Digital instrument with high sensitivity and accuracy, its application virtually all businesses. But there was more because of its failure
factors, and the randomness of big problems, not too many rules to follow, repair difficult. Therefore, I will work in practice for many
years accumulated experience in sorting out some repairs for the professional colleagues in the reference.
First, the repair methods
After the failure should be outside looking in, the easier issues first, split up, major breakthroughs. The methods can be divided into
the following categories:
1. Feeling Method Failure with sensory judgments directly, through visual inspection, can be found such as the disconnection, sealing
off, take the line short circuit breaking fuse pipe, burned components, mechanical injury, printed circuit copper foil Alice and fracture,
etc. starting; can touch the batteries, resistors, transistors, monolithic block of temperature rise, can refer to the schematic to identify
the cause of abnormal temperature rise. In addition, the hand can also check whether the loosening of components, integrated circuit leg is
plugged in firmly, whether the cassette switch; can hear and smell are no different sound, smell.
2. Measuring the voltage measurement of the key points of the operating voltage is normal, you can quickly find the point of failure. If
test A / D converter voltage, reference voltage.
3. Short circuit method have stated before, check A / D converter methods are generally used in short-circuit method, this method is weak
and in the repair of the instruments used slightly more power.
4. Disconnecting the suspicious part of the law or the unit from the machine and disconnect the circuit if the fault has disappeared,
that failed in the disconnected circuit. This method is mainly suitable for short circuit of the situation.
5. Measuring element method when the fault has been reduced to a place or a few components, it can measure its online or offline. When
necessary, replace with good components, If the fault disappears, indicating that components are bad.
6. Interference method using induced voltage as the interference signal the body to observe the changes of liquid crystal display,
commonly used in checking the input circuit and the display section is intact.
Second, repair tips
Failure of an instrument should first check and discrimination of the fault phenomena are common (all functions can not be measured), or
personality (individual features or individual range), and different conditions, symptomatic solution.
1. If all files are not working, should focus on checking the power circuit and A / D converter circuit. Check the power supply, you can
remove the laminated battery, press the power switch, with a positive test table table pen then the power of negative, then the power is
negative table pen (in terms of the digital multimeter), switch to the secondary tube measurement file, if the display is the diode forward
voltage, then the power supply is good, if the deviation is large, the description of power supply problems. If there is open, focus on
examination of the power switch and battery lead and so on. If a short circuit, you need to use open circuit method, and gradually disconnect
the power supply components with focus on checking operational amplifiers, timers and A / D converter. If a short circuit, generally more
than damage to an integrated component. Check the A / D converter and the basic table at the same time, the equivalent analog multimeter DC
header, the specific inspection methods:
(1) the measured range of the table to the low DC voltage;
(2) Measurement A / D converter voltage is normal. According to the table used in A / D converter models, corresponding to the V + pin
and the COM pin, the typical measured and compared to the value of its match.
(3) Measurement A / D converter reference voltage, the commonly used benchmark for digital multimeter or voltage typically 100mV 1V,
which measured between VREF + and COM DC voltage, if the deviation from 100mV or 1V, via external potentiometer adjustment.
(4) Check the display of the input number is zero, the A / D converter positive terminal and negative terminal IN + IN-shorted, the input
voltage Vin = 0, instrument displays "00.0" or "00.00."
(5) Check the screen, all light strokes. TEST pin and the test-side power supply terminal V + is shorted, so logically into a high-
potential, all the digital circuit to stop working. On each stroke because they all add a DC voltage, so all the strokes of light on the bit
table shows the "1888" on the bit table shows the "18888." If there is the phenomenon of missing strokes, check the A / D converter
corresponding to the output pin and conductive plastic (or line), and whether there is poor contact between the monitor and disconnect
situation.
2. If the individual files in question, indicating that A / D converter and power supply are functioning properly. For DC voltage,
resistance of a resistor divider file sharing; AC and DC current sharing shunt; AC voltage and AC current share of a AC / DC converter;
Others, such as Cx, HFE, F, etc. by different independent component converter . Understanding of the relationship between them, according to
the power diagram, it is easy to find fault position. If the measurement is not accurate small-signal or display large number of beats, then
check the range switch key contact is good.
3. If there is instability in measurement data, and is always a cumulative increase of short circuit A / D converter input, the data show
the situation is not zero, then the base capacitance of 0.1μF is usually caused by poor performance.
Based on the above analysis, digital multimeter repair basic sequence should be: DC voltage digital meter head → → → DC current AC
current AC voltage → → resistance profile (including the buzzer and diode check is the pressure drop) → Cx → HFE , F, H, T and so on. But
not too mechanical, can see some obvious problems, can be handled. However, when making adjustments, be sure to follow these procedures.
In short, a failure multimeter, after appropriate tests, the first site of failure may occur, and then find the fault location circuit
plans for replacement and repair. Digital multimeter is the result of more sophisticated instruments, replacement of components must use the
same device parameters, especially the replacement of A / D converter must be carefully selected by the manufacturer of the blocks, otherwise
it will not reach the required errors and accuracy. New for the A / D converter, is also required by the inspection method described above,
must not no doubt because of new and confidence.
At present, many manufacturers produce digital multimeter, the quality also has advantages and disadvantages of the quality of double-
sided copper complex in the repair is not easy to find. Dielectric strength of resin board is not enough, mainly the error in the measurement
of high voltage large repair with the change of partial pressure resistance is distinguished. In such cases, the best circuit method is used
to find the point of failure. Carbonation of the burnt part to be cleaned, the insulation requirements. Encountered due to the transition
from the double-sided hole broken connections caused by the input signal can not easily switch bad and confusing and difficult to separate
the phenomenon, such failures should adopt the short circuit method to find the point of failure.
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