In detail: do-it-yourself repair of the 830 multimeter from a real master for the site my.housecope.com.
Visually I discovered the absence of one terminal, apparently the battery was taken out without caring about the health of the board. The fuse is intact, the resistors are normal - so for checking I put the position of the voltmeter, connect the probes - 0.00 on the display. Ohmmeter too, ammeter, etc. I decided to remove the fee, and now:
I found a burned-out track near the terminal with the battery, sometimes this track is on fire, but the fuse is intact.
I connected it as I could and started assembling, I want to draw special attention of inexperienced home repair lovers to these bearings, which can be lost during quick disassembly, and without them you cannot see a clear switch.
Collected - it works. There was a lot of joy, opened the second, and surprise knew no bounds.
As a result, + 2 testers in 25 minutes, having collected both, tested them for performance - they function like new!
On the right is my tester and next to it there are two - now mine too :) It remains to figure out why I need 3 of them now, but that's another story. I wish everyone to be attentive to any technique before giving up on it, because often the repair consists in the simplest actions to restore contacts.
It is impossible to imagine a repairman's workbench without a handy, inexpensive digital multimeter.
This article describes the device of the 830 series digital multimeters, its circuit, as well as the most common malfunctions and how to fix them.
Currently, a huge variety of digital measuring instruments of varying degrees of complexity, reliability and quality are produced. The basis of all modern digital multimeters is an integrated analog-to-digital voltage converter (ADC). One of the first such ADCs suitable for constructing inexpensive portable measuring instruments was a converter based on the ICL7106 microcircuit manufactured by MAXIM. As a result, several successful low-cost models of 830 series digital multimeters have been developed, such as M830B, M830, M832, M838. DT can be used instead of the letter M. This instrument series is currently the most widespread and most repeatable in the world. Its basic capabilities: measuring direct and alternating voltages up to 1000 V (input resistance 1 MΩ), measuring direct currents up to 10 A, measuring resistances up to 2 MΩ, testing diodes and transistors. In addition, in some models there is a mode of sound continuity of connections, temperature measurement with and without a thermocouple, generation of a meander with a frequency of 50 ... 60 Hz or 1 kHz. The main manufacturer of this series of multimeters is Precision Mastech Enterprises (Hong Kong).
 |
Video (click to play). |
The basis of the multimeter is the ADC IC1 of the 7106 type (the closest domestic analogue is the 572PV5 microcircuit). Its structural diagram is shown in Fig. 1, and the pinout for the version in the DIP-40 package is shown in Fig. 2. The 7106 core can be preceded by different prefixes depending on the manufacturer: ICL7106, ТС7106, etc. Recently, more and more often used are chipless microcircuits (DIE chips), the crystal of which is soldered directly to the printed circuit board.
Consider the circuit of the Mastech M832 multimeter (Fig. 3). Pin 1 of IC1 supplies a positive 9V battery supply voltage, and Pin 26 supplies a negative battery supply. Inside the ADC there is a 3 V stabilized voltage source, its input is connected to pin 1 of IC1, and the output is connected to pin 32. Pin 32 is connected to the common pin of the multimeter and is galvanically connected to the COM input of the device. The voltage difference between pins 1 and 32 is approximately 3 V over a wide range of supply voltages - from nominal to 6.5 V.This stabilized voltage is fed to an adjustable divider R11, VR1, R13, and from its output to the input of microcircuit 36 (in the mode of measuring currents and voltages). The divider sets the potential U at pin 36, equal to 100 mV. Resistors R12, R25 and R26 perform protective functions. Transistor Q102 and resistors R109, R110 and R111 are responsible for indicating the discharge of the battery. Capacitors C7, C8 and resistors R19, R20 are responsible for displaying the decimal points of the display.
Operating input voltage range Umax directly depends on the level of the regulated reference voltage at pins 36 and 35 and is
The stability and accuracy of the display is dependent on the stability of this reference voltage.
The display N readings depend on the input voltage U and are expressed as a number
A simplified circuit of the multimeter in the voltage measurement mode is shown in Fig. 4.
When measuring DC voltage, the input signal is fed to R1… R6, from the output of which, through a switch [according to the scheme 1-8 / 1… 1-8 / 2), it is fed to the protective resistor R17. This resistor also forms a low-pass filter when measuring AC voltage together with the capacitor C3. Then the signal goes to the direct input of the ADC microcircuit, pin 31. The potential of the common pin, generated by the 3 V stabilized voltage source, pin 32, is fed to the inverse input of the microcircuit.
When measuring AC voltage, it is rectified by a half-wave rectifier on diode D1. Resistors R1 and R2 are selected so that when measuring sinusoidal voltage, the device shows the correct value. ADC protection is provided by the divider R1 ... R6 and the resistor R17.
A simplified circuit of the multimeter in the current measurement mode is shown in Fig. 5.
In the mode of measuring direct current, the latter flows through the resistors R0, R8, R7 and R6, which are switched depending on the measuring range. The voltage drop across these resistors through R17 is fed to the ADC input, and the result is displayed. ADC protection is provided by diodes D2, D3 (in some models they may not be installed) and fuse F.
A simplified circuit of the multimeter in the resistance measurement mode is shown in Fig. 6. In the resistance measurement mode, the dependence expressed by the formula (2) is used.
The diagram shows that the same current from the voltage source + U flows through the reference resistor and the measured resistor R "(the currents of inputs 35, 36, 30 and 31 are negligible) and the ratio of U and U is equal to the ratio of the resistances of the resistors R" and R ^. R1..R6 are used as reference resistors, R10 and R103 are used as current setting resistors. Protection of the ADC is provided by thermistor R18 (some cheap models use conventional 1.2 kΩ resistors), transistor Q1 in zener diode mode (not always installed) and resistors R35, R16 and R17 at inputs 36, 35 and 31 of the ADC.
Continuity mode The dialing circuit uses IC2 (LM358), which contains two operational amplifiers. A sound generator is assembled on one amplifier, and a comparator on the other. When the voltage at the input of the comparator (pin 6) is less than the threshold, a low voltage is set at its output (pin 7), which opens the switch on the transistor Q101, as a result of which a sound signal is emitted. The threshold is determined by the divider R103, R104. Protection is provided by resistor R106 at the comparator input.
All malfunctions can be divided into factory defects (and this happens) and damage caused by erroneous actions of the operator.
Since multimeters use tight wiring, shorts of elements, poor soldering and breakage of the leads of elements, especially those located at the edges of the board, are possible. Repair of a faulty device should begin with a visual inspection of the printed circuit board. The most common factory defects of M832 multimeters are shown in the table.
The LCD display can be checked for proper operation using a 50.60 Hz AC voltage source with an amplitude of several volts.As such a source of alternating voltage, you can take the M832 multimeter, which has a meander generation mode. To check the display, put it on a flat surface with the display up, connect one probe of the M832 multimeter to the common terminal of the indicator (bottom row, left terminal), and apply the other probe of the multimeter alternately to the rest of the display. If it is possible to get the ignition of all segments of the display, then it is serviceable.
The above malfunctions may also appear during operation. It should be noted that in the DC voltage measurement mode, the device rarely fails, because well protected from input overloads. The main problems arise when measuring current or resistance.
Repair of a faulty device should begin with checking the supply voltage and the ADC's operability: the stabilization voltage is 3 V and there is no breakdown between the power pins and the common ADC output.
In the current measurement mode when using the V, Q and mA inputs, despite the presence of a fuse, there may be cases when the fuse blows out later than the safety diodes D2 or D3 have time to break through. If a fuse is installed in the multimeter that does not meet the requirements of the instructions, then in this case the resistances R5 ... R8 may burn out, and this may not appear visually on the resistances. In the first case, when only the diode breaks through, the defect appears only in the current measurement mode: the current flows through the device, but the display shows zeros. In case of burnout of resistors R5 or R6 in the voltage measurement mode, the device will overestimate the readings or show an overload. When one or both resistors are completely burned out, the device does not reset in voltage measurement mode, but when the inputs are closed, the display is set to zero. When the resistors R7 or R8 burn out on the current measuring ranges of 20 mA and 200 mA, the device will show an overload, and in the 10 A range - only zeros.
In resistance measurement mode, faults usually occur in the 200 ohm and 2000 ohm ranges. In this case, when the voltage is applied to the input, the resistors R5, R6, R10, R18, the transistor Q1 can burn out and the capacitor C6 can break through. If the transistor Q1 is completely broken, then when measuring the resistance, the device will show zeros. In case of incomplete breakdown of the transistor, the multimeter with open probes will show the resistance of this transistor. In voltage and current measurement modes, the transistor is short-circuited by a switch and does not affect the multimeter readings. With a breakdown of capacitor C6, the multimeter will not measure voltage in the ranges of 20 V, 200 V and 1000 V or significantly underestimate the readings in these ranges.
If there is no indication on the display, when there is power to the ADC, or there is a visually noticeable burnout of a large number of circuit elements, there is a high probability of damage to the ADC. The ADC's serviceability is checked by monitoring the voltage of the 3 V stabilized voltage source. In practice, the ADC burns out only when a high voltage is applied to the input, much higher than 220 V. Very often, cracks appear in the compound of the open-frame ADC, the current consumption of the microcircuit increases, which leads to its noticeable heating ...
When a very high voltage is applied to the input of the device in the voltage measurement mode, a breakdown may occur in the elements (resistors) and on the printed circuit board, in the case of the voltage measurement mode, the circuit is protected by a divider on the resistances R1.R6.
For cheap models of the DT series, long part leads can be shorted to the screen located on the back cover of the device, disrupting the operation of the circuit. Mastech does not have such defects.
A source of a stabilized voltage of 3 V in an ADC for cheap Chinese models can in practice give a voltage of 2.6-3.4 V, and for some devices it stops working already at a voltage of a supply battery of 8.5 V.
The DT models use low quality ADCs and are very sensitive to the C4 and R14 integrator chain ratings. High-quality ADCs in Mastech multimeters allow the use of elements of close denominations.
Often, in DT multimeters, with open probes in the resistance measurement mode, the device approaches the overload value for a very long time ("1" on the display) or is not set at all. It is possible to "cure" a poor-quality ADC microcircuit by reducing the value of the resistance R14 from 300 to 100 kOhm.
When measuring resistances in the upper part of the range, the device "flips" the readings, for example, when measuring a resistor with a resistance of 19.8 kOhm, it shows 19.3 kOhm. It is "treated" by replacing the capacitor C4 with a capacitor of 0.22 ... 0.27 μF.
Since cheap Chinese firms use low-quality unpackaged ADCs, there are frequent cases of broken pins, and it is very difficult to determine the cause of the malfunction and it can manifest itself in different ways, depending on the broken pin. For example, one of the indicator leads is off. Since multimeters use displays with static indication, then to determine the cause of the malfunction, it is necessary to check the voltage at the corresponding pin of the ADC microcircuit, it should be about 0.5 V relative to the common pin. If it is zero, then the ADC is faulty.
There are malfunctions associated with poor-quality contacts on the biscuit switch, the device works only when the biscuit is pressed. Firms that make cheap multimeters rarely coat the tracks under the rocker switch with grease, which is why they quickly oxidize. Often the tracks are dirty. It is repaired as follows: the printed circuit board is removed from the case, and the switch tracks are wiped with alcohol. Then a thin layer of technical petroleum jelly is applied. Everything, the device is repaired.
With DT series devices, it sometimes happens that the alternating voltage is measured with a minus sign. This indicates an incorrect installation of D1, usually due to incorrect marking on the diode body.
It happens that manufacturers of cheap multimeters put low-quality operational amplifiers in the sound generator circuit, and then when the device is turned on, a buzzing buzzer is heard. This defect is eliminated by soldering a 5 μF electrolytic capacitor parallel to the power supply circuit. If this does not ensure the stable operation of the sound generator, then it is necessary to replace the operational amplifier with the LM358P.
Often there is such a nuisance as battery leakage. Small drops of electrolyte can be wiped off with alcohol, but if the board is heavily flooded, then good results can be obtained by washing it with hot water and laundry soap. After removing the indicator and unsoldering the buzzer, using a brush, for example, a toothbrush, you need to thoroughly soap the board on both sides and rinse it under running water from the tap. After repeating the wash 2.3 times, the board is dried and installed in the case.
Most recently manufactured devices use DIE chips ADCs. The crystal is installed directly on the PCB and is filled with resin. Unfortunately, this significantly reduces the maintainability of the devices, because when the ADC fails, which is quite common, it is difficult to replace it. Unpackaged ADCs are sometimes sensitive to bright light. For example, if you work near a table lamp, the measurement error may increase. The fact is that the indicator and the board of the device have some transparency, and light, penetrating through them, enters the ADC crystal, causing a photoelectric effect. To eliminate this drawback, you need to remove the board and, after removing the indicator, glue the location of the ADC crystal (it is clearly visible through the board) with thick paper.
When buying DT multimeters, you should pay attention to the quality of the switch mechanics; be sure to rotate the multimeter's rocker switch several times to make sure that the switching occurs clearly and without jamming: plastic defects cannot be repaired.
Sergey Bobin. "Repair of electronic equipment" No. 1, 2003
Experts advise starting the search for the cause of the malfunction with a thorough examination of the printed circuit board, since short circuits and poor soldering are possible, as well as a defect in the leads of the elements along the edges of the board.
Factory defect in these devices is manifested mainly on the display. There can be up to ten types of them (see table). Therefore, it is better to repair digital multimeters using the instructions that come with the device.
The same breakdowns can occur after operation. The above malfunctions may also appear during operation. However, if the device operates in constant voltage measurement mode, it rarely breaks.
The reason for this is its overload protection. Also, the repair of a faulty device should begin with checking the supply voltage and the operability of the ADC: the stabilization voltage is 3 V and there is no breakdown between the power pins and the common ADC output.
Experienced users and professionals have repeatedly stated that one of the most likely causes of frequent breakdowns in an instrument is poor manufacturing. Namely, soldering contacts with acid. As a result, the contacts are simply oxidized.
However, if you are not sure what kind of breakdown caused the inoperative state of the device, you should still contact a specialist for advice or help.
Banned
