Eico 465 User manual

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SECTION 1 FEATURES AND SPECIFICATIONS
•
1-1. Physical and electrical specifications
1- 2. Control features and facilities
1-3. Specifications
1-4. Controls and input terminals
1-5. Notes on controls and terminals
SECTION 2 OPERATION
SECTION 3 APPLICATION
3-1. General
3-2. Waveform investigation
3-3 . Waveform display
3-4. Lissajous patterns
3-5. Servicing TV receivers
SECTION 4 MAINTENANCE
4-1.
General
4-2. Access to chassis
4-3. Vertical amplifier adjustments
4-4. Horizontal amplifier adjustments
4-5 . Frequency compensation
Vertical channel
Horizontal channel
4-6. Calibration voltage adjustment
4-7 . Horizontal TV sweep trimmer adjustment
4- 8. Troubleshooting
4-9 . Tube Replacement
4-10. Fuse Replacement
1 COPYRIGHT ©
1969 EICO ELECTR ON IC INST RUMENT COMPANY, I nc .
-

SECTION 1 FEATURES AND SPECIFICATIONS
1-1. PHYSICAL AND ELECTRICAL SPECIFICATIONS
a. DC to 8 MHz (+l, -5 dB) o f. the vertical amplifier is more than enough needed for both color
and monochrome TV service. The horizontal amplifier frequency response is from DC to 1 MHz
(+l, - 3 dB).
Both amplifiers feature push-pull amplification throughout for minimum distortion,
while the direct-coupled design eliminates low-frequency phase shift or response fall off. This DC
design also permits making true voltage measurements of any waveform.
b. A pair of zener-controlled voltage reference sources, and their associated adjustment poten-
tiometers provide an accurate source of calibration voltages for both the horizontal and vertical
channels.
c. Using a 1500-volt accelerating potential provides a sharp, bright, trace free from blooming.
A mu-metal CRT neck shield minimizes the effects of external magnetic fields.
d. Distortionless vertical and horizontal gain and drift-free V & H positioning up to many times
the actual viewing screen diameter.
e. Automatic sync limiter and amplifier provides rock-steady sync signals for the sweep circuits.
f. Full retrace blanking is provided to remove objectionable background clutter.
g. Edge-lit calibration grid on a green filter screen has its illumination level controlled by front -
panel control.
h. Human-engineered front panel features all controls in logical order and grouped for easiest use.
i. Front-panel switches enable switching either vertical or horizontal channel to either AC or DC
response.
j. Heavy-duty power supply reduces generated heat and makes for a long component life.
k. Each channel is provided with its own internal voltage calibrator.
1-2. CONTROL FEATURES & FACILITIES
a. Both the vertical and horizontal channels use a four-position, frequency compensated input
decade attenuator, plus a calibration position. A concentric gain control is effective in all positions,
including the calibration position. A slide-switch is provided for each channel to select either direct
coupling (DC) or capacitive coupling (AC).
b. Both amplifiers can be balanced at any time via a pair of balance controls that can be adjusted
through a pair of small holes on the front panel.
c. The horizontal sync selector permits selection of either internal positive or negative, external,
60 Hz, or provides a 60-Hz sweep. A sixth position passes an external signal into the horizontal
amplifier.
d. Sweep-range selection from 10 Hz to 100 kHz in four overlapping ranges, plus a preset TV
vertical and horizontal sweep positions (30 and 7875 Hz respectively). A concentric sweep vernier
permits adjustment within any range.
e. Scale illumination, and CRT intensity, focus, and astigmatism are all controlled by front panel
controls.
f. An intensity modulation input is provided on the rear panel.
2

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a. Vertical amplifier:
Frequency response: DC to 8 MHz (+l, - 5 dB)
Sensitivity: 12 mV per cm r. m. s.
Input impedance: 1 megohm shunted by 35 pF
Calibration voltage: z ener-controlled 200 mV peak-to-peak, ±1%
Input scale attenuation: . 05, . 5, 5, and 50 volts/cm
b. Horizontal amplifier:
Frequency response: DC to 1 MHz, 1-l, -3 dB
Sensitivity: 17 mV per cm r. m. s.
Input Impedance: 1 megohm shunted by 35 pF
Ollibration voltage: zener-controlled 500 mV peak-to-peak, ± 1%
Input scale attenuation: . 05, . 5, 5, and 50 volts/cm
c. Sweep r anges :
10-100 Hz, 100 Hz- 1 kHz, 1-10 kHz, 10-100 kHz, plus fixed TV vertica1(30 Hz), and horizontal
(7875 Hz).
d.
Intensity modulation:
3 volts r. m. s. blanking
Input impedance 2. 2 megohms
e. Tube complement:
l-12AZ7, l-12AU7, 2-6AU8, 2-12BY7, 3-6B L 8; 3 silicon rectifiers, 1 silicon HV rectifier,
2 zener diodes, 5DEP1 CRT.
f. Power supply:
ll7 - volts AC, approximately 140 watts
g. Size:
12 1/2 high x 8 1/ 2 wide x 17 1/2 deep
h. Wei ght: 27 lb s.
1- 4. CONTROLS AND INPUT TERMINALS
The Model 465 has its controls and input terminals grouped in logical arrangement as for use.
All controls for either channel are vertically grouped on each side of the front panel, th e sync and
sweep controls are in the middle segment, while the CRT controls are grouped alongside the CRT
screen.
a. The AC pow er on/off switch is located on the SCALE ILLUMination control and is operated by
rotating this control clockwise from the OFF position . This also turns up the scale illumination.
3

b. The INTENSITY, FOCUS, and ASTIGmatism controls determine the brightness and sharpness
of the visible trace. The INTENSITY control determines the brightness, while both FOCUS and
ASTIGMATISM determine the sharpness. These three controls interact to a slight extent, therefore,
a change in any control setting may require a change in all.
c. In both amplifier groupings, the topmost control (V /CM) selects the amount of input attenuation ,
in conjunction with its concentric gain control. In both cases, the calibration voltage position is
indicated in red. Both POSITION controls adjust the location of the trace on the screen. The HORIZ
position control moves the trace to the right or left, while the VERTICAL control adjusts the trace
up and down.
Each input attenuator provides four degrees of attenuation; resulting in either . 05, . 5, 5, or
50 volts-per-cm deflection. Each input is obtainable with either AC or DC coupling by operation of
either AC-DC switch located near the bottom of each channel section . The use of direct coupling
(UC) prevent phase shifts and amplitude distortions of low-frequency waveforms and also provides
a DC reference line to be established for accurate amplitude measurement. AC coupling is used
when observing a small a. c. signal superimposed on a relatively large d. c. voltage . At the CAL
position, the square-wave calibration voltage is fed directly to the amplifier, bypassing the decade
attenuator switch, but not the GAIN control. By setting the vertical GAIN control for a vertical trace
size of four major divisions (4 cm), the basic 50 mV /cm sensitivity is obtained when setting the VERT
AMPLIFIER V/cm switch to the. 05 position and sensitivities of. 5, 5, and 50 volts/cm at the other
three positions provided that the VERT AMPLIFIER V /CM GAIN control is not changed.
By setting the HORIZ AMPLIFIER V /CM switch to the CAL position, and setting the GAIN
control for a trace of 10 divisions long (10 cm), the basic 50 mV /cm sensitivity is obtained for the
horizontal channel when resetting the HORIZ AMPLIFIER V/ CM switch to the . 05 position, and
sensitivities of . 5, 5, and 50 volts/cm at the other three positions provided that the HORIZ AMPLI-
FIER V/ CM GAIN control is not changed.
d. Both vertical and horizontal channels GAIN controls can be adjusted for any desired signal
height, or trace length, if accurate calibration is not needed.
e. Both VERT AMPLIFIER and HORIZ AMPLIFIER lNPUT and GND terminals will accept standard
scope connectors, probe leads, or plain wires.
f. The SYNC SELECTOR switch has four sync positions to permit sync selection for the sweep
generator. At the 60Q position, an a. c. signal of power-line frequency is taken from the power
supply and applied to the sweep generator to synchronize it at power-line frequency. At the EXT
position, an external signal applied to the HORIZ AMPLIFIER INPUT binding post syncs the sweep
generator. At both "+-" and "-" positions, the synchronizing signal is derived internally from the
vertical (signal) amplifier. At"+-", synchronization occurs on the positive-going edge of the applied
input signal. At"-", from the negative-going portion of the applied vertical channel signal. At the
60Q HORIZ position, an a. c. voltage is taken from the power supply and used as the input to the
horizontal amplifier to form the sweep. In the EXT HORIZ position, the signal applied to the HORIZ
AMPLIFIER INPUT binding post forms the sweep.
g. The SWEEP RANGE switch selects the frequency band over which the concentric vernier (VERN)
control can be varied for frequency adjustment of the internal linear sweep, or the preset VERT TV
or HORIZ TV positions, designed to eliminate the need for repeated adjustment of the SWEEP RANGE
switch when working on TV sets that use two widely spaced frequencies for vertical and horizontal
circuits. In the four numbered positions, the numbers above the positions markers indicate the upper
and lower frequency limits of the band (approximately), for that particular position. The convenience
provide by the VERT TV and HORIZ TV positions is as follows: if at the VERT TV position, the
SWEEP RANGE VERNier is set to display two full cycles of a 60 Hz signal to obtain a sweep frequency
of 30 Hz, placing the SWEEP RANGE switch in the HORIZ TV position will result in an automatic 7875
Hz
display without any adjustment of the VERNier .
4

11. rtu externa1 vonage tor the purpose of int ensity (Z axis) modulation may be applied between tbe
rear-panel intensity modulation pin jack and chassis ground. Do not apply more than 3 volts peak to
this input or the life of the CRT may be greatl y shortened. Do not allow the CRT grid to swing positive
(indicated by a noticeable defocussing of the trace).
1-5. NOTES ON CONTROLS AND TERMINALS
a. Proper trace definition will be obtained only if the FOCUS and ASTIG controls are correctly
adjusted, and the scope is not operated in strong magnetic fields such as those found near large power
transformers, radio transmitters, and power-generating equipment. These strong magnetic fields
may distort the electron beam that produces the trace.
b. A sharply focussed line, or a small dot of high intensity, should not be permitted to remain
stationary on the screen for any period of time (more than a 1/2 minute or so) or the screen may be
burned. A trace of excessively high intensity will burn the screen in 3 to 5 minutes, therefore, do
not keep the INTENSITY control turned up for extended periods of time. Burned portions of the screen
will no longer fluoresce and are useless for observation. If it is required to have a fixed trace on the
screen for a long period of time, reduce the intensity of the trace to minimum.
c. When either AC-DC switch is placed in the DC position, the d. c. component of any signal fed to
U:e
amplifier w ill be amplified along with the a. c. component of the signal. The direction of trace
movement is UP for a positive voltage and DOWN for a negative voltage. Therefore, when going from
observation of a pure a. c. signal to one containing d. c. (or vice versa), it may be necessary to use
the POSITION control to bring the trace back on the screen. Any d. G· component has no effect when
the AC-DC switch is in the AC position.
d. A trace can be expanded horizontally and / or vertically to several times full screeh without dis-
tortion by using the amplifier V /CM switch and concentric GAIN control. Likewise, ' trace positioning
is several times screen diameter to permit examination of any portion of the expanded trace.
e. An independent DC BALance control is provided for each channel. These are accessible through
small holes on the front panel. When they are properly adjusted, there should be no shifting of the
trace (no signal applied) when the GAIN control is rotated from minimum to maximum. Detailed
alignment will be given in the MAINTENANCE section.
f. The vertical channel voltage calibration developes a negative-going signal. This means that if
the top of the calibration square wave is set to the horizontal center line of the calibration grid, (using
the vertical channel POSITION control), the horizontal center line becomes the "zero center" for the
calibration grid (assuming that the DC balance is correct). Under these circumstances, direct read-
ing of AC, DC, or AC superimposed on DC voltages can be made. The horizontal center line is zero
voltage, while trace points above the center line are positive, and below the center line are negative.
Once calibration has been made, then operation of the V /CM control can be used to determine the
signal voltage level. Pure DC voltages are determined by noting the distance above the horizontal
center line (for positive signals) or below the center line for negative levels, and multiplying the
displacement in cm (smallest grid division is 2 mm, major division is 1 cm), by the sensitivity of
the particular V /CM switch position. For example - an unknown DC voltage applied to the VERT
AMPLIFIER INPUT and GND binding posts at the . 05 position of the V /CM switch causes a downward
trace displacement of 2 1/2 small grid division (4 1/2 mm). Since the deflection is downward, the
voltage is negative. As the sensitivity is 50 mV / cm in the . 05 position, the DC voltage is 25 mV.
AC waveforms with or without a DC component can be read as to absolute voltages at any point, and
including the value of any DC component. For example - an unknown voltage applied to the vertical
channel at the . 5 position, displays a waveform having a positive peak 4 major divisions above the
horizontal center line and a negative peak 2 major divisions below the horizontal center line. As the
sensitivity at the . 5 position is 500 mV /cm, the positive peak is 2 volts and the negative peak is 1
volt (3 volts peak-to-peak). When the AC-DC switch is in the AC position, the positive peak moves
1 1/2 major divisions (7 1/2 minor divisions) downward so that the positive peak is now only 2 1/2
major divisions (12 1/2 minor divisions) above tre horizontal center line. Since 1 major division is
500 mV at the . 5 position, the DC component of the voltage is 11/2 cm x 500 mV /cm or 750 mV and
positive. If the signal under observation is a sine-wave only, the r. m. s. value may be calculated by
dividing the peak-to-peak value by 2. 8.
5

SECTION 2 OPERATION
To obtain the best r es ults with your scope, it is advisable to beco me acquainte d with the functions
and correct usa ge of the panel controls and terminals , by mak i ng some si mple t ests . These tests
will also assure you that the instrument is in perfect wor ki ng or der. Do not a tt em pt this procedure
with kits before all fina l checks have been completed and all initial adjustments have been completed
as explained in the MAINTENANCE section.
a. Set the SC ALE ILLUM control to OFF, INTENSITY, FOCUS, and ASTIG to the center of their
range .
b. On
the VERT AM PLIFIER section, set the V /CM s wit ch to 50 and the GA IN cont r ol fully counter-
clockwise . Set POSITION near the center of its range .
c. On
the HORIZ A MPLIFIER section , set the V /CM switch to . 05 and the GAIN cont rol fully
counterclockwi se. The POSITION control should also be set about half way .
d.
Set the S YN C SELECTOR control to EXT . HORIZ. Set the SWEEP RANGE and VERN control
to any position.
e. Insert the power cord into a nominal ll7-volt, 60 -H z outlet.
WARNING
This instrument will not operate, and will be seriously damaged, if connected to an y ot her
type of
power sou r ce (such as d. c., 25-Hz, 200/240 - volts a. c. ), unless fitted wi th spe ciali zed
power supply components.
f. Rota te the SCALE ILLUM off the OFF position and note the click as the associated switch is
operated. The small pilot lamp at the bottom of the fron t panel should glow. The two scale illum i-
natfon lamps will start to glow and this glow can be increased by r otating the SCALE ILLUM control
more clockwise. Set the scale illumination at a low le v el.
g. As the unit warms up, a bright spot should appear. If the spot does not appear, it may be
necessary to rotate the INTENSITY control slightly more clockwise, and possibly make some ad-
justment to both POSITION controls.
h. Using the POSITION controls, center the spot on the screen.
i. Adjust the FOCUS and ASTIG controls for the sharpest image . Note that there is some inter -
action between the INTENSITY, FOCUS, and ASTIG controls , therefore, all three will have to be ad-
justed for the sharpest results.
j. Set the SYNC SELECTOR switch to either"+" or "- ". Advancing the HORIZ AMPLIFIER GAIN
control clockwise should cause the dot to extend into a horizontal line. This is the horizontal linear
sweep. Reset the SYNC SELECTOR to EXT HORIZ and note that the trace drops back to a dot. Any
signal, even touching the HORIZ AMPLIFIER INPUT terminal , will cause a trace to appear whose
length is a function of the HORIZ . AMPLIFIER V /CM switch and GAIN control.
k. Set the SYNC SELECTOR back to 60n HORIZ. The length of the horizontal line seen on the
screen is also a function of the setting of the HORIZ AMPLIFIER V /CM switch and its associated
GAIN control. This trace is a 60-Hz sine sweep and is used for certain set alignments.
1. Set the SYNC SELECTOR to "+-", and HORIZ AMPLIFIER V/ CM and GAIN controls for a useful
long tr ace. Place the SWEEP RANGE switch in the 10 - 100 position, and the associated VERN control
fully counterclockwise.
m. Set the VERT AMPLIFIER V /CM switch to the CAL position. Adjust the VERT and HORIZ
AMPLIFIER GAIN control until the square-wave pattern covers about two thirds of the screen. Center
the pattern using the POSITION controls. Lock the pattern on the trace by adjustment of the SWEEP
RANGE VERN control until a single square-wave is displayed on the screen, and remains stationary .
6

NOTE: In rotating the SWEEP RANGE VERN control, the pattern slows down as certain
critical frequencies are approached, and then it appears to reverse direction when a critical frequency
is passed. At these critical frequencies, a clear square-wave pattern can be discerned. These criti-
cal sweep frequencies are sub-multiples of the signal frequency, or the signal frequency itself (when
only one cycle is displayed). The pattern may be locked in at any sub-multiple of the signal frequency
when it is desired to view more than one cycle of the applied signal. The sweep frequency is equal to
the signal frequency divided by tl ie number of complete cycles displayed on the screen. For example
- if two complete cycles of the 60-Hz signal are displayed, the sweep frequency is 30 Hz.
At very low sweep frequencies, flickering of the trace is normal due to the slow writing speed
of the electron beam, and the persistence of the screen. Taken together, these may be insufficient
to cause the beam motion to blend into a clear fixed image.
n. Adjust the SWEEP RANGE VERN until the single square wave pattern locks on the positive-going
leading edge, which should not be visible. Placing the SYNC SELECTOR in the "-"position should
cause the pattern to move over until sync takes place on the negative transition.
o. If it is desired to observe the operation of the intensity modulation facility, connect the output
of an audio generator to the rear panel intensity jack and front panel GND. Be sure that the INTENSITY
control is set no higher than is necessary. Set the audio generator to 600 Hz and bring up the generator
o utput until the displayed single pattern breaks up into segments. Do not increase the generator's
ou tput above the level required to do this. Fine adjust the generator frequency carefully until the seg-
me nt s become stationary. If the segm,ents are counted, it will be found that there are 10. This indi-'
cat es that the ratio between the frequency of the intensity modulation (audio generator) and the vertical
ampl ifier input (in this case 60 Hz) is 600/60 or 10. Therefore, the intensity modulation can be used
for introducing timing markers on the trace, or for determining the frequency of an unknown signal.
CAUTION
When
using the INTENSITY modulation, the INTENSITY control should first be set
at minimum (counterclockwise} and then turned clockwise just enough to obtain normal
intensity. If this is done, the possibility of applying excessive intensity modulation
signal voltage will be minimized.
SECTION 3 APPLICATION
3-1. GENERAL The oscilloscope are the electronic "eyes" of the technician and enable him to see
waveforms that are totally invisible to his normal eyes. The scope is capable of displaying high-speed
electrical waveforms of both continuous and transient nature from a few cycles per second to many
millions of cycles per second.
3-2. WAVEFORM INVESTIGATION When the output of the scope's internal sweep generator is fed
to the horizontal channel, the pattern on the CRT screen is actually a graph displaying the variation
with time of the instantaneous amplitude of the signal applied to the vertical channel. The horizontal
sweep time can be varied so as to present one or more full cycles on the screen.
3-3. WAVEFORM DISPLAY It is generally most convenient to use a time base that varies linearly
with time, so that equal intervals of time are represented on the screen by equal intervals of distance
along the horizontal axis.
If the frequency of the observed signal is exactly equal to the sweep frequency, one complete
vertical channel cycle will be observed on the screen. If the frequency of the applied signal is twice
the horizontal sweep frequency, two complete cycles will be seen on the screen, and so on.
Fig. 1 is projection drawing of a simple sine wave applied to the vertical channel and a linear
(with time} sawtooth applied to the horizontal channel. Fig. 2 is a similar drawing showing the re-
sultant pattern when the frequency oi the sawtooth is one-half that used in Fig. 1. In both figures,
7

points that occur simultaneously are numbered the same. The circle represents the CRT screen. If
simultaneous projections were drawn from every point on each waveform, the intersections would
trace out the sine waves shown within the circles. The sections of the sawtooth between 1 and 4 of
Fig. 1, and between 1 and 9 of Fig. 2 are the sweep sections during which the actual CRT display is
produced. The sawtooth sections between 4 and 5 of Fig. 1, and between 9 and 10 of Fig. 2, are the
sections during which the electron beam is returned very rapidly to the starting point at the left of
the screen. This return tra ~ e is prevented from appearing on the screen by a built-in blanking circuit.
1......,__ I I _/'I
l~I
I I I I
tlz: 4:
I I 1
TllEI: 21 3 60 SEC.
FIGURE 1
t
TIME
I
I
I
110
I
1
JO SEC.
1~- -...,.-~
FIGURE 2
3-4. LISSAJOUS PATTERNS: Another type of fundamental pattern is obtained when both the vertical
and horizontal deflection voltages are sine waves that are related in frequency when one frequency is
a whole number of times greater than the other; or when one frequency is a simple fraction of the other.
When one or the other of these conditions is fulfilled, stationary closed-loop patterns are obtained.
These patterns are called Lissajous figures after a 19th century French scientist. They are parti-
cularly useful in determining the frequency ratio between two sine wave signals. If the frequency of
one
signal is known, the frequency of the other signal can be easily determined from the frequency
ratio. Usually the known signal is applied to the horizontal channel and the unknown signal to the
vertical channel. The shape of the pattern changes with the phase relationship between the known and
unknown signals. For example, all the patterns shown in Fig. 3 (and those intermediate) are possible
with a frequency ratio of 1:1 if the phase differences indicated exist.
2: 1
3: 1 4 :1
1: 2
1: 3 1:4
I 0
\J \
0
D° 45°
900
135° 1811"
OO@{XID8§~
FIGURE 3
FIGURE 4
In general, to determine frequency ratio from the Lissajous figure, count the number of points
of tangency to horizontal and vertical lines, drawn or imagined (see Fig. 4). Points of tangency at
the top of the figures result from the unknown frequency applied to the vertical channel. Those at the
side of the figure result from the known frequency applied to the horizontal axis.
V axis freq
H axis freq
V pts of tangency
H pts of tangency
As an example, take Fig. 4c, which shows four points of tangency at the top and one point at the
side. This indicates that the unknown frequency applied to the vertical axis is four times the known
frequency. In Fig. 4f, one point of tangency at the top and four at the side indicate that the unknown
frequency is one-fourth the known frequency.
8

A square-wave generator such as the E IC O Model 377 can be used to rapidly check amplifiers
for frequency response, phase shift, transi ent r e sponse, deficient design, or faulty components.
AUDIO
GENERATOR
EQUIPMENT
~
TO BE TESTEDr ~ SCOPE
FIGURE 5
~ ~ -\b -%- _r;rr
~ ~- ~ ~-
FIGURE 6
First, for a basis of comparison, the square wave output from the Audio Generator is directly
viewed on the scope. The horizontal sweep of the scope should be adjusted so that at least two full
cycles can be seen on the screen. (Fig. 6a shows one full cycle of a perfect square wave). The
scope is then connected to the output of the amplifier under test so that the amplified square wave can
be viewed on the screen. Possible output wave shapes are shown in Fig. 6b to 6i, and the significance
of each waveshape is explained below.
Fig. 6b shows "rounding" of the leading edge of square wave. This indicates a drop off in gain
at high frequencies. "Rounding" will generally be observable when there is a substantial drop in the
gain by the tenth harmonic (br less) . Therefore , if a 2KHz square wave fed to the amplifier is re-
produced on the scope without "rounding", the amplifier is flat to 10 x 2KHz =20KHz.
Fig. 6c shows the effect of increased low-frequency gain and Fig. 6d shows the effect of de-
creased low-frequency gain of the square wave frequency. Fig. 6e indicates lowered gain at a narrow
frequency band .
The effect of phase shift in the amplifier is shown in Figs. 6f and 6g. If, at low frequencies,
there is phase shift in the leading direction, the square wave will be tilted as in Fig. 6f. If there is
phase shift in the lagging direction, the top of the square wave will be tilted as in Fig. 6g. The steep-
ness of the tilt is proportional to the amount of phase shift. Phase shift is not important in audio ampli-
fiers, although the ear is not entirely insensitive to it. In television and scope amplifiers, however,
phase shift should not be tolerated.
Fig. 6h shows the pulse output from the amplifier that results when the square wave has under-
gone differentiation . This will happen when the grid resistor, or the coupling capacitor is too low in
value, or if the coupling capacitor is partially open. Lastly, Fig. 6i, shows a square wave with damped
oscillations following the leading edge. This results when a square wave is fed to an amplifier in which
distributed capacities and lead inductances resonate to produce "shock oscillations" . In television and
scope amplifiers it may result from an undamp ed peaking coil.
High-fidelity audio amplifiers may be given a rapid check by testing first with a square wave of
fundamental frequency not less than 3 to 4 times the low-frequency limit of the amplifier (3dB point),
and then with a square wave of fundamental frequency which may be anywhere between 1/100 to 1/ 10 of
the high-frequency limit of the amplifier depending upon how many harmonics are considered necessary
to produce an acceptable version of a square waveform. Usually, square waves of fundamental frequency
from 40 to 60 Hz and 1000 to 2000 Hz are employed to cover the range up to 20, 000 Hz.
To
insure correct results, the following is suggested: Connect the proper value of load across
the amplifier output terminals; use low-capacitance cable for connecting the generator to the amplifier
input; set the generator output to an ample value but be sure not to overload the amplifier. The square-
wave signal is fed to the amplifier input and the scope is connected across the amplifier load. Use the
internal linear sweep to observe the waveform . Note that tone controls have a very marked effect on
square wave response and should be set to the "flat" positions unless it is desired to observe their ef-
fect. Note, also , t hat low-fidelity and p.a. amplifiers will not reproduce the square waveform.
Video amplifiers may be square wave tested in the same manner as described for testing audio
amplifiers. The test frequencies might be 60 Hz for the low end, and 25, 000 Hz for the high end.
9