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Calibration ——- Type
3A74
6. Check
for
Grid
Current
and
Microphonics
(All Channels)
This step checks the Input CF tube V423 of each channel
for grid current and
checks
both
V423 and
V433
in each
channel
for microphonics
under
no-signal
conditions.
a. Set
channel
1 MODE
switch
to
NORM.
(Check
that
the other
MODE
switches
are set to OFF.)
b. Set channel
1 AC-GND-DC switch to GND and note the
amount of vertical trace shift, if any.
Trace shift due
to grid
current
should
not exceed
one
minor division.
c. Tap lightly on the front panel of the Type 3A74; at the
same
time, check for excessive
microphonics.
Ampli-
tude of the microphonics should not exceed one major
division.
d. Set channel
1 MODE
switch to OFF.
e.
Using
steps 6a through
6d as a guide, check
channels
2, 3 and
4 individually
for grid current and
micro-
phonics.
7. Adjust Output Amplifier Gain
a. Set
the
oscilloscope
Calibrator,
or equivalent,
for an
output of 0.1
volt peak-to-peak.
b. Connect a coaxial cable with BNC cable with BNC plug
connectors
on each
end
(item 4) between
the Calibra-
tor
output
connector
and
the
channel
1
connectors.
(Use a coaxial connector adapter, item 13, to complete
the connections
if required.}
Set channel!
1 AC-GND-DC
switch
to DC.
Place channel
1 MODE
switch
to NORM.
e.
Set the time-base Time/Div. switch to .1 mSec.
Rotate channel
GAIN
ADJ. control fully clockwise.
g. Connect the probe from a
test oscilloscope (item 4) to
the emitter of Q463
(see Fig.6-1).
NOTE
To differentiate between the two oscilloscopes used
in this calibration
procedure, the term 'test oscillo-
scope"
is item
4 while
the
term
"oscilloscope"
is
item 1.
h. Rotate the Type 3A74 VAR. GAIN
contro! so the cali-
brator
waveform
displayed
on
the
test
oscilloscope
crt is 2 volts peak-to-peak in amplitude.
Connect the probe from the test oscilloscope to pin 6
lead of V464 (see Fig. 6-2).
NOTE
This step
sets the voltage
gain
of the Output
Am-
plifier 2nd
Stage
for a nominal
gain
of 20.
k. Connect
the probe
from
the test oscilloscope
to pin
6 lead of V474 (see Fig. 6-2) and check that the cali-
brator
waveform
displayed
on
the
test oscilloscope
is 40 volts peak-to-peak, within a tolerance of 2 volts.
|.
Disconnect
the
probe
from
the
pin 6 lead of V474.
8. Adjust
Input Amplifier Gain
(All Channels)
a. Remove
the flexible-cable
plug-in
extension.
b. Insert the Type 3A74
directly into the vertical plug-in
opening
of the oscilloscope.
c.
Rotate
channel
1
VAR.
GAIN
control
to
the
CAL.
detent
position.
d. With
0.1-volt calibrator signal applied
to channel
1,
rotate channel
1 GAIN ADJ. control to obtain a deflec-
tion of exactly five major divisions.
NOTE
Use
the
channel
1
POSITION
control
for aligning
the
display
with
the
graticule
markings
to aid
in
setting
the gain
accurately.
e.
Set channel
1 MODE
switch
to OFF.
f.
Disconnect the signa! from the channel 1 Input connect-
or and apply the signal to channe! 2.
g. Set channel
2 AC-GND-DC switch to DC and the MODE
switch to NORM,
h. Adjust channel
2 GAIN ADJ. control for proper vertical
deflection as explained in step 8d.
Then, set channel
2 MODE
switch to OFF.
i.
Apply the signal to channel 3.
Using steps d through
h as a guide, adjust channel
3 GAIN
ADJ. control for
proper deflection.
Then set channel
3 MODE switch to
OFF
and
repeat the procedure
for channel
4.
j.
After adjusting
channel
4 GAIN
ADJ.
control,
leave
channel
4 MODE
switch set to the NORM
position.
9. Check
VAR.
GAIN
Control
(All
Channels)
a. With
the
display
five
major
divisions
peak-to-peak,
rotate channel 4 VAR. GAIN
control clockwise out of
the detent or CAL.
position so the control
is set for
minimum
gain.
The display should be no more than
2 major divisions.
This indicates that the attenuation
ratio of the VAR.
GAIN
control
is at least 2.5 to 1.
b.
Continue to rotate the channel 4 VAR. GAIN
control in
a clockwise
direction
and
check
for smooth
electrical
operation
throughout
its entire
range.
(If rotation
of
the VAR.
GAIN
control causes erratic jumping of the
trace,
clean
or replace
the control.)
c. After checking the control, set the control to the CAL.
detent position and set channel
4 MODE
switch to OFF.
d. Apply the calibrator signal to channel 3 Input and set
channel
3 MODE
switch to NORM.
e.
Adapt
steps 9a through
9c for checking
the channel
3
VAR.
GAIN
control.
Then, check the VAR. GAIN
con-
trols for channels
2 and
1.
f.
Disconnect
the coaxial
cable.
10.
Check
Alternate
Mode
Operation
a. Set channel
2 MODE
switch to NORM.
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