Theory Of Dc Balance And Distortion Adjustments; Dc Balance And Distortion Adjustments - HP 202A Operating And Service Manual

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 a t any point between 68 and 85 volts.
6) Repeat step 3 if you replace either V5 o r 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 f o r a new tube f o r either
V5 o r V6.
7) Test the regulated output voltage a t 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 - 1 0 T H E O R Y O F D C BALANCE A N D
D I S T O R T I O N ADJUSTMENTS
The output AMPLITUDE control
is
located a t 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 a r e 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 s e t
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 a s the cathode of V4.
The dc levels a t the input to the amplifier a r e 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 a r e made, the
dc component between the output terminals will
remain at a very low value, independent of amplitude
setting o r 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 b 6 s
levels of the shaper diodes a r e not variable and
therefore, the triangle input to the shaper can have
one and only one c o r r e c t magnitude and average
level.
Figure 4-2 shows the situation a t the shaper when
the two reference levels a r e 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 a r e simul-
taneously satisfied by optimum settings of the same
adjustments.
4-11 D C BALANCE A N D D I S T O R T I O N
ADJUSTMENTS
The following test procedure re@ires a dc voltmeter
with an input resistance of a t least 100 megohms
such a s an @Model 410B. In addition, the volt
-
meter must not
be
grounded a s the common side of
the m e t e r must be connected to points within the
202A that a r e not a t 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 "+" o r 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 f o r
a
voltmeter indication of "0 volts".
5) Move the DC volts probe to the a r m of Rl18 and
adjust R118 f o r an indication of approximately "0
volts".