Test Instructions
are strictly supervised by HAMEG. The selection of a cathoderay 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 squarewave 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 sinewave 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.
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 pumping 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 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
Subject to change without notice
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 amplifier + 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 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 counterclockwise, 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
