23
Subject to change without notice
Test Instructions
General
These Test Instructions are intended as an aid for checking
the most important characteristics of the HM304 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. Depress AUTO SET for default settings. 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 HM304 has very good brightness.
Any reduction of this brightness can only be judged visually.
However, decreased brightness may be the result of wrong
settings or reduced high voltage. The latter is easily
recognized by the greatly increased sensitivity of the vertical
amplifier. Correct setting means, that the HOLD OFF control
should be turned to the left stop; the X-MAG. x10 function
should be switched off; a medium time coefficient should be
selected; line triggering (~ indicated) should be used only
with a suitable TIME/DIV. 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 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
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.
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 nor-
mal 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. 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
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 0.2Vpp ±1% and 2Vpp are
present at the output sockets 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 0.12div. (3%) are permissible. If a x10 probe is
connected between the 2V output and Y input, the same
display height should result.
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 pro-
be is faulty or incorrectly compensated. If necessary the
measuring 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 a fast risetime (max. 5ns). The signal coaxial cable (e.g.
HZ34) must be terminated at the vertical input of the
oscilloscope with a resistor equal to the characteristic
impedance of the cable (e.g. with HZ22). Checks should be
made at 100Hz, 1kHz, 10kHz, 100kHz and 1MHz, the
deflection coefficient should be set at 5mV/div. with DC input
coupling (Y variable control in CAL. position). In so doing, the
square pulses must have a flat top without ramp-off, spikes
and glitches; no overshoot is permitted, especially at 1MHz
and a display height of 4-5div.. At the same time, the leading
top corner of the pulse must not be rounded. In general, no
great changes occur after the instrument has left the factory,
and it is left to the operators discretion whether this test is
undertaken or not. A suited generator for this test is HZ60
from HAMEG.
