Sect.
IV
Page
5
5) Measure the voltage between pin 5 of tube V6
and
B-.
This voltage should be about
+
7 5 volts.
Variations in OA3 tubes can cause this voltage to
fall at any point between 68 and 85 volts.
6) Repeat step 3
if
you replace either V5 or V6.
The characteristics of cold-cathode regulator tubes
drift during about the
first
72 hours of operation.
This drift can affect the 202A output. A 72 hour
aging
is
recommended for a new tube for either
V5 or V6.
7) Test the regulated output voltage at pin 5 of tube
V5 while varying line voltage between 103 and 127
volts. The regulated voltage
will
normally not change
by more than
*l%.
Check power supply tubes and
components if the change
is
excessive.
4-10 THEORY OF DC BALANCE AND
DISTORTION ADJUSTMENTS
The output AMPLITUDE control
is
located at the
input to the output amplifier.
If
the dc component
at the output terminals
is
to be zero for all settings
of the AMPLITUDE control the dc levels at the ends
of
the AMPLITUDE control must be the same and
also equal to the average level of the input wave.
From the schematic wiring diagram,
it
is
seen that
the common connection between the two sections of
the control
is
connected to the cathode of V4. The
level of this point
can
be adjusted to the desired value
by R54. The signal impedance of this point
is
very
low compared with the magnitude of the AMPLITUDE
control impedance, and therefore, the cathode of
V4
has
virtually zero signal.
When R49, R51, R54, and R60 are adjusted properly,
there
is
no dc component across either section
of
the AMPLITUDE control.
When the FUNCTION switch
is
in the squarewave
position, there
is
no signal input to one section
of
the AMPLITUDE control, hence, the tap on that
section merely carries the constant bias level set
by the cathode
of
V4. The other section
is
connected
through a network to the clamp section of the bi-
stable circuit. R22
d
this network adjusts the aver-
age level
of
the squarewave applied
to
the amplitude
control to the same value as the cathode
of
V4.
The dc levels at
the
input to the amplifier are inde-
pendent of AMPLITUDE control setting. The dc
levels of the two output terminals may
be
adjusted
to
be
equal
by
R65. Control R65 varies the dc level
of the signal on one grid of the second stage of the
amplifier. When these adjustments are made, the
dc component between the output terminals will
remain at a very low value, independent
of
amplitude
setting or waveform selected.
Control R49 varies the level to which the output of
the integrator rises in
a
positive direction and A51
varies the level
of
the negative excursion. The b6s
levels of the shaper diodes are not variable and
therefore, the triangle input
to
the shaper can have
one and only one correct magnitude and average
level.
Figure 4-2 shows the situation at the shaper when
the two reference levels are properly adjusted.
Figure 4-2B shows the effect of having the reference
levels adjusted for too large a magnitude, but with
the proper average value. Figure 4-2C shows the
effect of having reference levels adjusted for a
triangle of the proper magnitude, but incorrect
average level. This indicates
a
close relationship
between correct frequency calibration and minimum
distortion. In fact, the two conditions are simul-
taneously satisfied by optimum settings of the same
adjustments.
4-11 DC BALANCE AND DISTORTION
ADJUSTMENTS
The following test procedure re@ires a
dc
voltmeter
with an input resistance of at least 100 megohms
such as an
@Model 410B. In addition, the volt
-
meter must not
be
grounded as the common side of
the meter must
be
connected to points within the
202A
that
are not at ground potential. A Distortion
Analyzer and an Oscilloscope will also be required.
A 20 minute warm-up
is
recommended before you
start
this procedure. You should also adjust the
power supply
as
outlined in paragraph 4-9.
1) Adjust the insulated 410B voltmeter to indicate
0.5 on the
1
volt range with the dc leads shorted.
Use either the
"+" or the
"-"
position of the SEL-
ECTOR switch
--
whichever one
will
permit the
0.
5
setting
with
the ZERO
ADJ.
control.
This
meter
indication will be called
"0 volts1' in the remaining
portion of this procedure.
2) Connect the
COMMON
lead from the voltmeter
to the common junction of AMPLITUDE controls
R93A and R93B (violet wire).
3) Connect the DC volts probe
to
the opposite end
of
R93A. This
is
a
slate wire connected to the AM-
PLITUDE control.
4) Set the FUNCTION switch to TRIANGULAR and
adjust R54 for
a
voltmeter indication of "0 volts".
5) Move the DC volts probe to the arm of Rl18 and
adjust R118 for an indication of approximately
"0
volts".
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