HP 202A Service manual

Errata
Title & Document Type: 202A Low Frequency Function Generator
Manual Part Number: N/A
Revision Date: 1956
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this manual copy. The HP XXXX referred to in this document is now the Agilent XXXX.
For example, model number HP8648A is now model number Agilent 8648A.
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SPECIFICATIONS
r
FREQUENCY RANGE: 0.008 to 1200 cps in five decade ranges with wide overlap ateach
dial extreme.
DIAL ACCURACY: Within *2% from "1. 2" to "121q on dial; *3% from ".8" to "1.2".
FREQUENCY STABILITY: Within
4% including warm-up drift and line voltage variations of
*lo%.
OUTPUT WAVEFORMS: Sinusoidal, square, and triangular. Selected by panel switch.
MAXIMUM OUTPUT VOLTAGE: At least 30 volts peak-to-peak across rated load (4000 ohms) for
all
three waveforms. (10.6 volts RMS for sinewave. )
FREQUENCY RESPONSE: Constant within *0.2 db over entire frequency range at rated output
and load.
lNTERNAL IMPEDANCE :
Approximately 40 ohms over the entire range.
DISTORTION: Less than 1%on
all ranges except X100. Less than 2%rms on X100.
OUTPUT SYSTEM: Can
be operated either balanced orsingle-ended. Output system
is direct-coupled; dc level of output voltage remains stable over
long periods of time. DC adjustment available on front panel.
HUM LEVEL: Lessthan 0.05%
at rated output.
SYNC PULSE
:
10 volts peak negative, less than 5 microseconds duration. Sync
pulse occurs atcrestof sinewave and with corresponding positions
on other waveforms.
POWER: Operates from ll51230V
-+lo%, 5011000 cycles source. Requires
175 watts.
DINENSIONS: Cabinet Mount: 20-314" wide, 12-112" high, 14-112" deep.
Rack Mount: 19" wide, 10-l/21qhigh, 14-114" deep.
WEIGHT: Cabinet Mount: 48 lbs; shipping weight, approximately 84 lbs.
Rack Mount: 36 lbs; shipping weight, approximately 74 lbs.
ACCESSORIESAVAILABLE: For rack mount style: End Frames with handles for bench use.
Specify
@No. 17 End Frames.

CONTENTS
SECTION I GENERAL DESCRIPTION Page
1 . 1 General ........... 1-1
SECTION II OPERATING INSTRUCTIONS
2
. 1 Inspection .......... II . 1
2 . 2 Controls and Terminals . II . 1
2 . 3 230-Volt Operation ......
. \
11 . 1
. 2 . 4 Operation .......... II 1
. . 2 . 5 Single-Ended Output II 2
. ......... . 2 6 Balanced Output II 2
. . .......... 2 7 Sync Out 11-2
SECTION m THEORY OFOPERATION
3
-1 General ........... III
3 . 2 Bi-Stable Circuit ........ III
3 . 3 Linear Integrator ........ III
3 . 4 Sine Synthesizer and Function Selector Switch . . III
3 . 5 Output System ......... III
3 . 6 Sync Pulse Output ........ III
3-7 Powersupply ......... III
SECTION IV MAINTENANCE
General
........... IV-1
Power Supply
......... IV . 1
Function Generator (bi-stable circuit and integrator) . IV . 2
Sine Synthesizer and Function Selector . IV
. 2
Output Amplifier
........ IV . 3
sync Out ..........IV
. 3
Tube Replacement
........ IV . 3
Tube Replacement Chart
. IV . 4
Power Supply Regulator Adjustment . N . 4
Theory of DC Balance andDistortion Adjustments
.
IV . 5
DC Balance and Distortion Adjustments . . . lV . 5
Adjust Squarewave Amplitude . IV . 8
Frequency Ratio and Calibration Procedure . . IV-8
Replacement of R58 Potentiometer . IV . 9
SECTION V TABLE OFREPLACEABLE PARTS
5 . 1 Table of Replaceable Parts . V . 1

Sect. I Page 1
1-1 GENERAL
The Model 202A Low Frequency Function Generator
is a compact, convenient, and versatile source of
transient-free test voltages between 1200 and .008
cycles per second. It is useful for any general
purpose low frequency testing application and
is
particularly valuable in the testing of servo systems,
geophysical equipment, vibration and stability char-
acteristics of mechanical systems, electro-medical
equipment, and for the electrical simulation of
mechanical phenomena. Three types of output wave-
form are available; sine, squareandtriangular.
Also, a sync output pulse
is available forexternal use.
The Model 202A Low Frequency Function Generator
contains a type of relaxation oscillator that
is par-
ticularly advantageous for the generation of very low
frequencies. Both a triangular and
a squarewave
voltage function of time are inherent in the oscillating
system. Also, a sinewave function
is produced
by synthesis from the triangular wave.
Output amplitude and distortion are virtually in-
dependent of the frequency of operation. This type
SECTION I
GENERAL DESCRIPTION
of oscillating system in inherent^^ a constant am-
plitude device sothat no A.
V. C. system, with as-
sociateddelay in stabilization after frequency changes,
is required
The frequency range from .008 to 1200 cycles per
second
is covered in 5 bands. The frequency dial
is linear.
The output system
is a direct-coupled amplifier
system designed for either single ended or balanced
output. It has good stability with respect to direct
current in the output and very low hum level. Both
the FUNCTION selectro switch and the AMPLlTUDE
control are soarranged that the characteristics of
the amplifier are independent of their position. The
internal impedance of the output amplifier
is approx-
imately 40 ohms, and the unit
is rated to deliver
atleast 30 volts peak-to-peak to a 4000 ohm load.
A negative peak sync pulse of 10 volts into a 2500
ohm load
is also provided. It has a duration of less
than
5 microseconds and occurs atthe crest of the
sinewave and atcorresponding positions with the
other functions.

Sect. I1 Page 1
2-1 INSPECTION
After the instrument is unpacked, the instrument
should be carefully inspected for damage received
in transit.
If any shipping damage is found, follow
the procedure outlined in the "Claim for Damage in
Shipment" page atthe back of the instruction book.
2-2 CONTROLS AND TERMINALS
RANGE
This switch is used to select the desired frequency
range to be covered
by the frequency dial.
FUNCTION
This switch
is used to select any one of the three
types of output waveform.
FREQUENCY
This dial
is calibrated directly in cycles per second
for the X1 frequency range of the oscillator. The
knob just below the dial escutcheon
is directly con-
nected to the frequency varying element. The lower
knob
is a mechanical vernier for fine adjustment
of the frequency.
AMPLITUDE
This control adjusts the amplitude
of the oscillator
voltage admitted
to the amplifier and, therefore, the
output of .the instrument. This control is calibrated
from 0 to 100 in arbitrary units.
POWER
This toggle switch controls the power supplied to
the instrument from the power line.
FUSE
The fuseholder, which is located on the panel, con-
tains the power line fuse. Refer to the Table
of
Replaceable Parts for the correctfuse rating.
SECTION II
OPERATING INSTRUCTIONS
OUTPUT
This group consists of three terminals. The one
marked "G" is connected directly to the instrument
chassis.
The other two terminals, vertically aligned,
are the OUTPUT terminals. With respect to the
ground terminal each of these outputs has equal
magnitude of signal, but they are 180" out of phase
with each other. The internal impedance between
the two OUTPUT
terminals is appmximately 40 ohms.
SYNC OUT
The Sync Out terminals are single-ended and have
an internal impedance of about 2,000 ohms.
Power Cable
The three-conductor power cable
is supplied with
a three-prong plug. The third prong
is a round
off -set pin which provides
a chassis ground. An
adapter may be obtained to permit use of this plug
with two-conductor receptacles.
2-3 230-VOLT OPERATION
This instrument is shipped from the factory with
the power transformer primaries connected in
parallel for
115 v operation, unless otherwise spec-
ified
on the order. If 230 v operation is desired, the
primaries will have to be connected in series as
shown in "Transformer Details" on the schematic
wiring diagram
of the Power Supply Section.
2-4 OPERATION
The following step-by-step procedure should be
used as a guide when operating this instrument.
1) Turn the POWER switch to ON. Allow thirty
seconds for oscillations to start. The instrument
will operate nearly within specifications after a
few minutes warm-up. It will be within specifi-
cations after
30 minutes.

Sect. I1 Page 2
2) Set the
RANGE and FREQUENCYcontrols for the
desired frequency. The frequency dial scale must
be multiplied by the multiplying factor indicated by
the RANGE switch setting to obtain the oscillator
frequency. Example: 4 (on dial scale)
x .l (multi-
plying factor indicated by RANGE switch setting)
= .4 cycles/sec.
3) Set the FUNCTION switch for the desired output
waveform.
4) Connect the equipment under test to the OUTPUT
terminals.
5) Adjust the AMPLITUDE Controlfor thedesired
output voltage. Because the frequency response
is rated k0.2 db, the output amplitude may be mea-
sured
at any convenient frequency and the output
level will be correct (within these limits) for any
other frequency.
must
be connected to one of the OUTPUT terminals,
and the strappedpair will then be the ground side of
the output.
2-6 BALANCED OUTPUT
Connect the two OUTPUT binding posts to the equip-
ment being supplied. The "G" binding post may
then
be connected to the chassis of the equipment
being driven. Under these conditions the internal
impedance of the Model 202A from either OUTPUT
terminal to ground
is 7900 ohms in series with a 1pf
capacitor (C29). A maximum dc voltage of 400 volts
may
be applied between either OUTPUT terminal
and the "G" terminal without damaging the 1 pf
capacitor (C29). The 40 ohms internal impedance
(resistive) will shunt the impedance existing between
the two signal inputs of the system being driven.
Under circumstances where the connection places
the Model 202A
in serieswith a path carrying cur-
rent, distortion of the Model 202A output will occur
if greater than 10 ma peak current is caused to flow
through the Model 202A output system.
NOTES
When small output voltages are required it may
be
To minimize distortion in the output waveform, EQUIPMENT
always use the lowest RANGE when the overlap
of the FREQUENCY dial permits a choice. SUPPLIED
-------------
groundorno
- signal point
desirable to use
an externalattenuator. This is
2-5 SINGLE-ENDED OUTPUT
Figure 2-2. Balanced Output Connections
The terminal
marked "(3" is isolated from the actual
because the hum and noise
in the output is nearly
constant with output amplitude.
OUTPVT terminals. For single-ended operation "(3'
2-7 SYNC. OUT
@
The SYNC. OUT is a negative pulse of less than
0
5 microseconds duration and at least 10 volts peak
EQUIPMENT into
a 2,500 ohm load. It occurs on one of the sine
BEING and triangular crests and atthe rise orfallof the
SUPPLIED squarewave.
It occurs atthe positive crests with
respect to one of the OUTPUT terminals and at the
negative crestof the other. Therefore,
it can be
changed by 180" with respect to the output system
-
-
by reversing connections to the two OUTPUT ter-
RO
minals, which are otherwise completely interchange-
able. The SYNC. OUT terminal marked "G1'
is
FYgwe 2-L Single-Ended Output Co~ections directly connected to the chassis.
0

Sect. 111 Page 1
SECTION Ill
3-1 GENERAL
Figure 3-1 depicts the general scheme of the @Model
202A and indicatesthe waveforms produced. The bi-
stable circuit consists of a flip-flop circuit capable
of producing a square-wave output at point
A, pro-
vided
it is triggered at the proper time. This is
done by including in the bi-stable circuit, a two-way
comparator circuit which produces the proper trig-
gers for the flip-flop whenever the switching signal
becomes equal to either the "plus switchingreference"
or the "minus switching reference". The triangular
switching signal returned to the bi-stable circuit
THEORY OF OPERATION
is that seen between points B and D. The conversion
of square wave to triangular wave takes place in
the integrator unit which
is carefully designed to
produce an accurate integral of the applied square
wave. The bi-stable circuit and linear integrator
are loop coupled in such a manner that the resulting
relaxation oscillator
is suitable for very low fre-
quency operation.
The sinewave output
is taken from a point C between
the triangular voltage at point
B and the average
level atpoint
D. The resistance between B and C
is fixed, and the network between C and D is a
1
OUTPUT
Figure 3-1. Model 202A Function Generator
-
+ SWITCHING REF
0
- 8+
CR12
-
CRlO
@
=
OUTPUT
AMPLIFIER
CRIl AVE FtfH vOLT4GE FROM
@TO @
CR13
VOLTAGE FROM
+
SWITCHING SIGNAL
+
-SWITCHING REF
, ,,

Sect. III Page 2
non-linear system which synthesizes a sinewave
from the triangular wave. This network consists
of a group of biased diodes arranged in such a man-
ner that at certain predetermined voltage levels they
begin to conduct, therefore, providing shunt paths
from C to
D. Each additional shunt path reduces
the slope of the triangle in the proper amount so
that the wave
is shaped to approximate a sinewave.
This approximation
is asshown, and the degree to
which a sinewave may be approached depends on
the number of diodes. Thus there are available
the sinewave C, triangular wave B, and square-
wave A functions with respect to D to be selected
and brought to the OUTPUT terminals through the
output amplifier. The output amplifier has a differ-
ential input and push-pull output.
3-2 BI-STABLE CIRCUIT
Figure 3-2 shows the details of the bi-stable circuit
and includes the integrator in block form in order
to indicate the bilateral connection from integrator
output to comparator input.
The portion of the diagram composed of V1, V2
and V3 is the "bi-stable circuit". Actually, this
circuit
is a combination of two circuits. If capa-
citors C10 and C13 are disconnected so that there
is no possibility of inductive coupling from grids
to cathode of V1 and V2, the remaining circuit
is
the well-known "flip-flop" or Eccles-Jordan trigger
circuit. The other circuit which appears in the bi-
stable circuit
is a voltage comparator known as the
"Multiar". The multiar is a circuit which employs
a regenerative loop to produce a pulse when the
two input voltages are equal. There are two
of these
in the bi-stable unit. One multiar
is composed of
V1, V3A and T2, and the other of V2, 3B and T1.
The cathode of V3A and the plate of V3B are con-
nected to reference voltages derived from the volt-
age regulator tubes V5 and V6. The triangular.
wave
is applied to the plate of V3A and the cathode
of V3B. As the voltage on the plate of V3A rises
towards the plus switching reference, V1
is con-
ducting, but when V3A conducts, a negative pulse
is formed on the grid of V1 which flips the Bi-Stable
Unit to its other stable state and starts the voltage
on the cathode of V3B towards the minus switching
Figure
3-2.
Details of Bi-Stable Circuit and Switching System
Bf
-
$
R20
4 I
SEC13
* B-
.0-"-,,
LINEAR
-
~'j1133! V3B
IE
INTEGRATOR
+REF
T
+
-
-REF
-
T
- .-.
*

Sect. III Page 3
reference. When V3B conducts the Bi-Stable Unit
is flipped back to its original state, completing one
cycle of operation.
Voltage regulator tubes V5 and V6 are connected
by a voltage divider from which the switching refer-
ence voltages aretaken. They also provide the
limiting voltages applied to tubes V7 and V8 which
are seen to be a push-pull clamping system. In-
asmuch asthe integrator output
is directly related
to the input, it
is seen that the magnitude of square-
wave applied must be carefully controlled. Al-
though only the squarewave appearing at the plate
of Vl
is needed to drive the integrator, the clamp
is made push-pull to prevent excessive current
variations in the regulator tubes. The action of
V7B and V8B is such that if the applied waveform
has peak excursions in excess of the potentials on
the remaining cathode and plate, these being deter-
mined
by regulator tubes V5 and V6, a current will
flow through R20 which drops the voltage to very
nearly the potential of the regulated element of the
conducting section of the diode. The action of the
other diodes
is the same, but 180" out of phase,
inasmuch asthey are coupled to the plate of V2.
In this way, waveforms appearing on the clamped
sides of R21 and R20 are assured to be of equal
magnitude aswell as
180" out of phase, and further
the average of dc level of the squarewave
is ac-
curately controlled.
3-3 LINEAR INTEGRATOR
Consider the block diagram of the linear of feed-
back integrator asshown in Figure 3-3. Starting
with the output voltage E,,
it is seen that if the gain
of the amplifier
is high, then the signal appearing
at the junction of R and
C (the amplifier input)must
be small. For a fixed output Eoasthe gain is in-
creased the resultant signal at the input of the am-
plifier becomes arbitrarily small. Since the voltage
at the junction at R and
C is arbitrarily small, a
squarewave applied to the input will cause a constant
current in R. Because the current charging and dis-
charging C
is constant, except for direction, the
voltage across C will be triangular. Since there
is virtually no signal at the junction of R and C the
output voltage must also be triangular.
In this case the frequency of the applied
signal is so
low that the amplifier used must be direct coupled.
There
is a net voltage rise between input level and
output level in a dc amplifier. In this particular
application the average output level
is determined
asthe average of the "plus reference" and "minus
reference" levels, since the output excursion
is
limited to these levels. B this level does not coincide
with the average level of the applied squarewave,
then the positive and negative excursions of the
squarewave will not be equal, resulting in unequal
rise and fall rates of the output triangle. Because
the squarewave input
is generated from the triangular
output by the bi-stable circuit, the net result is that
under such conditions the squarewave
is really a
rectangular wave. The resulting rectangular wave
has an average value just equal to that demanded
of the amplifier input
by virtue of the pre-set output
level. The average levels of the input and output
are stabilized by the use of a differential amplifier
that has high gain to the difference between the volt-
age applied to its inputs but little or no gain to any
voltage change common to both inputs.
Figure 3-4 shows how this
is done. The right hand
grid of the differential amplifier V15,
is the signal
input and
is driven through R by the rectangular
wave appearing on the FREQUENCY control. The
average voltage of this rectangular wave
is depen-
dent on the clamping levels and the ratio of "on" to
"off" time. When the system
is adjusted for equal
on-off times (squarewave) the average
is just the
average of the clamping levels. The left hand grid
has no signal because the voltage divider which in-
cludes the balance control
is connected to the no-
signal sides of the clamping tubes. However, any
change in the clamping level changes the average
level appearing on both input grids in the same
amount. Due to the large common cathode resistors
of V15 and V16 a common mode change has very
little effect. The input to the left hand grid has
another function.
If the balance control R60, is
varied slightly, the output of the amplifier will show
a considerable change in average level; and therefore
Figure 3-3. Generalized Miller or Feedback Integrator
-
-
-
-
€9,
Eour
ID-"-5.
AMP
t a
-

Sect. III Page 4
Figure 3-4. Simplified Linear Integrator
the average level of the output can be adjusted to
exactly the voltage midway between the "reference"
levels.
This control then servesadequately to adjust
the triangular wave balance which in turn equalizes
the on-off time of the squarewave. The signals
appearing atthe plates of the first tube V15, are
180" out of phase and nearly equal in magnitude.
These signals arealso very nearly the difference
between the inputs on the two grids. Since there
is no signal on the left grid, the only signal into
the amplifier
is that at the junction of R and C, which
is the condition originally required. The second
stage
is a push-pull amplifier employing the signals
from the plates of the previous stage. Again the
common cathode resistance
is very high, but there
is very little degeneration of the push-pull input.
The gain of the system to changes common to both
grids
is about one-half while the gain to voltages
appearing between the input grids
is something over
250. Finally C
is fed back to the signal grid from
the cathode
of V17A which is 180" out of phase with
the signal input.
The cathode follower
is used asan isolation stage
between the integrator and the bi-stable circuit.
This completes
the oscillating loop with its inherent
production of both square and triangular functions.
3-4 SINE SYNTHESIZER AND FUNCTION
SELECTOR SWITCH
The triangular wave from the linear integrator is
connected to R94. In the SINE position of the FUNC.-
TION selector switch (53) the other end of R94 is
connected to the sine synthesizing diodes and to
R93B, one half of the dual AMPLITUDE potentio-
meter. The synthesized sinewave signal appears
asthe difference signal between points C and
D,
but an error signal which appears at D with respect
to
B- also appears at C with respect to B-. This
composite signal
is applied to a differential amplifier
in the output circuit.
The plus and minus switching references in the
bi-stable unit are adjusted sothat the ratio of the
triangular wave amplitude tothe conduction voltages
of
the synthesizer diodesproduces the least distortion
of the sinewave. This adjustment also fixes the
average voltage at C and
is equal to the average
of the plus and minus switching references.
The dc voltages
at D, and the cathode of V4 are
adjusted to be the average of the plus and minus
switching references. Since
these voltages are equal
there
is no change in DC level applied to the Output
Amplifier
as the AMPLITUDE control is varied.

Sect. 111 Page 5
El-STABLE
f 225 VDCREG .
ID.I.6M
t75 VDC TO OUTPUT
t75 VDC
REG AMPLIFIER
REG.
Figure 3-5. Sine Synthesizer and Function Selector
(A) Waveform from integrator output to B-. Triangular
regardless of function selector position.
w
(B) Waveform from@to B- with selector switch in sine
position. Note distortion especially at peaks.
(C) Waveform from@to B- with selector switch in sine
position. This
is the distortion component present
in waveform (B) above.
(D) Waveform fromato@ (i. e. : difference between
waveforms (B) and
(C) above. ) This is the approxi-
mated sinewave.
RO
Figure 3-6. 50 IL Waveforms

Sect. 111 Page 6
The sinewave
is approximated by varying the shunt
resistance across R93B
is steps determined by the
diode synthesizing network. The waveform slope,
at first,
is just that determined by R94, R93B and
the input waveform. When the first diode conducts
R93
is shunted by a predetermined amount, decreas-
ing the slope. Each diode in turn decreases the
slope until all the diodes are conducting and the
triangular wave
has reached its crest. The triangular
wave starts down, the diodes stop conducting in turn
until the triangular wave has reached its crest. The
triangular wave
starts down, the diodes stop conduct-
ing in turn until the triangular wave reaches the
average level. The other half-cycle is formed in
the same manner, but by the diodes that are biased
to shape the negative excursion.
It can
be shown that using seven segments to approx-
imate one half cycle of the sinewave results in ap-
proximately 116%rms distortion. However, variations
in the diodes limit the practical result to about 1%
rms distortion.
In the triangular wave position of the FUNCTION
selector switch the non-linear load consisting of
the diode network
is replaced by R95 sothat the
combination R94 and R95
is a simple linear divider
for all voltage levels. It
is adjusted to give equal
sine and triangular wave peak magnitude. The
squarewave
is connected to the FUNCTION selector
switch through the divider R59 and R22 which adjusts
the average voltage of the squarewave to the voltage
at the cathode of V4. In the squarewave position
of the selector switch, R63 parallels R93B to adjust
the amplitude of the squarewave to be equal to the
amplitude of the sinewave and the triangular wave.
3-5 OUTPUT SYSTEM
The output system consistsof three stages as shown
in Figure 3-7. The first Stage V18
is a dual triode
acting
as a pair of separate cathode followers. These
cathode followers isolate the signal input from the
output stage. Any dc unbalance at the output ter-
minals can be corrected by varying R65.
The second stage V19
is a differential amplifier.
The difference between the two signals at its grids
appears at both plates in nearly equal magnitudes
and 180" out of phase. This effect
is due to the large
common cathode resistance. In this stage ampli-
fication takes place and also the signal difference
E minus F is converted to push-pull voltages. The
third stage V20
is another pair of cathode followers.
The signals appearing at the plates of V19 are
Figure
3-7. Output Amplifier System of Model 202A

Sect. 111 Page 7
R44 R41
CZO
Figure 3-8. Sync Output Circuit of Model 202A
attenuated before being applied to
the cathode follower
grids. The smallshunt capacitors on the upper sides
of the dividers improve the high frequency response
of the amplifier. The voltages appearing at the
cathode follower output terminals are equal in mag-
nitude and 180" out of phase. Negative feedback
is
used to reduce distortion, lower the output impedance
and improve stability. This improved stability
applies not only to the signal output, but to the dc
level at the output terminals.
The symbol for chassis or ground
is used for the
first time in the output terminal network R98, R99
and C29. In all other description the reference level
for operation has been
B-, and in the Model 202A
the
B- line is completely isolatedfrom the chassis.
Thus, the chassis ground
is available for whatever
connection
is desired. It
is possible to consider
the two output terminals as a transformer output
and further to balance this apparent transformer to
chassis by making R98 equal to R99. The capacitor
C29 insulates the apparent transformer secondary
from ground.
If single-ended operation is desired
the ground connection can be tied to either output
terminal without affecting the amplifier.
3-6 SYNC PULSE OUTPUT
The output sync pulse is obtained from the bi-stable
circuit
V1 and V2. On the minus switching reference
atthe plate of multiar diode V3, one positive pulse
and one negative pulse appear for every cycle of
operation. These pulses are coupled to the grid
REGULATOR
FlER
f 375 VOLTS REG
.-0 +P25 VOLTS REG
-
rs REG
Figure 3-9. Model 202A Power Supply

Sect. 111 Page 8
of the sync pulse amplifier, V17, through an RC
coupling which lowers the average voltage on the
grid to B-.
In the absence of pulses, V17 is biased
to cut-off by the bleeder to B+. When a positive
pulse appears at the grid, it momentarily turns
V17 "on", thus, inducing a large voltage swing in
the pulse transformer primary. The resistor and
diode in the secondary remove the positive excursion,
resulting in a negative pulse at the
SYNC OUT ter-
minals.
3-7 POWER SUPPLY
The Power Supply is a full wave rectifier and regula-
tor which supplies
+ 375 volts. The + 75 volt and
+ 225 volt regulated outputs are taken from a voltagc
divider across the + 375 volt supply. The main
requirement on the three regulated voltages
is very
low impedance at low frequencies. Reasonable vari-
ations in the actual voltages do not affect the output
frequency orwaveform.

Sect. IV Page 1
SECTION IV
MAINTENANCE
4-1 GENERAL 4-2 POWER SUPPLY
All dc voltages are measured with respect to B- and
not with respect to the chassis. The B- points in
the instrument areconnectedwith black
hook-up wire.
Most of the following analyzing and adjustment pro- After power supply parts replacements or adjust-
cedures require the measurement of dc voltages or ments, a final check of regulated voltages should
the observation of waveforms. To obtain accurate
be made. See Power Supply Regulator Adjustments
results, use a voltmeter with an input resistance of in paragraph 4-9.
CAUTION
100 megohms ormore. The @Model410B Vacuum
Tube Voltmeter
is recommended.
Isolate alltest equipment from the main chassis or
gromd Otherwise, both
B- and one side of the output
may be connected to the main chassis through the
test equipment.
I€ this happens, one cathode resistor
in output stage V20 will be shorted and the tube will
be severely damaged.
--------------
TABLE 4-1
Whenever pssiblethe instrument frequency should
be set to approximately 50 cycles/sec. to permit
the use of a capacitor in series with the acvoltmeter
or oscilloscope to eliminate the dc component.
Interaction between most of the circuits of the Model
202A makes a fairly definite procedure for trouble
shooting necessary. For example,
a fault in the
oscillator section may easily cause considerable
voltage deviations in the output system. Therefore,
it
is more desirable to divide the instrument into
five sections asfollows and consider each in turn.
4-2 Power Supply
4-3 Function Generator
4-4 Sine Synthesizer and Function Selector
4- 5 Output Amplifier
4-6 Sync Out
SYMPTOM
Instrkentinoperative
(Indicator lamp won't
light, no output volt-
age).
Instrument inoperative
(Indicator lamp lights,
no output voltage).
Instrument inoperative
(normalvoltageatV21).
(Extremely low or no
voltage between Y5,
pin 5 and B-).
Instrument inoperative
(normal
+ 37W reg-
ulated) (+225V reg-
ulated, off voltage).
(+ 7W regulated, off
voltage)
Instrument inoperative
(No
+ 225 regulated
+ 75 regulated voltages,
V5 and/or V6 not
ionized).
1
CAUSE AND/OR
REMEDY
Blown fuse,
F1.
Measure resistance
from V21 socket
(pins 2 or
8) to B-.
55,000 ohms or
more replace V21.
If less than 55,000
ohms clear short
circuit in filter or
regulator circuits
then replace V21.
Defective 6AU5
tubes (V22, V23).
Capacitor C6 short
circuited.
Defective OA2 tube
(V5).
Defective OA3 tube
(V6).
Opencircuit in R62,
R84, R85, R91, or
R92.

Sect. lV Page 2
4-3 FUNCTION GENERATOR
(bi-stable circuit and integrator)
A. REPAIR ANALYSIS OFFUNCTION GE3lERATOR
me
voltage, then a simple test should be made to deter-
mine whether the fault
is in the integrator or the
bi-stable circuit. This test
is asfollows:
1) Connect a high resistance dc voltmeter between
B- andpin 3 of tube V17.
2) Set the RANa switch to the
X. 01 position. Dis-
connect the lead from the center lug of the variable
resistor R58. Temporarily connect this lead to
pin 5, V6
(+ 75 Reg. ).
TABLE 4-2. (CONT'D)
3) After this connection
is made, the voltage indi-
cated by the voltmeter should slowly climb until
it
is over 200 volts. 4-4 SINE SYNTHESIZER _ANDFUNCTION
SELECTOR
4) Remove the lead from the + 75 Reg. supply and
connect
it to pin 2, V5 (+225regulated). The volt- When the trouble has been corrected in the Sine
meter indication should now drop slowly
to less than Synthesizer and Function Selector, the following
140 volts. Disconnect the lead from V5 and return checks should
be made to determine if the instru-
it to the original connection on R58. ment
is again functioning correctly.
SYMPTOM
Same symptoms as
above when frequency
dial
is set near low
frequency end.
Triangle not linear.
7
5) If the instrument meets the above voltage re-
quirements, then the integrator section
is functioning
normally and the fault
is confined to the bi-stable
circuit.
If the instrument does not pass the test,
then the trouble
is in the integrator.
CAUSE AND/OR
REMEDY
Try replacement
tubes for V15, V16,
and/or V17.
Replace tubes V15,
V16, V17. Check
- DC Balance.
After all defective parts have been replaced and
the necessary adjustments made, an oscilloscope
should be connected between pin 3, tube V17 and
B- to see
if a good triangular waveform is obtained
on all ranges.
,
TABLE 4-2.
1) Sine Wave
- Observe the waveform between pin 2,
V18 and B- with oscillator set to 50 cycles/sec. and
the AMPLITUDE control at maximum. Set the
FUNCTION switch in the SINE position. The wave-
form should
be substantially sinusoidal and approx-
imately 30 volts peak-to-peak. See Figure 3-6B.
SYMPTOM
No output voltage (Power
Supply Section normal,
no triangle voltage be-
tween V17, pin 3 and B-
on any range).
Same symptoms asabove
on one or more ranges.
Observe the waveform between pin 7, V18 and B-
with the same conditions asabove. The waveform
should be similar to Figure 3-6C and approximately
1volt peak-to-peak.
CAUSE AND/OR
REMEDY
Replace V1,
V2,
V3, Vl5, V16,
or
V17. If tube
replacement fails
to curethetrouble,
seeanalysispro-
cedure following
a this chart.
Check RANGE
switch contacts,
components, and
connections.
Check C14-C18
for excessive
leakage.
2) Triangular Wave
- Observe the waveform be-
tween Pin 2, V18 and B- with the oscillator set
to
50 cycles/sec. and the AMPLITUDE control at max-
imum Set the FUNCTION switch in the TRIANGULAR
position. The waveform should be triangular and
approximately 30 volts peak-to-peak.
Observe
the waveform between pin 7, V18 and B- with
same conditions asabove. The waveform should be
triangular and approximately
1 volt peak-to-peak.
3) Square Wave
- Observe the waveform between
pin 7, V18 and
B- with the oscillator set to 50 cycles1
sec. and the AMPLITUDE control at maximum.
Set the FUNCTION switch to the SQUARE position.
The waveform should be square and approximately
30 volts peak-to-peak.
The dc voltage across the OUTPUT terminals should
be adjustable to zero under any operating conditions
by means of R65.