix :
patula —
4) eG
yy
o
Rotor blade
EXTEND
Tuning
|
FRo----
¢
Spatula
ade
(GGK-066)
Stator
Rotor blade
Fig.5
Adjustment of Tuning Capacitor
ht
. Connect
up the test instruments
as shown
in
Fig. 7.
. Set the FUNCTION switch to FM.
. Turn the MODE/FM
MUTE
switch to OFF.
. Set the dial pointer of the SX-680 to 106MHz.
. Set the FM Signal Generator (FM SG) output
to 106MHz
at 106dB,
with 100% modulation
(75kHz deviation) for 400Hz. This modulation
setting
should
be used for all the following
adjustments.
6. Adjust TC,
to give the maximum
reading on
the multitester (with DC 500uA range).
7. Adjust the primary (lower) core of T, so that
the
AM-FM
meter
(the
FM
tuning
meter)
indicator needle reads dead center.
8. Set the FM SG output to 15dB.
9. Adjust
TC,
and
TC,
to give the maximum
reading on the multitester.
oP
wd
14
10. Set the SX-680 dial pointer to 90OMHz.
11. Set the FM SG output to 90MHz at 106dB.
12. Adjust
VC;
for
maximum
reading
on
the
multitester.
13. Set the FM SG output to 15dB.
14. Adjust VC,
and VC,
so that the multitester
reading reaches a maximum.
15. Repeat the adjustments of VC,, VC, and VC,
when the dial pointer is at 90MHz, and of TC;,
TC,
and
TC,
when
the
dial
pointer
is at
106MHz,
alternately until adjustment
at the
one
frequency
requires
no
further
re-adjust-
ment at the other frequency.
16. Adjust the core
of T, to give the maximum
reading on the multitester.
17. De-tune
the SX-680
(to a position
at which
only inter-station
noise is heard),
and adjust
the
primary
(lower)
core
of T,
so that the
AM-FM
meter indicator needle reads at dead
center.
18. Set the FM SG output to 98MHz at 66dB.
19. Tune the SX-680 to a dial reading of 98MHz
observe that the AM-FM meter is reading cor-
rectly at the center of the meter scale.
20. Adjust the secondary
(upper)
core
of T, to
give a minimum level of distortion.
21. Repeat the steps detailed in section 17 to 20
for the adjustment of the primary and secon-
dary cores of T, until both requirements are
fully met.
22. Set the SX-680 dial indication and FM SG fre-
quency for 98MHz.
23. Set the MODE/FM
MUTE
switch to the ON,
and check the muting threshold. If the muting
threshold is more than 28dB, cut the jumper
lead A.
Multiplex Decoder
24. Connect a MPX Signal Generator (MPX SG) to
the external
modulator
terminals
of the FM
SG.
25. Connect
the 19kHz
pilot output signal from
the MPX SG to the X-plates (horizontal input)
of an oscilloscope, and use a probe to connect
the Y-plates (vertical input) to the TP, termi-
nal.
26. Set the FM SG output to 66dB, unmodulated.
27. Adjust VR,
to freeze motion
of the resulting
Lissajous' figures.
NOTE:
Lissajous' figure adopts the general form shown in Figure
6 due
to the fact that the MPX
SG
19kHz
pilot signal
output
is a sine
wave,
and
the
TP,
terminal
carries
a
76kHz
saw
tooth
wave as determined
by the adjustment
of VR,.
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