Filter
Top Products
Maintenance
-
494A/{g4Ap
Servtce
Vol.
I
Troubleshooting
and
Galibrating
the
16-20
MHz
Phase
Lock
Section
Replacing
oscillator
components
in
this
section
may
alter
sweep
linearity
and
the
oscillator
frequency.
Thi
following
checks
and
calibration
aid
in
repairing
and
retuning
the assembly.
1. Preliminary
_
a.
Test
equipment
setup
is shown
in
Figure
6-23.
Remove
and
install
the
Center
Frequency
Control
board
on
an
extender
board.
b.
Switch POWER
on
and
set
the
FREO
SPAN/D|V
to
1
MHz.
2.
Check
Voltages
-
a.
Check all input
voltages
at
the
feedthrough
capa_
cJtors
in
the
housing
wall.
RefEr
to Figure
OZb
or
ine
data
printed
on
the
tid.
The
vottage
LEVEL
at the
sweep
and
tuns
input
lines
should
be
0 V
+O.OS
V with
the
FREQ
SPAN/D|V
)500 kHz.
b.
Switch
the POWER
off.
Remove
the tid
frorn
the
mu-metal
housing
assembly
to
gain
access
to internal
circuitry.
c.
Switch
POWER
on,
then
check
the
internal
regu-
lated
voltages;
+12
V *0.4
V at
C2201,
-12V
*O.i
V
at
C2202,
and
+S.2
V
*0.2S
V
at
TplO109 (see
Figure
6-16).
Check
the
output
of
the
shaper
at
Tpl0gg
for
a level
between
+0.9
V and
-0.3
V
(0
V i0.g
V).
3.
Setting
Center Frequency
a.
Connect a
frequency
counter
to
Tp2O35
and
note
the frequency.
tf
the
frequency
is
within
S0 kHz
of
18
MHz
no
correction
is
necessary; proceed
to
part
4
(Setting
Tune
Sensitivity
and
nangay.
tf
outside
the
range
proceed
as
follows:
(1)
Turn
POWER
off.
Unsotder
and
remove
one end
of
Shaper
Offset
resistor
R1070.
Unsolder
the wire
strap
between
T2092
and
T1091,
at
the
T1091
end.
and
lift
it free.
(2)
Solder a
flexible
wire
jumper
to the
T2092
end;
then,
by
means
of
a a
plastic
tuning
tool,
attach
the
free
end
to one
of
the
three
pads
for
T1091
and
note
the frequency
readout
of
the counter.
ll
this flexible
wire
touches
ground
whil€
the
circuit
is
operating,
the
supply
regulators
can
be
damaged.
The
regulators
are
not
protected
against
a short
circuit.
(3)
lf one
of
the
pads
provides
a
frequency
that
is
within
the range
of
17.5
MHz *0.2S
MHz,
solder
the
wire
strap
to
this
pad.
lf
the frequ€ncy
is
still
out-
side
the range,
movE the
jumper
to
the
other pad
for
T2092
and repeat
the
procedure.
Frequency
must
equal
17.5
MHz +0.25
MHz.
b. Turn
POWER
OFF. Replace
R1070 with
a
10
turn
25 kO
potentiometer
in series
with
a
S ko resistor.
c. Turn
POWER
on
and
with
the counter
connected
to
TP2035, adjust
th€
potentiometer
for
a
frequency
readout
of 18
MHz *50
kHz.
d.
Turn
POWER off,
measure
the total resistance
valu6 and teplace
R1070 with
a
lyo resistor
of
thls
measured value.
Switch
POWER
on
and recheck
the
frequency
to
ensure
that
it is 18 MHz *50
kHz.
Disconnect
the
counter
from
TP2035.
4.
Setting Tune
Sensitivity
and Range
a.
Center
the Fine Tune adjustment
R4040,
on
the
Center Frequency
Control
board
and
the
2nd LO
Sweep
adjustment
Rl067,
on the
Span
Attenuator
board
(Figure
6-24).
b.
Decrease
the
FREQ
SPAN/D|V
to 200 kHz
or
less. The 2nd
LO
is now in
the
center
of
its
tune
range.
c.
Press <Blue-SHIFT>
10
dB/DlV
to disabte
frs-
quency
corrections.
Press <Blue-SHIFT>
PULSE
STRETCHER
and select item #0
(ALTERNATE
FRE-
QUENCY DfSPLAY),
then item
#2
(znd
LO FRE-
OUENCY DISPLAY).
Beadout will
now indicate
the
2nd
LO
frequency.
d.
Tune
the
2nd
LO to one end of
its range
where
the frequency readout
no
longer changes.
Note
the frE-
quency
and measure
the
voltage
on
the
Tune
Line at
the
input
feed-through
capacitor
(Figure
6-20).
e. Tune
the 2nd LO
to the other end of its range
and
again note
the frequency and
the
new
voltage
read-
ing.
f.
Calculate
the frequency change
per
vott
(fre-
quency
range versus
voltage
range).
Frequency
change
per
volt should
equal 128.0 kHz
*10o/o or range
between 115.2
kHz and 140.8 kHz.
g.
lf
the
frequency/volt change
is low, decrease
ihe
vafue of R2072
(Figure
6-20).
h. Press <BIue-SHIFT>
PULSE
STRETCHER
and
menu item
#1.
then
select
the 2nd LO for
calibration.
Perform
the
procedure
that
is called out for adjusting
the Fine Tune
Range R4040 and Fine
Tune Sensitivity
R3040
to
calibrate
the
2nd LO
tuning range.
o
o
o
O
)
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o
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a
a
o
a
o
o
o
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O
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t
o
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O
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O
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I
o
6-40