Hameg HM304 Manual

Hameg HM304 Manual Manual pdf 23 page image

23Subject to change without noticeTest InstructionsGeneralThese Test Instructions are intended as an aid for checkingthe most important characteristics of the HM304 at regularintervals without the need for expensive test equipment.Resulting corrections and readjustments inside theinstrument, indicated by the following tests, are described inthe Service Instructions or on the Adjusting Plan. They shouldonly be undertaken by qualified personnel.As with the First Time Operation instructions, care should betaken that all knobs with arrows are set to their calibratedpositions. Depress AUTO SET for default settings. It isrecommended to switch on the instrument for about 20minutes prior to the commencement of any check.Cathode-Ray Tube: Brightness and Focus,Linearity, Raster DistortionsNormally, 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 wrongsettings or reduced high voltage. The latter is easilyrecognized by the greatly increased sensitivity of the verticalamplifier. Correct setting means, that the HOLD OFF controlshould be turned to the left stop; the X-MAG. x10 functionshould be switched off; a medium time coefficient should beselected; line triggering (~ indicated) should be used onlywith a suitable TIME/DIV. setting (e.g. 2ms/div.). The controlrange for maximum and minimum brightness (intensity) mustbe such that the beam just disappears before reaching theleft hand stop of the INTENS. control (particularly when inXY mode), while with the control at the right hand stop thefocus and the line width are just acceptable.With maximum intensity the timebase fly-back must on noaccount be visible. Visible trace fault without input signal:bright dot on the left side or decreasing brightness from leftto right or shortening of the baseline. (Cause: IncorrectUnblanking Pulse.)It should be noted that with wide variations in brightness,refocusing is always necessary. Moreover, with maximumbrightness, no „pumping“ of the display must occur. Ifpumping does occur, it is normally due to a fault in theregulation circuitry for the high voltage supply. The presettingpots for the high voltage circuit, minimum and maximumintensity, are only accessible inside the instrument.A certain out-of-focus condition in the edge zone of the screenmust be accepted. It is limited by standards of the CRTmanufacturer. The same is valid for tolerances of theorthogonality, the undeflected spot position, the non-linearityand the raster distortion in the marginal zone of the screen inaccordance with international standards (see CRT data book).These limit values are strictly supervised by HAMEG. Theselection of a cathode-ray tube without any tolerances ispractically impossible.Astigmatism CheckCheck whether the horizontal and vertical sharpness of thedisplay 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 turningthe FOCUS control, then an adjustment of the astigmatismcontrol is necessary. A potentiometer of 47kΩ is providedinside the instrument for the correction of astigmatism. Acertain 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 AmplifierBoth of these characteristics are substantially determinedby the input stages of the amplifiers.The symmetry of both channels and the vertical final amplifiercan be checked by inverting Channel I and II (depress thecorresponding INV pushbutton). The vertical position of thetrace should not change by more than 0.5div. However, achange of 1div. is just permissible. Larger deviations indicatethat changes have occurred in the amplifier.A further check of the vertical amplifier symmetry is possibleby checking the control range of the Y-POS. controls. A sine-wave signal of 10-100kHz is applied to the amplifier input. Whenthe Y-POS. control is then turned fully in both directions fromstop to stop with a display height of approximately 8div., theupper and lower positions of the trace that are visible shouldbe approximately of the same height. Differences of up to1div. are permissible (input coupling should be set to AC).Checking the drift is relatively simple. 20minutes afterswitching on the instrument, set the baseline exactly on thehorizontal center line of the graticule. The beam position mustnot change by more than 0.5div. during the following hour.Calibration of the Vertical AmplifierTwo square-wave voltages of 0.2Vpp ±1% and 2Vpp arepresent at the output sockets of the calibrator (CAL.) If adirect connection is made between the 0.2V output and theinput of the vertical amplifier (e.g. using a x1 probe), thedisplayed signal in the 50mV/div. position (variable control toCAL.) should be 4div. high (DC input coupling). Maximumdeviations of 0.12div. (3%) are permissible. If a x10 probe isconnected between the 2V output and Y input, the samedisplay height should result.With higher tolerances it should first be investigated whetherthe cause lies, within the amplifier or in the amplitude of thesquare-wave signal. On occasions it is possible that the pro-be is faulty or incorrectly compensated. If necessary themeasuring amplifier can be calibrated with an accuratelyknown DC voltage (DC input coupling). The trace positionshould then vary in accordance with the deflection coefficientset.With variable control in the attenuator sector fully counter-clockwise, the input sensitivity is decreased at least by thefactor 2.5 in each position. In the 50mV/div. position, thedisplayed calibrator signal height should vary from 4div. to atleast 1.6div.Transmission Performance of theVertical AmplifierThe transient response and the delay distortion correctioncan only be checked with the aid of a square-wave generatorwith a fast risetime (max. 5ns). The signal coaxial cable (e.g.HZ34) must be terminated at the vertical input of theoscilloscope with a resistor equal to the characteristicimpedance of the cable (e.g. with HZ22). Checks should bemade at 100Hz, 1kHz, 10kHz, 100kHz and 1MHz, thedeflection coefficient should be set at 5mV/div. with DC inputcoupling (Y variable control in CAL. position). In so doing, thesquare pulses must have a flat top without ramp-off, spikesand glitches; no overshoot is permitted, especially at 1MHzand a display height of 4-5div.. At the same time, the leadingtop corner of the pulse must not be rounded. In general, nogreat changes occur after the instrument has left the factory,and it is left to the operators discretion whether this test isundertaken or not. A suited generator for this test is HZ60from HAMEG.