Hameg HM 407A Manual

38 Subject to change without noticepanel can withdrawn forward. When the chassis is insertedinto the case later on, it should be noticed that the case has tofit under the flange of the front panel. The same applies for therear of the case, on which the rear cover is put.Caution !During opening or closing of the case, the instrumentmust be disconnected from all power sources formaintenance work or a change of parts or components.If a measurement, trouble-shooting, or an adjustmentis unavoidable, this work must be done by a specialist,who is familiar with the risk involved.When the instrument is set into operation after the case has beenremoved, attention must be paid to the acceleration voltage forthe CRT approx. -2025V and to the operating voltages for bothfinal amplifier stages 175V and 146V. Potentials of these voltagesare on the PS-Board, the CRT-PCB and on the main PCB. They arehighly dangerous and therefore precautions must be taken. Itshould be noted furthermore that shorts occurring on differentpoints of the CRT high voltage and unblanking circuitry willdefinitely damage some semiconductors. For the same reason itis very risky to connect capacitors to these points while theinstrument is on.Capacitors in the instrument may still be charged, even when theinstrument is disconnected from all voltage sources. Normally,the capacitors are discharged approx. 6 seconds after switchingoff. However, with a defective instrument an interruption of theload is not impossible. Therefore, after switching off, it isrecommended to connect one by one all terminals across 1kΩ toground (chassis) for a period of 1 second.CautionHandling 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). If any work has been carried out,the complete instrument (with case closed and POWERbutton depressed) must undergo a safety test, including avoltage test with 2200V DC, between accessible parts to bothmains/line supply terminals. This test is dangerous and requiresan adequately trained specialist. The maximum impedancebetween each accessible metallic part and the safety earthconnector at the 3-pole power connector must be less than0.1Ω.Operating VoltagesThe operating voltages ( +12V, -6V, -2025V) are stabilized bythe switch mode power supply. The +12V supply is furtherstabilized and used as a reference voltage for -6V stabilization.These different operating voltages are fixed voltages, exceptthe +12V, which can be adjusted. The variation of the fixedvoltages greater than 5% from the nominal value indicates afault. Measurements of the high voltage may only beaccomplished by the use of a sufficient highly resistive voltmeter(>10MΩ). You must make absolutely sure that the electricstrength of the voltmeter is sufficiently high. It is recommendedto check the ripple and also the interaction from other possiblesources. Excessive values might be very often the reason forincomprehensible faults.Maximum and Minimum BrightnessA variable resistor (100kΩ), located on the CRT PCB, is usedfor this adjustment procedure. It may only be touched by aproperly insulating screwdriver (Caution! High voltage!). Correctadjustment is achieved, when the trace can be blanked in XYanalog mode and, in addition, when the requirement describedin the Test Instructions 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. +71V when the trace is in center position)should firstly be checked, because this voltage will affect theastigmatism correction. While the adjustment is being carriedout (with medium brightness and a 1MHz square-wave signal),the upper horizontal square-wave tops are firstly focussedwith the FOCUS control. Then the sharpness of the verticallines are corrected with the 47kΩ Astigm. pot. The correctionshould be repeated several times in this sequence. Theadjustment is optimized, when the FOCUS knob exclusivelybrings no improvement of the sharpness in both directions.Trigger ThresholdThe internal trigger threshold should be in the range 0.3 to0.5div display height.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 traced bythe display of signal or ripple voltages. As noted before, theregulated high voltage and the supply voltages for the finalstages are highly dangerous. Therefore it is recommended touse totally insulated extended probe tips, when trouble-shooting the instrument. Accidental contact with dangerousvoltage potentials is then unlikely. Of course, these instructionscannot thoroughly cover all kinds of faults. Some common-sense will certainly be required, when a complex fault has tobe 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 making goodcontact and are not shorting to an adjacent circuit. Especiallyinspect the connections between 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 separate voltagesof each plate pair are almost equal then (Y approx. 71V and Xapprox 90V). If the separate voltages of a plate pair are verydifferent, the associated circuit must be faulty. An absenttrace in spite of correct plate voltages means a fault in the CRTcircuit. Missing deflection plate voltages is probably caused bya defect in the power supply.Service Instructions

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