Reparationstips för digital multimeter
För ett defekt instrument, kontrollera först och avgör om felfenomenet är vanligt (alla funktioner kan inte mätas) eller individuellt (enskilda funktioner eller individuella intervall), och särskilj sedan situationen och lös den symptomatiskt.
⒈ If all gears cannot work, focus on checking the power supply circuit and A/D converter circuit. When checking the power supply part, you can remove the laminated battery, press the power switch, connect the positive test lead to the negative power supply of the meter under test, and connect the negative test lead to the positive power supply (for digital multimeters), switch the switch to the diode measurement gear, if the display shows If it is the forward voltage of the diode, it means that the power supply part is good. If the deviation is large, it means that there is a problem with the power supply part. If there is an open circuit, focus on checking the power switch and battery leads. If there is a short circuit, you need to use the circuit breaking method to gradually disconnect the components that use the power supply, focusing on checking operational amplifiers, timers, and A/D converters. If a short circuit occurs, more than one integrated component is generally damaged. The A/D converter can be checked at the same time as the basic meter, which is equivalent to the DC meter of an analog multimeter. The specific inspection method is as follows:
⑴The range of the meter under test is turned to the lowest level of DC voltage;
(2) Mät om A/D-omvandlarens arbetsspänning är normal. Jämför det uppmätta värdet med dess typiska värde enligt A/D-omvandlarmodellen som används i tabellen, motsvarande V plus-stiftet och COM-stiftet.
(3) Mät referensspänningen för A/D-omvandlaren. Referensspänningen för den vanliga digitala multimetern är vanligtvis 100mV eller 1V, det vill säga DC-spänningen mellan VREF plus och COM mäts. Om den avviker från 100mV eller 1V kan den justeras med en extern potentiometer.
⑷ Check the displayed number of input zero, short{{0}}circuit the positive terminal IN plus of the A/D converter with the negative terminal IN-, make the input voltage Vin=0, and the meter displays "00.0" or "00.00".
⑸ Check the full bright stroke of the display. Short-circuit the test terminal TEST pin with the positive power supply terminal V plus to make the logic ground become a high potential, and all digital circuits stop working. Because DC voltage is applied to each stroke, all strokes are bright and the alignment table displays "1888", and the alignment table displays "18888". If there is a missing stroke, check the corresponding output pin of the A/D converter and the conductive glue (or connection), and check whether there is poor contact and disconnection between the A/D converter and the display.
⒉ If there is a problem with individual files, it means that the A/D converter and the power supply part are working normally. Because DC voltage and resistance files share a set of voltage divider resistors; AC and DC current share a shunt; AC voltage and AC current share a set of AC/DC converters; others such as Cx, HFE, F, etc. are composed of independent different converters . Understand the relationship between them, and then according to the power diagram, it is easy to find the fault location. If the measurement of the small signal is inaccurate or the displayed number jumps greatly, focus on checking whether the contact of the range switch is good.
3. If the measured data is unstable, and the value always increases cumulatively, the input terminal of the A/D converter is short-circuited, and the displayed data is not zero, which is generally caused by the poor performance of the 0.1μF reference capacitor.
According to the above analysis, the basic sequence of repairing the digital multimeter should be: digital meter head→DC voltage→DC current→AC voltage→AC current→resistance gear (including buzzer and checking diode positive voltage drop)→Cx→HFE , F, H, T, etc. But it should not be too mechanical. Some obvious problems can be dealt with first. But when making adjustments, be sure to follow the above procedure.
Kort sagt, en defekt multimeter, efter korrekt testning, måste först analysera de möjliga delarna av felet, och sedan hitta felplatsen enligt kopplingsschemat för utbyte och reparation. Eftersom den digitala multimetern är ett relativt exakt instrument måste ersättningskomponenterna ersättas med komponenter med samma parametrar, speciellt ersättningen av A/D-omvandlaren måste använda det integrerade blocket som har blivit strikt skärmat av tillverkaren, annars kommer det att vara fel och kommer inte att uppfylla kraven. Noggrannhet. Den nyligen ersatta A/D-omvandlaren måste också kontrolleras enligt metoden som beskrivs ovan, och får inte litas på på grund av den nya.
