Tektronix 2A61 User manual

INSTRUCTION
MI ANI WAL Serial Number 27.3 2.3
Tektronix, Inc.
S.W. Millikan Way @ P. ©. Box 500 @ Beaverton, Oregon 97005 @ Phone [phone] @ Cables: Tektronix
070-0328-01 367

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General Information
The Type 2A61 is a high sensitivity differential amplifier
designed for use with all Tektronix 560-series Oscilloscopes.
The amplifier provides calibrated deflection factors from
10 pV/division to 20 mV/division in a 1-2-5 sequence.
The upper and lower cutoff frequencies of the amplifier
are adjustable. In addition, a line frequency notch filter can
be switched in to minimize signals in the power line fre-
quency region.
SECTION I!
Type 2A61
CHARACTERISTICS
A trace restorer pushbutton returns the trace if it is driven
off the CRT graticule area by a large transient.
A front panel control, the Input Selector, can connect the
amplifier input to an internal common mode signal for a
check of common mode rejection; ground the inputs of the
amplifier for a no-signal reference display; select either
Input A or B; or select A minus B.
The following characteristics apply over an ambient tem-
perature range of 0° C to 50° C. Warm-up time to reach
specified performance is 20 minutes at 25° C, 5° C.
Characteristic
Performance Requirement
Supplemental Information
Deflection Factor
0.01 mV/div to 20 mV/div, =5%
GAIN AD§J control properly set and VARI-
ABLE at CALIB. Steps in 1-2-5 sequence.
VARIABLE range
>2.5 to 1
Maximum Bandwidth
<0,06 Hz to >0.3MHz, at 20 mV/div
<0.1 Hz to >0.1 MHz, at 0.01 mV/div
FREQ RESPONSE TO
switch (High Frequency —3dB
point)
Selectable in fixed steps. Within 30%
of indicated frequency at 300-100 kHz (de-
pending upon mV/div switch setting}, 60
kHz, 6kHz, 600 Hz and 60 Hz.
Risetime (10-90%}. (Based on cutoff fre-
quency within +30% of front-panel fre-
quency indication.)
60 kHz 4.5 to 8.3 ps
6 kHz 45 to 83 ys
600 Hz 0.45 to 0.83 ms
60 Hz 45 to 8.3 ms
FROM switch (Low Frequency —3
dB point)
Selectable in fixed steps. Within +30%
of indicated frequency at 0.06 to 0.1 Hz (de-
pending upon mV/div switch setting}, 0.6
Hz, 6Hz, 60Hz and 600 Hz.
Decay Time Constant (Based on cutoff fre-
quency within +30% of front-panel fre-
quency indication).
600 Hz 0.21 to 0.37 ms
60 Hz 2.1 to 3.7 ms
6 Hz 21.0 to 37.0 ms
0.6 Hz 0.21 to 0.37 s
POSITION control Range
> +6 divisions
Short Circuit Noise
<3.5 uV (RMS), referred to input, (0.01 mV/
div and maximum frequency response).
2220 uN peak to peak. Input at GND.
Grid Current
A, —B input grid current, 10nA (nano-
amperes) max.
Input R and C
10 Megohms, +1%, paralleled by approxi-
mately 100 pF
For each input to ground
Line Filter
Attenuation at 60 Hz
>50:1 (with 60Hz filter}
Insertion Loss
<15% at and above 1 kHz (with 60 Hz
filter)
Filters for 50Hz and 400Hz available
(60 Hz standard)
Common Mode Rejection Ratio
50,000:1 below 10 kHz, 5V peak to peak
common mode sine-wave input.
MILLIVOLTS/DIV switch set to .05
Maximum Common-Mode
Voltage
Input
+5V
A—B mode
1-1

Characteristics—Type 2A61
Characteristic
Performance Requirement
Supplemental Information
Dynamic Range
+15 divisions minimum, about the zero-
input
level, with the VARIABLE control at
CALIBRATED
For other settings of the VARIABLE, the
dynamic range is reduced in proportion to
the gain, and on-screen distortion may ap-
pear.
Differential Input DC Bias
+0.1 V maximum
May be superimposed on the AC input sig-
nal with no change in instrument perform-
ance.
Overload Recovery Time
2 minutes maximum {with +0.1 V DC bias)
A recovery period of at least 2 minutes
should be allowed after applying maximum
allowable bias, before any measurement is
attempted.
DC Balance DC BAL adjustable for no trace shift with | Inputs at ground
operation of MILLIVOLTS/DIV Control
throughout its range
1-2

Type 2A61
SECTION 2
OPERATING INSTRUCTIONS
Introduction
This section gives a brief description or front panel con-
nectors, controls and signal overload indicator.
INPUT CONNECTOR The INPUT connector is designed to
mate with the low noise input cable
supplied with the instrument. Pin
connections are shown on the front
panel.
The INPUT SELECTOR is a five posi-
tion rotary switch that selects the
method of coupling A and B Inputs
to the amplifier.
A—B: Connects the Amplifier to A
and —B inputs for differential oper-
ation.
GND: Connects the Amplifier to
ground without grounding A and
—B Inputs.
INPUT SELECTOR
CM: Connects the Amplifier to a com-
mon 5 volt peak to peak sine-wave
at power line frequency for a com-
mon-mode rejection check.
A: Connects the + side of the Ampli-
fier to the A Input and connects the
— side of the Amplifier to ground.
—B: Connects the — side of the
Amplifier to the B Input and connects
the + side of the Amplifier to
ground.
NOTE
A signal applied to the A Input will be displayed
in the same polarity, positive upward, with the
INPUT SELECTOR switch in the A position, The same
signal applied to —B Input will be displayed
inverted with the INPUT SELECTOR switch in the
—B position.
MILLIVOLTS/DIV. Selects calibrated deflection factors
when VARIABLE is in the CALIB.
position.
MILLIVOLTS/DIV. Provides continuously variable de-
VARIABLE flection factor to at least 2.5 times
the setting of MILLIVOLTS/DIV.
switch.
CAUTION
When the VARIABLE control is in the minimum gain
(fully counterclockwise) position and the POSI-
TION control is at either end of its range, severe
signal distortion may appear on the screen for cer-
tain settings of the MILLIVOLTS/DIV. control.
©i
DC BAL. Front panel screwdriver adjustment.
Adjusts for minimum trace shift as the
MILLIVOLTS/DIV. switch is rotated
throughout its range.
DIFF BAL. Front panel screwdriver adjustment.
Sets optimum common-mode rejec-
tion.
LINE FREQUENCY With FILTER switch set to IN, signals
FILTER at power line frequency are attenu-
ated approximately 34 dB, The chart,
Fig. 2-1, shows —3 dB points of the
Filter.
FREQ RESPONSE FROM: (Black knob) Sets the approxi-
mate lower 3dB down frequency
limit. See Chart, Fig. 2-1.
TO: (Red knob) Sets the approximate
upper 3 dB down frequency limit. See
Chart, Fig. 2-1.
GAIN ADJ. Screwdriver adjustment for changing
the gain of the Type 2A61 to com-
pensate for differences in deflection
factors of different indicator units.
POSITION Provides vertical positioning of the
display. Clockwise rotation moves
the trace up. Counterclockwise rota-
tion moves the trace down.
TRACE RESTORER The red button in the center of the
POSITION knob. Returns the trace
to its original position if the trace is
driven off the CRT screen by signal
transient or control operation. Neon
indicator lights whenever the display
is driven beyond the range of the
POSITION control.
GENERAL OPERATING INFORMATION
Signal Connections
Whenever possible, make all signal connections to the
Type 2A61 INPUT connector with the low-noise input cable
supplied with the instrument. If signal connections are made
to the Type 2A6] with leads other than those supplied, be
sure to use shielded cable.
When making single-signal measurements be sure to
establish a common ground between the chassis of the device
under test and the Type 2A61. The common ground provides
a reference for the voltage measurements. No ground con-
nection is required in differential measurements, but one
should be maintained as a safety measure.
2-1

Operating Instructions—Type 2A61
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Make certain that any signal carrying leads are positioned
away from stray electrostatic or AC magnetic fields that
might induce voltages into the input circuit of the Type 2A61.
This ‘is important because the Type 2A61 is a low-level, high
gain system in which stray signal of a millivolt or less can
cause serious measurement error.
The circuits under test should operate as closely as possible
to normal operating conditions. The amount of loading that
the input circuit of the Type 2A61 imposes on the circuit
under test must be considered. This is especially important
when the circuit under test offers a high source impedance.
Each input of the Type 2A61 has a resistance of 10 MQ,
+1%, shunted by about 100 pF. Due to the capacitive
component in the input circuit of the Type 2A61, loading
on the circuit increases with frequency. For example, the
input impedance of the Type 2A61 with an input frequency
of 3kHz is about 530kQ but with an input frequency of
50 kHz, input impedance decreases to about 32 k0.
It is important to note that shielded cable adds capacit-
ance to the input circuit of the Type 2A61. A 42-inch length
of 509 coaxial cable with BNC connectors, such as Tek-
tronix part number 012-0057-00, will add about 110 pF to the
input capacitance.
Setting the GAIN ADJ Control
The GAIN ADJ control should be checked and/or adjusted
each time the Type 2A61 is used with a new or different
indicator unit, and occasionally during regular operation,
to insure reliability of measurement.
To set the GAIN ADJ contro! proceed as follows:
1. Plug the Type 2A61 into the left-hand plug-in compart-
ment of a Tektronix 560-series Oscilloscope and tighten the
securing knob. Turn on the power and allow about 5 minutes
for the system to reach operating temperature.
2. Free run the oscilloscope sweep at 2ms/diy and con-
nect a suitable patch cord between the oscilloscope calibrator
and the A terminal of the Type 2A61 INPUT connector. Set
the
oscilloscope calibrator for a 50 mV (0.05 volt} input.
3. Set the Type 2A61 front panel controls as follows:
INPUT SELECTOR A
MILLIVOLTS/DIV 10
VARIABLE CALIB
FREQ RESPONSE
FROM .06 to .1 Hz
TO 1 to .3 MHz
FILTER : OUT
4. Check for exactly five major graticule divisions of
deflection. If the deflection is not correct set the GAIN ADJ
control for five major divisions. Use the Type 2A61 POSI-
TION control as necessary to keep the display centered.
Setting the DIFF BAL Control
The DIFF BAL control (differential balance) should be set
occasionally during regular operation of the TYPE 2Aé6l,
particularly prior fo making critical differential measurements.
Either of two methods may be used to set the DIFF BAL
control. The first of the methods to be described uses the
®
Operating Instructions—Type 2A61
internal 5 volt peak to peak line frequency source. The
second uses an external 5 volt peak to peak signal.
To set the DIFF BAL control by the first method, proceed
as follows:
Method 1
1. Plug the Type 2A61 into the left-hand plug-in compart-
ment of a Tektronix Type 560 Series oscilloscope and tighten
the securing knob. Turn on the oscilloscope power and
allow about 5 minutes for the system to warm up.
2. Set the Type 2A61 front panel controls as follows:
INPUT SELECTOR CM
MILLIVOLTS/DIV |
VARIABLE CALIB
FREQ RESPONSE FROM—.6 Hz
TO—.1 MHz to .3 MHz
FILTER OUT
3. Trigger the oscilloscope time base from the line and set
the sweep rate to 10 ms/div.
4, Position the trace vertically to the graticule center line,
using the Type 2A61 POSITION control and/or the Trace
Restorer button. Adjust the DIFF BAL control for minimum
vertical deflection. One major division of deflection with
the mV/DIV switch set at .1 indicates a common mode rejec-
tion ratio of 50,000:1.
If the 1 division requirement for 50,000:1 rejection ratio
cannot be met by adjusting the DIFF BAL control, the Type
2A61 should be recalibrated. See Section 6.
Method 2
1. Plug the Type 2A61 into the left-hand plug-in compart-
ment of a 560-series Oscilloscope and tighten the securing
knob. Turn on the oscilloscope power and allow 5 minutes
for the system to reach operating temperature.
2. Set the Type 2A61 front panel controls as follows:
INPUT SELECTOR A—B
MILLIVOLTS /DIV J
VARIABLE CALIB
FREQ RESPONSE
FROM .06 Hz
TO .1 MHz to 3 MHz
FILTER OUT
3. Apply 5 volts peak to peak signal (preferably the signal,
or a signal similar to the signal to be rejected) to the A and
B INPUT terminals of the Type 2A61.
4. Set the oscilloscope time base at a sweep rate con-
sistent with frequency of the signal applied. Set the oscillo-
scope triggering for a stable display. Position the trace as
necessary with the Type 2A61 POSITION and/or Trace
Restorer button.
5. Adjust the Type 2A61 DIFF BAL control for minimum
vertical deflection. One major division of deflection with
the MILLIVOLTS/DIV at .1 indicates a common mode rejec-
tion ratio of 50,000:1 with a 5 volt peak to peak signal
applied.
2-3

Operating Instructions—Type 2A61
(B) Signal applied to —INPUT.
(C) Resultant display.
Fig. 2-2. Typical CRT displays showing differential rejection of common mode signal. Resultant waveform (C) displays the difference in
signals A and B.
Setting the DC BAL Control
The DC BAL control may need to be set occasionally dur-
ing regular operation of the Type 2A61. Proper DC balance
minimizes trace shift when rotating the MILLIVOLTS/DIV
switch from one position to another.
To set the DC BAL control, proceed as follows:
1. Plug the Type 2A61 into the left-hand plug-in compart-
ment of a Tektronix 560-series Oscilloscope and tighten the
securing knob. Turn the oscilloscope power on and allow
about 5 minutes for the system to reach operating tempera-
ture.
2. Free run the oscilloscope time base at a sweep rate
of 2 ms/div.
3. Set the Type 2A61 front panel controls as follows:
INPUT SELECTOR GND
MILLIVOLTS/DIV 5
VARIABLE CALIB
FREQ RESPONSE
FROM 60 Hz
TO 60 Hz
FILTER OUT
4. Vertically position the trace to the CRT graticule center
line using the Type 2A61 POSITION control.
5. Reset the MILLIVOLTS/DIV switch to .01.
6. Adjust the DC BAL control to return the trace to the
graticule center line.
2-4
7. Reset the MILLIVOLTS/DIV switch to .5 and repeat steps
4, 5 and 6 until there is no trace shift while rotating the
MILLIVOLTS/DIV switch between .5 and .01.
Fig. 2-3. Example of a measurement situation in which CMRR is only
1000:1.

Differential Voltage Measurements
Differential voltage measurements are made with the
INPUT SELECTOR switch set to A~B and the signals applied
to the A and B terminals of the INPUT connector. Under this
condition, only the voltage difference of the two signals is
amplified and displayed on the CRT. Common mode signals
(signals that are common in amplitude, frequency and phase)
are rejected and not displayed (see Fig. 2-2). Maximum com-
mon-mode rejection occurs when the MILLIVOLTS/DIV switch
is in the higher sensitivity settings (.5 through .01 positions).
Before making critical differential voltage measurements,
be sure to check the adjustment of the DIFF BAL control as
described under Setting the DIFF BAL Control with a signal
as nearly identical as possible in frequency and wave shape
to the signal you wish to reject.
Severe distortion may result from exceeding the maximum
input voltage ratings of the Type 2A61.
In differential measurements, the impedance of the signal
source is of primary importance in determining the resultant
Common Mode Rejection Ratio. The source impedance and
input impedance of the Type 2A61 form voltage dividers
which, if unbalanced, will cause loss of Common Mode Rejec-
tion Ratio. For instance, with a measurement situation such
as that of Fig. 2-3, the Common Mode Rejection Ratio with
respect to VCM is only 1000.1.
Frequency Response
Upper and lower frequency limits of the Type 2A6] are
variable with the FREQ RESPONSE controls. This feature is
useful in improving the signal to noise ratio of the display
(see Fig. 2-4). However, if the FREQ RESPONSE controls are
not properly set, attenuation or distortion of the signal of
interest
may
result.
A
square
wave,
for
example,
contains
a
wide
range
of
frequency
components.
The
effect
of
the
FREQ RESPONSE controls may appear to be more pro-
nounced
on
a
square
wave
than
with
sinsoidal
signals.
The
chart
of
Fig.
2-1
gives
the
effect
of
the
FREQ
RESPONSE
controls at various frequencies.
The line frequency filter provides selective attenuation
to
signals
at
or
near
the
line
frequency.
The
FILTER
switch
should be used with the same precaution as the FREQ
RESPONSE
controls.
When
the
FILTER
switch
is
set
to
IN,
avoid
the
tendency
to
apply
too
large
a
line
frequency
sig-
nal
to
the
INPUT
of
the
Type
2A61.
Maximum
applied
volt-
ages
for
the
Type
2A61
are
given
in
Section
1,
Characteristics,
and
are
applicable
regardless
of
the
setting
of
the
FILTER
switch.
Applications
The
Type
2A61
is
suitable
for
practically
any
application
that
requires
measurement
of
low
level
AC
signals.
The
high
input
sensitivity
and
isolation
from
ground
allows
direct
dis-
play
of
electroencephalographic
or
electrocardiographic
impulses
or
input-output
information
relayed
through
a
nerv-
ous
system.
®
Operating Instructions—Type 2A61
The Type 2A61 is also suitable for direct measurement of
outputs from dynamic strain measurement systems. It will
display outputs from resistive, inductive or capacitive bridges.
Inputs from two bridges (one applied to A input and the
other to B input) will allow measurement of strain at either of
two points or the differential strain between two points.
oor
ML 0.52 to coo:
~t. +—
Fig. 2-4. Typical CRT display showing improvement of signal to noise
through use of the FREQ RESPONSE controls.
2-5

Operating Instructions—-Type 2A61
If your application contains DC common-mode voltage
higher than the +5 volt maximum, or DC difference levels
greater than 100 mV, use blocking capacitors in series with
the inputs of the Type 2A61. If the dynamic signal amplitude
is large, a simple resistance divider may be needed to bring
the signal to a suitable level for measurement. However,
when using such dividers, the loading of the source by the
divider(s), the loading of the divider(s) by the Type 2A61
input resistance and capacitance, the amplitude error intro-
duced by inaccuracies in the divider components and the
noise generated by the divider(s) should all be taken into
account. Additionally, when using two such dividers in a
differential application, the resultant Common Mode Rejec-
tion Ratio depends on how accurately the dividers are
matched over the frequency range of interest.

Type 2A61
SECTION 3
CIRCUIT DESCRIPTION
Introduction
This section of the manual contain an electrical descrip-
tion of each circuit in the Type 2A61.
Complete schematic diagrams are given in the Diagrams
section. These diagrams should be refered to for electrical
values and relationships.
INPUT AMPLIFIER
Input Selector
Input signals are fed from the Input connector J401, to
cathode coupled amplifier tubes V424, V425 and V524, V525.
In the A position of the Input Selector switch the grids of
V424, V425 are connected to the A Input connector and the
grids of V524, V525 are connected to ground.
When switch SW401 is in the —B position, the grids of
V424, and V425 are grounded and —B Input is connected to
to
grids of V524 and V525. The circuit operation is identical
to that in the A switch positian except that the output signa!
is inverted and the input signal is connected to the B input
tubes.
In the A—B setting of the Input Selector switch, SW401,
both grids are connected to the Input Connector: A Input to
V424, V425 and B Input to V524, V525.
In the CM setting of the Input Selector switch, SW401,
both grids are connected to the junction of R402, R403. This
divider provides 5 volts, 60Hz* to both grids for checking
low frequency common mode rejection.
In the GND setting of the Input Selector switch, SW401,
both grids are connected to ground. The signal is not
grounded in this switch position.
First Stage
The first stage uses parallel Nuvistors V424, V425 and V524,
V525 to reduce noise in the Input Stage.
A constant current cathode supply, Q508, provides current
for the input nuvistors.
The constant current supply functions as follows: Zener
diode D509 sets the voltage at one end of R508 at —75
volts. The other end or R508 is set at approximately —11.6
volts by the —12.2 volts on Q508 base. As the voltage is
fixed at both ends of R508, the current through R508 must
be constant. This current is the only source for the cathodes
of the Input nuvistors. Input tube current is therefore a
function of current through R508 rather than collector volt-
age or impedance.
With the total cathode current fixed, any signal which
causes V424, V425 current to increase will cause V524, V525
1For instruments using 60 Hz power line frequency.
©1
current to decrease by an equal amount. The tranfer of cur-
rent will cause Q433 and @533 to receive balanced drive.
R501, INT BAL control, adjusts the cathode circuit resist-
ances on the two sides to correct for differences in tube bias
characteristics (tube replacement, aging and other Input
Amplifiers circuit unbalances).
Second Stage
The second stage consists of Q433 and Q533, emitter fol-
lowers with a common voltage-dropping collector resistor.
Compensating capacitors C424 and C524 balance the
impedance characteristics of the two sides of the stage.
R515, DIFF BAL, sets the low frequency common-mode
rejection of the circuit.
Third Stage
The third stage consists of Q434 and @534, collector
loaded amplifiers with emitter degeneration to set gain.
R440 is the emitter feedback resistor that determines the
gain of the Type 2A61 through mV/DIV switch position .5
through 20.
R438, 20mV GAIN, is the fine gain adjustment in series
with R440. R438 is used to set the emitter resistance of the
stage, and thus insure correct gain tracking as R440 is
switched.
Common Mode Rejection System
The input tubes are operated at constant current (ensured
by the constant current source, Q508) and also by the con-
stant plate voltage (by bootstrapping the effective plate
supply to the mean cathode voltage) in order to improve
the common mode dynamic range over which good rejec-
tion is maintained, and to reduce the dependence of com-
mon mode rejection upon dynamic tube characteristics.
lf small identical signals are applied to both inputs (com-
mon mode}, both sides of the input stage attempt to change
currents in the same direction. Since the total cathode cur-
rent cannot change, no signal current appears in the plate
circuits of the input tubes.
The only output that can appear in the plate circuit of the
input tubes results from applying a differential signal that
allows the balance of current through the tubes to shift.
The plate load resistors R424 and R524 for V424 and V524
are driven from the output of the “unity gain’ bootstrap
amplifier consisting of Q504 and Q514 and associated com-
ponents. The input to this amplifier is taken from the center
arm of R501, which represents the mean cathode voltage.
Since the tube current is constant, no common mode signals
3-1

Circuit Description—Type 2A61
appear across R424 and R524, so the mean plate-to-cathode
voltage is constant.
The gain of the bootstrap amplifer Q504 and Q514 is
determined in part by the voltage divider action of the
impedance of D516 and the load resistor R431. The loss of
gain due to the divider action may be offset by increasing
the gain of the basic amplifier. This gain adjustment, R502,
sets the overall gain of the bootstrap amplifier at 1.
For high frequencies, the Zener impedance and compensat-
ing gain adjustment are bypassed by C516, ensuring unity
gain.
The voltage changes at the terminals of C437 are identical
so the capacitor C437 and resistor R440 float with the volt-
age changes.
In differential operation C437 acts as a short circuit to the
signal allowing signal amplification to be determined by the
value of the degeneration resistor, R440.
FILTERS
Frequency Response Filter
Two sections of filtering are provided to attenuate
unwanted signals.
C443, C543, A through E, together with R450 and R550
comprise the FROM section of the Frequency Response Filter
and set the low frequency response.
C445, A through D, together with R435 and R535 comprise
the TO section of the Frequency Response Filter and set the
high frequency response.
Power Line Frequency Filter
The power line frequency filter, consisting of R452, C452,
R453, C453, R552, C552, R553 and C553 may be switched
into the circuit with the FILTER switch on the front panel.
Signals arrive at each output terminal via two paths; the
direct path, R452-C452 (R552-C552}, and the crossover path,
R553-C553 (R453-C453) which is driven from a source 180°
out of phase with the direct path source. These signals tend
to cancel and at one frequency the cancellation can be made
almost complete. Maximum rejection is at 60?Hz with
attenuation greater than 50:1.
OUTPUT AMPLIFIER
Trace Restoration
The high system sensitivity may cause large transients
resulting from swich operation or input signals to drive the
trace off the CRT. The large time constant in the low fre-
quency filter will then cause a long time delay before the
trace returns. Depressing the push button switch SW450, con-
centric with the POSITION control, will bleed off the capaci-
tor charge and quickly return the trace to the CRT.
First Section
The first section of the Output Amplifier is a two stage
feedback amplifier consisting of V464, Q474 and Q574. Its
2Also available at 50 Hz and 400 Hz.
3-2
gain is set by R476, R576 and switched resistor R470 which
controls the gain of the Type 2A61 from .01 millivolts/div
through .2 millivolts/div. The .01 millivolt GAIN control
R579, in parallel with R470, compensates for “‘less than
infinite” loop gain in the amplifier and acts as a gain track-
ing adjustment for R470.
The triode section of V484 and its shunt resistor R459
form a simple series voltage regulator which supplies the
first section of the output amplifier.
The output signal of the first section may be attenuated by
the two section VARIABLE control R482A and B which is used
in a balanced voltage divider connection.
The DC BAL adjustment, R465, compensates for differences
in the grid bias of V464A and B, and ensures that no trace
shift occurs when R470 is switched or when the VARIABLE
control is operated.
Second Section
Output from the Variable Gain control is fed to the grids
of V484A and V584A, a cathode coupled plate loaded con-
figuration.
The cathodes of V484A and V584A are coupled together
through R585 and Gain Adjust control R485, a cathode
degeneration control.
R485 sets the overall gain of the Type 2A61, and is adjusted
to match the Type 2A61 output to the deflection factor of the
CRT in the 560-series Indicator Oscilloscopes.
The cathode current of V484A and V584A is through R489
and the POSITION control, R488. R488 can vary the quies-
cent current of V484A and V584A to position the trace ver-
tically on the CRT.
V484A and V584A are coupled through pins 17 and 21 of
the interconnecting socket to the CRT vertical deflection
plates. L484 and L584 are shunt peaking inductances in the
plate circuits of V484A and V584A. Ferrite cores L483 and
L583 in the plate leads of the output tubes are parasitic
suppressors.
Trace Position Indicator
The DC balance of the output stage is sensed by Q494.
The cathode with the higher voltage drives Q494 through
either D491 or D591. The emitter of Q494 is returned to a
divider which stays at the average voltage of both cathodes.
When the trace is beyond the position control range, the
DC unbalance is sufficient to drive Q494 and light the neon
lamp, B494.
Trigger Amplifier
The
trigger
amplifier,
V584B,
is
driven
from
the
output
of
V584A
through voltage
divider
R586,
R587
and
R588.
R588,
INT
TRIG
DC
LEVEL,
sets
the
division
ratio
so
that
the
out-
put
of
V584B
is
zero
when
the
trace
is
centered
vertically
on
the
CRT.
V584B
is
a
cathode
follower
that
isolates
the
output
of
V584A
from
the
trigger
circuits.
The
trigger
sig-
nal
is
coupled
through
pin
11
of
the
interconnecting
socket
to
the
time
base
unit.

Type 2A61
SECTION 4
MAINTENANCE
Introduction
This section of the manual contains information for use in
preventive maintenance, corrective maintenance or trouble-
shooting of the Type 2A61.
PREVENTIVE MAINTENANCE
General
Preventive maintenance consists of cleaning, visual inspec-
tion, lubrication, etc. Preventive maintenance performed on a
regular
basis
will
help
prevent
instrument
failure
and
will
improve reliability of this instrument. The severity of the
environment to which the Type 2A61 is subjected will deter-
mine the frequency of maintenance.
Cleaning
The Type 2A61 should be cleaned as often as operating
conditions require. Accumulation of dirt in the instrument
can
cause
overheating
and
component
breakdown.
Dirt
on
components acts as an insulating blanket and prevents effi-
cient
heat
dissipation.
It
also
provides
an
electrical
conduc-
tion path.
The top and bottom covers of the indicating instrument
into
which
the
Type
2A61
is
plugged
provide
protection
against
dust
in
the
interior
of
the
instrument.
Operation
with-
out
these
covers
in
place
will
require
more
frequent
cleaning.
CAUTION
Avoid the use of chemical cleaning agents which
might
damage
the
plastics
used
in
this
instrument.
Some chemicals to avoid are benzene, toluene,
xylene, acetone and similar solvents.
Interior
Dust in the interior of the instrument should be removed
occasionally
due
to
its
electrical
conductivity
under
high
humidity
conditions.
The
best
way
to
clean
the
interior
is
to
blow
off
the
accumulated
dust
with
dry,
low
velocity
air.
Remove
any
dirt
which
remains
with
a
soft
paint
brush
or
a
cloth
dampened
with
a
mild
detergent
and
water
solution.
A cotton tipped applicator is useful for cleaning in narrow
spaces or for cleaning ceramic terminal strips.
Exterior
Loose dust accumulated on the outside of the Type 2A61
can be removed with a soft cloth or small paint brush. The
paint brush is particularly useful for dislodging dirt on and
around the front panel controls. Dirt which remains can be
removed with a soft cloth damped in a mild solution of water
and detergent. Abrasive cleaners should not be used.
©
Lubrication
The reliability of potentiometers, rotary switches and other
moving parts can be increased if they are kept properly
lubricated. Use a cleaning type lubricant (such as Tektronix
Part No. 006-0218-00) on shaft bearings and switch contacts.
Lubricate switch detents with a heavier grease (such as Tek-
tronix Part No. 006-0219-00}. Potentiometers should be lubri-
cated with a lubricant that will not affect electrical charac-
teristics (such as Tektronix Part No. 006-0220-00). Do not
over-lubricate. A lubrication kit containing the necessary
lubricants and instructions is available from Tektronix, Inc.
Order Tektronx Part No. 003-0342-00.
Visual Inspection
The Type 2A61 should be inspected occasionally for such
defects as broken connections, broken or damaged ceramic
strips, improperly seated transistors or nuvistors and heat
damaged parts.
The remedy for most visible defects is obvious; however,
care must be taken if heat damaged parts are located. Over-
heating is usually only a symptom of trouble. For this reason
it is essential to determine the actual cause of overheating
before the heat damaged part is replaced, otherwise the
damage may be repeated.
Transistor and Nuvistor Checks
Periodic checks of the transistors and nuvistors in the Type
2A61 are not recommended. The best check of transistor or
nuvistor performance is its actual operation in the instrument.
More details on checking transistor and nuvistor operation is
given under Troubleshooting.
Recalibration
To assure accurate measurement, check the calibration
of this instrument after 500 hours of operation or every six
months if used infrequently. Complete instructions are given
in the Calibration section.
The calibration procedure can also be helpful in localiz-
ing certain troubles in the instrument. In some cases minor
troubles, not apparent during normal use, may be revealed
and/or corrected by recalibration.
CORRECTIVE MAINTENANCE
General
Corrective maintenance consists of component replacement
and instrument repair. Special techniques or procedures
required to replace components in this instrument are
described here.
4-1

Maintenance—Type 2A61
Obtaining Replacement Parts
Standard Parts. Al! electrical and mechanical parts
replacement for the Type 2A61 can be obtained through
your local Tektronix Field Office or representative. However,
many of the standard electronic components can be obtained
locally in less time than is required to order them from Tek-
tronix, Inc. Before purchasing or ordering replacement parts,
consult the Parts List for value, tolerance and rating.
NOTE
When selecting replacement parts, it is important
to remember that the physical size and shape of a
component may affect its performane at high fre-
quencies. All replacement parts should be direct
replacements unless it is known that a different
component will not adversely affect instrument
performance.
Special Parts. In addition to the standard electronic com-
ponents, some special parts are used in the Type 2A61.
These parts are manufactured or selected by Tektronix, Inc.
to meet specific performance requirements, or are manufac-
tured for Tektronix, Inc. in accordance with our specifications.
These special parts are indicated in the Parts List by an
asterisk preceding the part number. Most of the mechanical
parts used in this instrument have been manufactured by
Tektronix, Inc. Order all special parts directly from your
Tektronix
Field Office or representative.
Ordering Parts. When ordering replacement parts from
Tektronix, include the following information:
1. Instrument Type.
2. A description of the part (if electrical, include the cir-
cuit number).
3. Tektronix Part Number.
4. Instrument Serial Number.
Soldering Techniques
WARNING
Disconnect the instrument from the power source
before soldering.
Ceramic Terminal Strips. Solder used on the ceramic
terminal strips should contain about 3% silver. Ordinary
tin-lead solder can be used occasionally without damage to
the ceramic terminal strips. Use a 40 to 75 watt soldering
iron with a Y, inch wide chisel-shaped tip. If ordinary solder
is used repeatedly, or if excessive heat is applied, the solder
to ceramic bond may be broken. Silver solder should be
available locally, or it can be purchased directly from Tek-
tronix, Inc.; order by Tektronix Part No. 251-0514-00.
Observe the following precautions when soldering ceramic
terminal strips:
1. Use a hot iron for a short time. Apply only enough heat
to make the solder flow freely.
2. Maintain a clean, properly tinned tip.
3. Avoid putting pressure on the ceramic terminal strip.
4-2
4. Do not attempt to fill the terminal strip notch with
solder; use only enough solder to cover the wires adequately.
5. Clean the flux from the terminal strip with a flux remover
solvent to maintain good enviromental characteristics.
Metal Terminals. When soldering metal terminals (e.g.,
switch terminals, potentiometers, etc), ordinary 60/40 solder
can be used. The soldering iron should have a 40 to 75 watt
rating with a Ys inch wide chisel-shaped tip.
Observe the following precautions when soldering metal
terminals:
1. Apply only enough heat to make the solder flow freely.
2. Apply only enough solder to form a solid connection.
Excess solder may impair the function of the part.
3. If a wire extends beyond the solder joint, clip off the
excess.
4. Clean the flux from the solder joint with flux remover
solvent to maintain good environmental characteristics.
Component Replacement
WARNING
Disconnect the instrument from the power source
before replacing components.
Ceramic Terminals Strip Replacement. A complete cer-
amic terminal strip assembly is shown in Fig. 4-1. Replace-
ment strips (including studs) and spacers are supplied under
separate numbers. The old spacers may be re-used if they
are not damaged.
Noth YU UU UU
Ceramic Strip
Chassis Spacer Stud Pin
Fig. 4-1, Ceramic terminal strip assembly.
To replace a ceramic terminal strip, first unsolder all con-
nections. Then the damaged strip can be pried or pulled
loose from the chassis. If the spacers come out with the
strip, remove them from the stud pins to be used for installa-
tion of the new strip.
After the damaged strip has been removed, place the
undamaged spacers in the chassis holes. Then, carefully press
the studs into the spacers until completely seated. If neces-
sary, use a soft mallet and tap lightly, directly over the stud
area of the strip.

Transistor and Nuvistor Replacement
Transistors and nuvistors should not be replaced unless
actually defective. If removed during routine maintenance
return them to their original sockets. Unnecessary replace-
ment of transistors or nuvistors may affect the calibration of
this instrument. When transistors or nuvistors are replaced,
check the operation of that part of the circuit which may be
affected.
Replacement transistors or nuvistors should be of the origi-
nal type or a direct replacement. The transistors should be
remounted in the same manner as the original.
Rotary Switches. Individual wafers or mechanical parts
of rotary switches are normally not replaced. If a switch is
defective, replace the entire assembly. Replacement switches
can be ordered either wired or unwired: refer to the Parts
List for the applicable part numbers.
When replacing a switch, it is recommended that the leads
and switch terminals be tagged with corresponding identifica-
tion tags as the leads are disconnected. Then use the old
switch as a guide for installing the new one. An alternate
method is to draw a sketch of the switch layout and record
the wire color to each terminal.
Maintenance—Type 2A61
TROUBLESHOOTING
Introduction
The following information is provided to facilitate trouble-
shooting the Type 2A61. Information contained in other
sections of this manual should be used along with the follow-
ing information to aid in locating the defective components.
Troubleshooting Aids
Diagrams. Circuit diagrams are given on foldout pages
in Section 9. The circuit number and electrical value of each
component in this instrument are shown on the diagram.
Important voltages and waveforms are also shown on the
diagram.
Switch Wafer Identification. Switch wafers shown on
the diagrams are coded to indicate the position of the wafer
in the complete switch assembly. The numbered portion of
the code refers to the wafer number counting from the front,
or mounting end of the switch, toward the rear. The letters F
and R indicate whether the front or rear of the wafer per-
forms the particular switching function. For example, a wafer
designated 2R indicates that the rear of the second wafer
is used for this particular switching function.
Composition Resistors:
Metal-Film Resistors:
Ceramic Capacitors:
G) Q) and G) —Ist, 2nd and 3rd significant figures;
(m) —multiplier; @) —tolerance;
Resistor and Capacitor Color Code
Signifi- Multiplier Tolerance
cant Resis- |Capaci-; — Resis- Capaci-
Color Figures | tors tors tors tors
Silver --- 10°? --- 10% ---
Gold --- 10-7 soe +5% ---
Black 0 1 ] --- +20% or
2 pF*
Brown 1 10 10 +1% 1% or
0.1 pF*
Red 2 10? 10? +2% +2%
Orange 3 10° 108 +3% +3%
Yellow 4 10% 10* +4, +100%
—0%
Green 5 10° 108 +05% | 45% or
0.5 pF*
Blue 6 10° 10° --- ---
Violet 7 --- --- --- ---
Gray 8 “+: 10 --- +80%
—20%
or 0.25 pF*
White 9 --- 10-7 --- +10% or
1 pF*
(none) --- --- --- +20% +10% or
1 pF*
(rc) —temperature coefficient. NOTE: @)
manufacturer and capacitor type. May not be present in some cases.
*For capacitance of 10 pF or less.
and/or €) color code for capacitors depends upon
Fig. 4-2. Resistor and ceramic capacitor color code.
4-3

Maintenance—Type 2A61
Wiring Color Code. All insulated wire used in the Type
2A6l is color-coded according to the E{A standard color code
(as used for resistors) to facilitate circuit tracing. The widest
color stripe identifies the first color of the code. Power sup-
ply voltages can be identified by three color stripes and
the following background color code: white, positive voltage;
tan, negative voltage. Table 4-1 shows the wiring color code
for the power supply voltages used in the Type 2A61. The
remainder of the wiring in the Type 2A61 is color coded with
two or less stripes or has a solid background with no stripes.
The color coding helps to trace a wire from one point in the
instrument to another.
TABLE 4-1
Background Ist 2nd 3rd
Supply Color Stripe Stripe Stripe
—100 Tan Brown Black Brown
—12.2 Tan Brown Red Black
+125 White Brown Red Brown
+300 White Orange | Brown Brown
6.3 VAC White Blue Brown
Resistor Color Code. A number of precision metal film
resistors are used in this instrument. These resistors can be
identified by their gray body color. If a metal film resistor
has a value indicated by three significant figures and a
multiplier, it will be color coded according to the EIA stand-
ard resistor color code. If it has a value indicated by four
significant figures and a multiplier, the value will be printed
on the body of the resistor. For example, a 333 kQ resistor
will be color coded, but a 333.5kQ resistor will have its
value printed on the resistor body. The color code sequence
is shown in Fig. 4-2.
Composition resistors are color coded according to the
EIA standard resistor color code.
Capacitor Marking. The capacitance values of common
disc capacitors and small electrolytics are marked in micro-
farads on the side of the component body. The white ceramic
capacitors used in the Type 2A61 are color coded in pico-
farads using a modified EIA code (see Fig. 4-2).
Troubleshooting Techniques
This troubleshooting procedure is arranged in an order
which checks the simple trouble possibilities before proceed-
ing with extensive troubleshooting. The first few checks
assure proper connections, operation and calibration. If the
trouble is not located by these checks, the remaining steps
aid in locating the defective component. When the defective
component is located, it should be replaced following the
replacement procedure given in this section.
1, Check the Associated equipment. Before proceeding
with troubleshooting of the Type 2A61, check that the equip-
ment used with it is operating correctly. Check that the sig-
nal is properly connected and that the interconnecting cables
or probes are not defective.
2. Check Control Settings. Incorrect settings of the con-
trols on the Time Base Plug-in, the 560 Series Indicator or
the Type 2A61 can indicate a trouble that does not exist.
For example, an incorrect setting of the Time/div Variable
control on the Time Base Plug-in will give erroneous risetime
4-4
measurements; incorrect settings of the mV/div and/or Vari-
able appears as incorrect gain; incorrect setting of the trig-
gering controls on the Time Base Plug-in might appear to be
defective trigger pickoff, etc.
If there is any question about the operation or correct
function of any control, consult the Operating Instructions
section of the manual for the instrument involved.
3. Check Instrument Calibration. Check the calibration
of the instrument or the affected circuit if the trouble exists
in one circuit. The indicated trouble may only be a result of
misadjustment or may be corrected by calibration. Complete
instructions are given in the Calibration section of this manual.
Individual calibration steps can be performed out of sequence.
However, if the circuit affects the calibration of other circuits
in the instrument, a more complete calibration will be nec-
essary. General Information in the Calibration section
describes how steps which interact are noted.
4. Visual Check. Visually check the circuit in which the
trouble is located. Many troubles can be located by visual
indications such as unsoldered connections, broken wires or
damaged components.
5. Check Resistances to Ground. Check the resistance
to ground on each of the pins on the Amphenol 24 pin con-
nector, Fig. 4-3. Table 4-2 shows pin numbers, approximate
resistance which should be seen and the use of each pin.
TABLE 4-2
Pin No. Approx Resistance Use
1 19 6.3 volts, AC
2 0 6.3 volts, AC
3,4 Inf Unused
5 0 Ground
6,78 Inf Unused
9 0 Ground
10 50 kQ +300 volts
11 40 ko Trigger Out
12 Inf Unused
13,14,15 30 kQ +125 volts
16 49 —12.2 volts
17 ; 50 ka Output
18,19,20 Inf Unused
21 50 ka Output
22 1kQ —100 volts {unreg)
23 10 kQ —100 volts (reg)
24 0 Gnd
NOTE
The DC resistance of any circuit is dependent on
several factors. Therefore, the resistances in the
table are approximations and not to be con-
sidered absolute values.
6. Check Voltages and Waveforms. Often the defective
component can be located by checking for the correct voltage
or waveform in the circuit. Typical voltages are given on
the Schematic Diagrams. To check waveforms apply a signal
to the input and trace the signal through the amplifiers. Any
waveform distortion or loss of signal is an indication of the
location of the trouble.

Amphenol 24 pin connector
Voltohmmeter
common lead
Voltohmmeter
Positive Lead
Fig, 4-3. Location of 24 pin Amphenol connector,
NOTE
Voltages given on the diagrams are not absolute
and may vary slightly between instruments. To
obtain operating conditions similar to those used
to take these readings, see instructions (in blue)
on the schematic,
a. Voltages. Voltage measurements should be made with
a 20,000 ohms/volt DC voltmeter, Accuracy of the voltmeter
should be within 3% on all ranges. Be sure that the test
prods are well insulated to prevent accidental shorting of
components,
b. Waveforms. Use a test oscilloscope that provides mini-
mum loading of the circuit under test. For example, an input
resistance of approximately 10 megohms paralleled by about
10 pF is obtained when using a 10% probe. The test oscillo-
scope should have a minimum deflection factor of 5mV/div.
7. Check Individual Components, The following pro-
cedures describe methods of checking individual components
in the Type 2461. Components that are soldered into place
can be checked most easily by disconnecting one end. This
eliminates incorrect measurement due to the effect of sur-
rounding circuitry.
a. Transistors and Nuvistors, The best check of transistor
and nuvistor operation is actual performance under operating
conditions. If a transistor or nuvistor is suspected of being
defective, it can best be checked by substituting a new com-
ponent or one which has been checked previously. However,
be sure that the circuit conditions are not such that a replace-
ment transistor or nuvistor might also be damaged. If sub-
stitute transistors or nuvistors are not available a dynamic
tester may be used [such as a Tektronix Type 570 or 575).
Static type testers are not recommended since they do not
check operation under simulated operating conditions. Nuvis-
tors should be replaced only in matched sets of four. Tek-
tronix part number for the matched set may be found in the
Electrical Parts List, Section 7.
b. Diodes, A diode can be checked for an open or shorted
condition by measuring the resistance between terminals.
©
Maintenance—Type 2A61
With an ohmmeter scale having an internal source of about
1.5 volts, the resistance should be very high in one direction
and very low when the leads are reversed.
CAUTION
Use an ohmmeter scale that has an internal current
less than 20 mA. High currents may damage the
diode.
c, Resistors. Resistors can be checked with an ohmmeter.
Check the Electrical Parts List for the tolerance of the resis-
tors used in this instrument. Resistors normally do not need
to be replaced unless the measured value varies widely from
the specified value,
d. Inductors. Check for open inductors by checking con-
tinuity with an ohmmeter. Shorted or partially shorted induc-
tors can usually be found by checking the waveform response
when high frequency signals are passed through the circuit.
Partial shorting often reduces high frequency response (rise
time).
e, Capacitors. A leaky or shorted capacitor can best be
detected by checking resistance with an ohmmeter on the
highest scale. Do not exceed the voltage rating of the capa-
citor, The resistance reading should be high after an initial
charge of the capacitor. An open capacitor can best be
detected with a capacitance meter or by checking whether
the capacitor passes AC signals.
8, Isolate the trouble to a section of the Amplifier,
Since most troubles that may occur in the Type 2Aé61 will
cause an unbalance, the following procedure is recommended
to isolate trouble to a section.
a. Short circuit (do not ground) the CRT Vertical Deflection
Plate pins on the 560-series Oscilloscope and note the posi-
tion of the trace vertically. This is the electrical center of
the CRT.
b. Remove the short circuit from the CRT Vertical Deflec-
tion Plate pins,
c. Adjust the Type 2A61 POSITION control to set the trace
to the CRT electrical center observed in step a.
d.
Connect V484A and V584A grids [pin 7) together with
a short clip lead. (If the clip lead causes oscillation in the
circuit, loop a couple of turns of the clip lead through a
ferrite bead.) Note the position of the trace vertically. The
trace should be at the CRT electrical center. If the trace is
not centered, trouble is indicated in V484, V584 or in the
connected circuitry.
e. If no trouble is indicated continue to work toward the
input of the amplifier, connecting together in turn, V464, V564
grids, Q434, Q534 bases, Q433, Q533 bases and V424,
¥524 grids, until the trouble section is located,
f. The section in which the trouble is located may now be
checked by tube or transistor substitution and/or voltage
measurement to locate the defective component.
NOTE
Waveform photographs are included on the Sche-
matic Diagrams, Section 9, to show typical signal
levels at various points in the circuit.

NOTES