31
Subject to change without notice
Test Instructions
Test Instructions
General
These Test Instructions are intended as an aid for checking
the most important characteristics of the instrument at
regular intervals without the need for expensive test
equipment.
As with the First Time Operation instructions, care should be
taken that all variable functions are set to their calibrated
positions. This is the case if AUTO SET had been pressed. It
is recommended to switch on the instrument for about 20
minutes prior to the commencement of any check.
Cathode-Ray Tube: Brightness and Focus,
Linearity, Raster Distortion
Normally, the CRT of the instrument has very good brightness.
Any reduction of this brightness can only be judged visually.
Decreased brightness may be the result of reduced high
voltage. In this case the sensitivity of the vertical amplifiers is
significantly increased.
The control range for maximum and minimum brightness
(intensity) is set so that the beam just disappears before
reaching the left hand stop of the INTENS. control (particularly
when in XY mode), while with the control at the right hand stop
the focus and the line width are just acceptable.
With maximum intensity the timebase fly-back must on no
account be visible. Visible trace fault without input signal: bright
dot on the left side or decreasing brightness from left to right or
shortening of the baseline. (Cause: Incorrect Unblanking Pulse.)
It should be noted that with wide variations in brightness,
refocusing is always necessary. Moreover, with maximum
brightness, no “pumping” of the display must occur. If pump-
ing does occur, it is normally due to a fault in the regulation
circuitry for the high voltage supply.
A certain out-of-focus condition in the edge zone of the screen
must be accepted. It is limited by standards of the CRT manu-
facturer. The same is valid for tolerances of the orthogonality, the
undeflected spot position, the non-linearity and the raster distor-
tion in the marginal zone of the screen in accordance with
international standards (see CRT data book). These limit values
are strictly supervised by
HAMEG
. The selection of a cathode-
ray tube without any tolerances is practically impossible.
Astigmatism Check
Check whether the horizontal and vertical sharpness of the
display are equal. This is best seen by displaying a square-
wave signal with the repetition rate of approximately 1MHz.
Focus the horizontal tops of the square-wave signal at normal
intensity, then check the sharpness of the vertical edges. A
certain loss of marginal sharpness of the CRT is unavoidable;
this is due to the manufacturing process of the CRT.
Symmetry and Drift of the Vertical Amplifier
Both of these characteristics are substantially determined by
the input stages of the amplifiers.
The symmetry of both channels and the vertical final amplifier
can be checked by inverting Channel I and II (depress the
corresponding INV pushbutton). The vertical position of the
trace should not change by more than 0.5div. However, a
change of 1div is just permissible. Larger deviations indicate
that changes have occurred in the amplifier.
A further check of the vertical amplifier symmetry is possible
by checking the control range of the Y-POS controls. A sine-
wave signal of 10-100kHz is applied to the amplifier input.
When the Y-POS. control is then turned fully in both directions
from stop to stop with a display height of approximately 8div,
the upper and lower positions of the trace that are visible
should be approximately of the same height. Differences of
up to 1div are permissible (input coupling should be set to AC).
Checking the drift is relatively simple. 20minutes after switch-
ing on the instrument, set the baseline exactly on the horizon-
tal center line of the graticule. The beam position must not
change by more than 0.5div during the following hour.
Check Vertical Amplifier
A square-wave voltage of 0.2Vpp ±1% is present at the
output socket of the calibrator (CAL.) If a direct connection is
made between the 0.2V output and the input of the vertical
amplifier (e.g. using a x1 probe), the displayed signal in the
50mV/div position (variable control to CAL.) should be 4div
high (DC input coupling). Maximum deviations of 0.16div (3%
vertical amp 1% calibrator amplifier = 4% theoretical
maximum error) are permissible.
With higher tolerances it should first be investigated whether
the cause lies, within the amplifier or in the amplitude of the
square-wave signal. On occasions it is possible that the probe
is faulty or incorrectly compensated. If necessary the meas-
uring amplifier can be calibrated with an accurately known DC
voltage (DC input coupling). The trace position should then
vary in accordance with the deflection coefficient set.
With variable control in the attenuator sector fully counter-
clockwise, the input sensitivity is decreased at least by the
factor 2.5 in each position. In the 50mV/div position, the
displayed calibrator signal height should vary from 4div to at
least 1.6div.
Transmission Performance
of the Vertical Amplifier
The transient response and the delay distortion correction can
only be checked with the aid of a square- wave generator with
