Subject to change without notice
23
General
These Test Instructions are intended as an aid for checking
the most important characteristics of the HM303 at
regular intervals without the need for expensive test
equipment. Resulting corrections and readjustments inside
the instrument, indicated by the following tests, are
described in the Service Instructions or on the Adjusting
Plan. They should only be undertaken by qualified
personnel.
As with the First Time Operation instructions, care should
be taken that all knobs with arrows are set to their
calibrated positions. None of the pushbuttons should be
depressed. TRIG. selector switch to
AC
.
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 Distortions
Normally, the CRT of the HM303 has very good brightness.
Any reduction of this brightness can only be judged
visually. However, decreased brightness may be the
result of wrong setting or reduced high voltage. The latter
is easily recognized by the greatly increased sensitivity of
the vertical amplifier. Right setting means, that the
HOLD
OFF
control should be turned to the left stop; the
X-MAG.
(x10)
button should be released; a medium time coefficient
should be selected; line triggering (~ position) should be
used only with a suitable
TIME/DIV.
switch setting (e.g.
2ms/div.
). The control range for maximum and minimum
brightness (intensity) must be such that the beam just
disappears before reaching the left hand stop of the
INTENS.
control (particularly when the
XY
button is
depressed), 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 pumping does
occur, it is normally due to a fault in the regulation circuitry
for the high voltage supply. The presetting pots for the
high voltage circuit, minimum and maximum intensity, are
only accessible inside the instrument (see Adjusting Plan
and Service Instructions).
A certain out-of-focus condition in the edge zone of the
screen must be accepted. It is limited by standards of the
CRT manufacturer. The same is valid for tolerances of the
orthogonality, the undeflected spot position, the non-
linearity and the raster distortion 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. If it is possible to improve this vertical sharpness by
turning the
FOCUS
control, then an adjustment of the
astigmatism control is necessary. A potentiometer of 47k
Ω
is provided inside the instrument for the correction of
astigmatism (see Service Instructions). 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
INVERT
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.
20 minutes after
switching on the instrument
, set the baseline exactly
on the horizontal center line of the graticule. The beam
position must not change by more than
0.5div.
during the
following hour.
Calibration of the Vertical Amplifier
Two square-wave voltages of
0.2V
pp
and
2V
pp
±1% are
present at the output sockets of the calibrator (
CAL.
) If a
direct connection is made between the
0.2V
output and the
Test Instructions
