Hameg HM304 Manual

Hameg HM304 Manual Manual pdf 26 page image

Subject to change without notice26Handling of the CRT needs utmost caution. The glass bulbmust not be allowed under any circumstances to come intocontact with hardened tools, nor should it undergo localsuperheating (e.g. by soldering iron) or local undercooling (e.g.by cryogenic-spray). We recommend the wearing of safetygoggles (implosion danger).The complete instrument (with case closed and POWERbutton depressed) is after each intervention undergo avoltage test with 2200V, DC, between accessible parts toboth mains/line supply terminals. This test is dangerous andrequires an adequately trained specialist.Operating VoltagesAll operating voltages ( +12V, -12V, +175V, +144V, -2025V)are stabilized by the switch mode power supply. The +12Vsupply is further stabilized and used as a reference voltagefor -12V stabilisation. These different operating voltages arefixed voltages, except the +12V, which can be adjusted. Thevariation of the fixed voltages greater than 5% from the no-minal value indicates a fault. Measurements of the highvoltage may only be accomplished by the use of a sufficienthighly resistive voltmeter (>10MΩ). You must make absolutelysure that the electric strength of the voltmeter is sufficientlyhigh. It is recommended to check the ripple and also theinteraction from other possible sources. Excessive valuesmight be very often the reason for incomprehensible faults.Maximum and Minimum BrightnessTwo variable resistors (470kΩ), located on the switch modepower supply PCB, are used for these adjustment procedures.They may only be touched by a properly insulating screwdriver(Caution! High voltage!). The adjustments may possibly haveto be repeated, because the functions of both variableresistors are dependent on each other. Correct adjustmentis achieved, when the trace can be blanked in XY mode and,in addition, when the requirement described in the TestInstructions are met.Astigmatism controlThe ratio of vertical and horizontal sharpness can be adjustedby the variable resistor of 47kΩ, located on the CRT PCB. Asa precaution however, the voltage for the vertical deflectingplates (approx. +80V when the trace is in center position)should firstly be checked, because this voltage will affectthe astigmastism correction. While the adjustment is beingcarried out (with medium brightness and a 1MHz square-wave signal), the upper horizontal square-wave tops are firstlyfocussed with the FOCUS control. Then the sharpness ofthe vertical lines are corrected with the 47kΩ Astigm. pot.The correction should be repeated several times in thissequence. The adjustment is optimised, when the FOCUSknob exclusively brings no improvement of the sharpness inboth directions.Trigger ThresholdThe internal trigger threshold should be in the range 0.3 to0.5div. display height. It is strongly dependent on thecomparator IC. If there are compelling reasons to replacethis comparator, it may be that triggering becomes too sen-sitive or too insensitive caused by the IC gain tolerances (seeTest Instructions: „Triggering Checks“). In extreme cases,the hysteresis resistor of the comparator should be changed.Generally, max. halving or doubling of this resistance valueshould be sufficient. A too small trigger threshold causedouble-triggering or premature trigger action due tointerference pulses or random noise. A too high triggerthreshold prevents the display of very small display heights.Trouble-Shooting the InstrumentFor this job, at least an isolating variable mains/line transformer(protection class II), a signal generator, an adequate precisemultimeter, and, if possible, an oscilloscope are needed. Thislast item is required for complex faults, which can be tracedby the display of signal or ripple voltages. As noted before,the regulated high voltage and the supply voltages for thefinal stages are highly dangerous. Therefore it isrecommended to use totally insulated extended probe tips,when trouble-shooting the instrument. Accidental contactwith dangerous voltage potentials is then unlikely. Of course,these instructions cannot thoroughly cover all kinds of faults.Some common-sense will certainly be required, when acomplex fault has to be investigated.If trouble is suspected, visually inspect the instrumentthoroughly after removal of the case. Look for loose or badlycontacted or discolored components (caused by overheating).Check to see that all circuit board connections are makinggood contact and are not shorting to an adjacent circuit.Especially inspect the connections between the PCBs, to frontchassis parts, to CRT PCB, to trace rotation coil (inside ofCRTs shielding), and to the control potentiometers andswitches on top of and beneath the PCBs. This visualinspection can lead to success much more quickly than asystematic fault location using measuring instruments. Priorto any extensive trouble-shooting, also check the externalpower source.If the instrument fails completely, the first and important step- after checking the power fuses - will be to measure thedeflecting plate voltages of the CRT. In almost any case, thefaulty section can be located. The sections represent:1. Vertical deflection. 2. Horizontal deflection.3. CRT circuit. 4. Power supply.While the measurement takes place, the position controls ofboth deflection devices must be in mid-position. When thedeflection devices are operating properly, the separatevoltages of each plate pair are almost equal then (Y approx.85V and X approx 86V). If the separate voltages of a platepair are very different, the associated circuit must be faulty.An absent trace in spite of correct plate voltages means afault in the CRT circuit. Missing deflection plate voltages isprobably caused by a defect in the power supply.AdjustmentsAs advised in the Operating, Test and Service Instructions,small corrections and adjustments are easily carried out withthe aid of the Circuit Diagrams and Adjusting Plan. However,a complete recalibration of the scope should not be attemptedby an inexperienced operator, but only someone withsufficient expertise. Several precision measuring instrumentswith cables and adapters are required, and only then shouldthe pots and trimmers be readjusted, provided that the resultof each adjustment can be exactly determined. Thus for eachoperating mode and switch position, a signal with theappropriate sine or square waveform, frequency, amplitude,risetime and duty cycle is required.