Dt 830b chassis 3425 resistor values. Multimeter circuits. How to measure AC voltage

Radio amateurs periodically encounter the problem of multimeter breakdown. Most often, the problem is that the multimeter was soldered using acid and the contacts simply oxidize. In this case, it is very easy to fix the problem, but there is a more serious problem, for example (as in my case), forgetting to discharge the capacitor, they put it in a digital multimeter and want to measure the capacitance, after which the tester refuses to measure anything at all.

Having opened the multimeter, we obviously will not see anything, since the microcircuit was killed by static. The microcircuit itself will most likely be with the numbers 324, as in the photo. fundamental diagram DT9205A Can.

But since the multimeter is made in China, then most likely we will not find any data on this microcircuit. So at first I didn’t find anything, but then I decided to look, adding not all the elements of the inscription of the microcircuit, but only numbers. And the result pleased - the microcircuit turned out to be lm324, or rather a Chinese copy, only with different letters. It is possible to change it to any other OS. If you have a radio store in your city, then you can quickly go there and buy this microcircuit, but if there is no such store (as in my case) or it is far away, and the capacitance meter is very necessary, then we change it to any available microcircuit that contains It has 4 operational amplifiers. If there are no quads - just put two microcircuits that contain 2 op-amps, as I did first.

True, it later turned out that with them the multimeter gives an error. This was due to the fact that the gain of my op amps was different from the gain of the lm324. But there was nowhere to go, as I said earlier, we don’t have radio stores, and ordering via the Internet is also not the best the best option- it will be necessary to wait a long time for the arrival of the order, and I decided to put others. Just a couple of days before the repair of the DT9205A multimeter, an order of five TL074s arrived.

True, I had them in a DIP package and so that it does not interfere with closing the lid DT9205A- Soldered it with wires.

Perhaps when you change the op-amp, even if it is lm324, the multimeter will show a little wrong. In this case, if the deviation is not very large, then this error is removed by a tuning resistor next to the microcircuit (shown by a red arrow), but since there may be deviations in the value of the capacitor, it is better to measure its capacitance on another multimeter and set yours to the same reading.

And finally, a couple of pictures of work after repair.

Enough time has passed since then - and the multimeter works without problems. I wish you all creative success! Article Author: 13265

Discuss the article REPAIR OF THE DT9205 MULTIMETER

Flux SKF

In any case, no matter how you dismantle this resistor from the board, the tubercles of the old solder will remain on the board, we need to remove it with a dismantling braid, dipping it in an alcohol-rosin flux. We put the tip of the braid directly on the solder and press it in, warming it up with a soldering iron tip until all the solder from the contacts is absorbed into the braid.

Dismantling braid

Well, then it’s a matter of technology: we take the resistor we bought in the radio store, put it on the contact pads that we freed from solder, press it down with a screwdriver from above and touch the soldering iron with a power of 25 watts, pads and leads located at the edges of the resistor, solder it in place.

Braid for solder - application

From the first time, it will probably come out crooked, but the most important thing is that the device will be restored. On the forums, opinions on such repairs were divided, some argued that due to the cheapness of multimeters, it makes no sense to repair them at all, they say they threw them away and went to buy a new one, others were even ready to go all the way and solder the ADC). But as this case shows, sometimes repairing a multimeter is quite simple and cost-effective, and any home craftsman can handle such a repair. Everyone! AKV.

A huge variety of digital measuring instruments varying degrees of complexity, reliability and quality. The basis of all modern digital multimeters is an integrated analog-to-digital voltage converter (ADC). One of the first such ADCs suitable for building inexpensive portable measuring instruments was the ICL71O6 chip converter manufactured by MAXIM. As a result, several successful inexpensive models digital multimeters of the 830th series, such as M830V, M830, M832, M838. Instead of the letter M, DT can stand. Currently, this series of devices is the most widespread and most repeated in the world. Its basic features: measurement of direct and alternating voltages up to 1000 V (input resistance 1 MΩ), measurement of direct currents up to 10 A, measurement of resistances up to 2 MΩ, testing of 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).

Scheme and operation of the device

The basis of the multimeter is ADC IC1 type 7106 (the closest domestic analogue is the 572PV5 microcircuit). Its block diagram is shown in fig. 1, and the pinout for the execution in the DIP-40 package is shown in fig. 2. The 7106 kernel may have different prefixes depending on the manufacturer: ICL7106, TC7106, etc. Recently, packageless microcircuits (DIE chips) have been increasingly used, the crystal of which is soldered directly to the printed circuit board.

Consider the circuit of the M832 multimeter from Mastech (Fig. 3). Pin 1 of IC1 is the positive 9V battery supply, pin 26 is the negative. Inside the ADC there is a stabilized voltage source of 3 V, its input is connected to pin 1 of IC1, and the output is connected to pin 32. Pin 32 is connected to the common terminal 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 wide range supply voltages - from nominal to 6.5 V. This stabilized voltage is supplied to the adjustable divider R11, VR1, R13, as its output - at the entrance microcircuits 36 (in the mode of measuring currents and voltages). The divider sets the potential U eg at pin 36, equal to 100 mV. Resistors R12, R25 and R26 perform protective functions. Transistor Q102 and resistors R109, R110nR111 are responsible for low battery indication. Capacitors C7, C8 and resistors R19, R20 are responsible for displaying the decimal points of the display.

Rice. 3. Schematic diagram of the M832 multimeter

Voltage measurement

A simplified diagram of the multimeter in voltage measurement mode is shown in fig. 4. When measuring DC voltage, the input signal is applied 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, in addition, when measuring AC voltage, together with the capacitor C3, forms a low-pass filter. Next, the signal is fed to the direct input of the ADC chip, pin 31. The potential of the common output generated by a stabilized voltage source of 3 V, pin 32 is applied 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 in such a way that when measuring a sinusoidal voltage, the device shows correct value. ADC protection is provided by R1…R6 divider and R17 resistor.

Current measurement

A simplified diagram of the multimeter in the current measurement mode is shown in fig. 5. In the DC measurement mode, the latter flows through the resistors RO, R8, R7 and R6, switched depending on the measurement range. The voltage drop across these resistors through R17 is fed to the input of the ADC, and the result is displayed. ADC protection is provided by diodes D2, D3 (may not be installed in some models) and fuse F.

Resistance measurement

A simplified diagram 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 +LJ flows through the reference resistor Ron and the measured resistor Rx (the currents of inputs 35, 36, 30 and 31 are negligible) and the ratio of UBX and Uon is equal to the ratio of the resistances of resistors Rx and Ron. R1….R6 are used as reference resistors, R10 and R103 are used as current-setting resistors. ADC protection is provided by the thermistor R18 [some cheap models use conventional resistors with a nominal value of 1 ... 2 kOhm), transistor Q1 in zener diode mode (not always installed), and resistors R35, R16 and R17 at inputs 36, 35 and 31 of the ADC.

Call mode

The continuity circuit uses an IC2 chip (LM358) containing two operational amplifiers. A sound generator is assembled on one amplifier, a comparator on the other. When the voltage at the input of the comparator (pin 6) is less than the threshold, its output (pin 7) is set to open the key on transistor Q101, as a result of which an audible signal sounds. The threshold is determined by the divider R103, R104. Protection is provided by resistor R106 at the input of the comparator.

Multimeter Defects

Factory defects of M832 multimeters

Manifestation of a defect	Possible reason	Troubleshooting
		Check elements C1 and R15













		Open connector pins



When measuring AC voltage, the readings of the device “float”, for example, instead of 220 V, they change from 200 V to 240 V


		Solder the pins of IC2


		To restore reliable contact you need: Fix conductive rubber bands; Wipe the corresponding contact pads on the printed circuit board with alcohol; Tear off these pins on the board

The health of the LCD display can be checked using an alternating voltage source with a frequency of 50 ... 60 Hz and an amplitude of several volts. As such an AC voltage source, 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 M832 multimeter probe to the common output of the indicator (bottom row, left output), and apply the other multimeter probe alternately to the rest of the display outputs. If you can get the ignition of all segments of the display, then it is working.

In the current measurement mode, when using the V, Ω and mA inputs, despite the presence of a fuse, there may be cases when the fuse burns 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 the event 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 is not reset in the voltage measurement mode, but when the inputs are closed, the display is set to zero. When resistors R7 or R8 burn out on the current measurement ranges of 20 mA and 200 mA, the device will show an overload, and in the range of 10 A - only zeros.

When applied to the input of the device, very high voltage in the voltage measurement mode, a breakdown may occur along the elements (resistors) and along the printed circuit board; in the case of the voltage measurement mode, the circuit is protected by a divider on resistances R1 ... R6.

A stabilized voltage source of 3 V in the 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 supply battery voltage of 8.5 V.

Often in DT multimeters with open probes in the resistance measurement mode, the device approaches the overload value (“1” on the display) for a very long time or is not set at all. You can “cure” a low-quality ADC chip by reducing the resistance value R14 from 300 to 100 kOhm.

When measuring resistances in the upper part of the range, the device “fills up” 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 microfarads.

With devices of the DT series, it sometimes happens that the alternating voltage is measured with a minus sign. This points to incorrect installation D1, usually due to incorrect markings on the body of the diode.

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, the buzzer buzzes. This defect is eliminated by soldering an electrolytic capacitor with a nominal value of 5 microfarads in parallel with the power circuit. If this does not ensure stable operation of the sound generator, then it is necessary to replace the operational amplifier with an LM358P.

Often there is such a nuisance as battery leakage. Small drops of electrolyte can be wiped 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 squeaker, using a brush, such as a toothbrush, you need to carefully lather the board on both sides and rinse it under running tap water. After washing 2...3 times, the board is dried and installed in the case.

In most of the devices produced recently, unpackaged (DIE chips) ADCs are used. The crystal is installed directly on printed circuit board and filled with resin. Unfortunately, this significantly reduces the maintainability of devices, because. when the ADC fails, which occurs quite often, it is difficult to replace it. Devices with unpackaged ADCs are sometimes sensitive to bright light. For example, when working 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 the light, penetrating through them, falls on the ADC crystal, causing a photoelectric effect. To eliminate this shortcoming, you need to remove the board and, having removed the indicator, glue the location of the ADC crystal (it can be clearly seen through the board) with thick paper.

M830 circuits ... The difference is not big DT830 or M830 ...

Everyone needs to know how to use measuring instruments.
Voltammeter - a universal device (shortly - "tester", from the word "test"). There are a lot of varieties. We will not consider them all. Let's take the most easily accessible Chinese-made multimeter DT-830B.

MULTIMETER DT-830B consists of:
- LCD display
-multiposition switch
- sockets for connecting probes
-panel for testing transistors
- back cover (it will be needed to replace the battery of the device, an element of the "Krona" type 9 volts)
Switch positions are divided into sectors:
OFF/on - device power switch
DCV - DC voltage measurement (voltmeter)
ACV - measurement of voltage overflow current (voltmeter)
hFe - switching sector for measuring transistors
1.5v-9v - checking the batteries.
DCA - DC current measurement (ammeter).
10A - sector of the ammeter for measuring large values of direct current (according to the instructions
measurements are taken within a few seconds).
Diode - sector for testing diodes.
Ohm - resistance measurement sector.

DCV sector
On this instrument The sector is divided into 5 ranges. Measurements are taken from 0 to 500 volts. We will meet a large DC voltage only when repairing a TV. With this device, high voltages you have to work very carefully.
When turned on to the "500" volt position, the HV warning lights up on the screen in the upper left corner. that the highest level of measurement is turned on and when large values appear, you need to be extremely careful.

Typically, voltage measurement is carried out by switching large positions of the range to smaller ones if you do not know the value of the measured voltage. For example, before measuring the voltage on battery cell phone or car, on which the maximum voltage of 3 or 12 volts is written, then we boldly set the sector to the “20” volt position. If we put it on a smaller one, for example, on "2000" millivolts, the device may fail. If we put it on a large one, the readings of the device will be less accurate.
When you do not know the value of the measured voltage (of course, within the framework of household electrical equipment, where it does not exceed the values of the device), then set the "500" volts to the upper position and take a measurement. In general, roughly measuring, with an accuracy of one volt, is possible at the "500" volt position.
If greater accuracy is required, switch to the lower position only so that the measured voltage does not exceed the value at the switch position of the device. This device is convenient in measuring DC voltage in that it does not require obligatory observance of polarity. If the polarity of the probes ("+" - red, "-" - black) does not match the polarity of the measured voltage / th, the "-" sign will appear on the left side of the screen, and the value will correspond to the measured one.

ACV sector
The sector has 2 positions on this type of device - "500" and "200" volts.
Handle 220-380 volt measurements with great care.
The procedure for measuring and setting positions is similar to the DCV sector.
DCA sector.
It is a DC milliammeter and is used to measure small currents, mainly in radio electronic circuits Oh. We don't need it yet.
To avoid damage to the device, do not put the switch on this sector, if you forget and start measuring the voltage, the device will fail.

Diode sector.
One position for testing diodes for breakdown (for a small
resistance) and open (infinite resistance). The principles of measurement are based on the operation of the ohmmeter. Same as hFE.
hFE sector
To measure transistors, there is a socket indicating in which socket which leg of the transistor should be placed. The transistors of both n - r - n and r - n -r conductivities are checked for breakdown, open circuit and for a greater deviation from the standard junction resistances.

Digital multimeter M832. Wiring diagram, description, characteristics

It's impossible to imagine a repairman's desktop without a handy inexpensive digital multimeter. This article discusses the design of the 830 series digital multimeters, the most common malfunctions and how to resolve them.

Currently, a huge variety of digital measuring instruments of varying degrees of complexity, reliability and quality is being 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 building inexpensive portable measuring instruments was the ICL71O6 chip converter manufactured by MAXIM. As a result, several successful inexpensive models of digital multimeters of the 830th series were developed, such as M830B, M830, M832, M838. Instead of the letter M, DT can stand. Currently, this series of devices is the most widespread and most repeated in the world. Its basic features: measurement of direct and alternating voltages up to 1000 V (input resistance 1 MΩ), measurement of direct currents up to 10 A, measurement of resistances up to 2 MΩ, testing of 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).

Scheme and operation of the device

Rice. 1. Structural diagram of ADC 7106

The basis of the multimeter is ADC IC1 type 7106 (the closest domestic analogue is the 572PV5 microcircuit). His structural scheme shown in fig. 1, and the pinout for execution in the DIP-40 package - in fig. 2. The 7106 kernel may have different prefixes depending on the manufacturer: ICL7106, TC7106, etc. Recently, packageless microcircuits (DIE chips) have been increasingly used, the crystal of which is soldered directly to the printed circuit board.

Rice. 2. ADC 7106 pinout in DIP-40 package

Consider the circuit of the M832 multimeter from Mastech (Fig. 3). Pin 1 of IC1 has a positive 9V battery supply voltage, pin 26 is negative. Inside the ADC there is a stabilized voltage source of 3 V, its input is connected to pin 1 of IC1, and the output is connected to pin 32. Pin 32 is connected to the common terminal of the multimeter and is galvanically connected to the COM input of the device.

The voltage difference between terminals 1 and 32 is approximately 3 V in a wide range of supply voltages - from nominal to 6.5 V. This stabilized voltage is supplied to the adjustable divider R11, VR1, R13, and its output is to the input of microcircuit 36 (in measurement mode currents and voltages).

The divider sets the potential U eg at pin 36, equal to 100 mV. Resistors R12, R25 and R26 perform protective functions. Transistor Q102 and resistors R109, R110nR111 are responsible for low battery indication. Capacitors C7, C8 and resistors R19, R20 are responsible for displaying the decimal points of the display.

Rice. 3. circuit diagram multimeter M832

The operating input voltage range Umax directly depends on the level of the adjustable reference voltage at terminals 36 and 35 and is:

The stability and accuracy of the display reading depends on the stability of this voltage reference. The display reading N depends on the input voltage UBX and is expressed as a number:

Consider the operation of the device in the main modes.

Voltage measurement

A simplified diagram of the multimeter in voltage measurement mode is shown in fig. 4. When measuring constant voltage the input signal is fed to R1 ... R6, from the output of which, through the switch (according to the scheme 1-8 / 1 ... 1-8 / 2), it is fed to the protective resistor R17. This resistor, in addition, when measuring AC voltage, together with the capacitor C3, forms a low-pass filter. Next, the signal is fed to the direct input of the ADC chip, pin 31. The potential of the common output generated by a stabilized voltage source of 3 V, pin 32 is applied to the inverse input of the microcircuit.

Rice. 4. Simplified diagram of a multimeter in voltage measurement mode

When measuring AC voltage, it is rectified by a half-wave rectifier on diode D1. Resistors R1 and R2 are selected in such a way that when measuring a sinusoidal voltage, the device shows the correct value. ADC protection is provided by R1...R6 divider and R17 resistor.

Current measurement

Rice. 5. Simplified diagram of a multimeter in current measurement mode

Resistance measurement

Rice. 6. Simplified diagram of a multimeter in resistance measurement mode

A simplified diagram 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 +LJ flows through the reference resistor Ron and the measured resistor Rx (the currents of inputs 35, 36, 30 and 31 are negligible) and the ratio of UBX and Uon is equal to the ratio of the resistances of resistors Rx and Ron. R1 .... R6 are used as reference resistors, R10 and R103 are used as current-setting resistors. Protection of the ADC is provided by the thermistor R18 [some cheap models use conventional resistors with a nominal value of 1 ... 2 kOhm), transistor Q1 in zener diode mode (not always installed) and resistors R35, R16 and R17 at inputs 36, 35 and 31 of the ADC.

Call mode

The continuity circuit uses an IC2 chip (LM358) containing two operational amplifiers. A sound generator is assembled on one amplifier, 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 key on transistor Q101, as a result of which an audible signal sounds. The threshold is determined by the divider R103, R104. Protection is provided by resistor R106 at the input of the comparator.

Multimeter Defects

All malfunctions can be divided into factory defects (and this happens) and damage caused by erroneous actions of the operator.

Since multimeters use dense mounting, element short circuits, poor soldering and breakage of element leads, especially those located along the edges of the board, are possible. Repair of a faulty device should be started with visual inspection printed circuit board. The most common factory defects of M832 multimeters are shown in the table.

Factory defects of M832 multimeters

Manifestation of a defect	Possible reason	Troubleshooting
When the device is turned on, the display lights up and then fades out.	Malfunction of the master oscillator of the ADC chip, the signal from which is fed to the LCD substrate	Check elements C1 and R15
When the device is turned on, the display lights up and then fades out. When the back cover is removed, the device works normally.	When the back cover of the device is closed, the contact helical spring rests on the resistor R15 and closes the master oscillator circuit	Bend or slightly shorten the spring
When you turn on the device in the voltage measurement mode, the display changes from 0 to 1	Integrator circuits are faulty or poorly soldered: capacitors C4, C5 and C2 and resistor R14	Solder or replace C2, C4, C5, R14
The device takes a long time to zero	Poor quality of the capacitor C3 at the input of the ADC (pin 31)	Replace SZ with a condenser with a low absorption coefficient
When measuring resistance, the display readings are set for a long time	Poor quality of capacitor C5 (zero auto-correction circuit)	Replace C5 with a low absorption condenser
The device does not work properly in all modes, the IC1 chip overheats.	The long terminals of the connector for testing transistors are shorted together	Open connector pins
When measuring AC voltage, the readings of the device "float", for example, instead of 220 V, they change from 200 V to 240 V	Loss of capacitance of the capacitor C3. Possible poor soldering of its findings or simply the absence of this capacitor	Replace SZ with a serviceable capacitor with a low absorption coefficient
When turned on, the multimeter either constantly beeps, or vice versa, is silent in the connection dialing mode	Poor soldering of the U2 microcircuit pins	Solder the pins of IC2
Display segments disappear and reappear	Poor contact of the LCD display and the contacts of the multimeter board through conductive rubber inserts	To restore reliable contact you need: fix conductive rubber bands; wipe the corresponding contact pads on the printed circuit board with alcohol; irrigate these contacts on the board

The serviceability of the LCD display can be checked using an alternating voltage source with a frequency of 50 ... 60 Hz and an amplitude of several volts. As such an AC voltage source, 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 M832 multimeter probe to the common output of the indicator (bottom row, left output), and apply the other multimeter probe alternately to the rest of the display outputs. If you can get the ignition of all segments of the display, then it is working.

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 operability: the stabilization voltage is 3 V and the absence of a breakdown between the power outputs and the common output of the ADC.

In resistance measurement mode, faults typically occur in the 200 ohm and 2000 ohm ranges. In this case, when voltage is applied to the input, resistors R5, R6, R10, R18, transistor Q1 can burn out and capacitor Sat breaks through. If transistor Q1 is completely broken, then when measuring resistance, the device will show zeros. With an incomplete breakdown of the transistor, the multimeter with open probes will show the resistance of this transistor. In the voltage and current measurement modes, the transistor is short-circuited by the switch and does not affect the multimeter readings. In the event of 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 if a large number of circuit elements are visually burnt out, there is a high probability of damage to the ADC. The serviceability of the ADC is checked by monitoring the voltage of a stabilized voltage source of 3 V. 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 frameless ADC compound, 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 along the elements (resistors) and along the printed circuit board; in the case of the voltage measurement mode, the circuit is protected by a divider on resistances R1 ... R6.

For cheap models of the DT series, long leads of parts can be shorted to the screen located on the back of the device, disrupting the operation of the circuit. Mastech does not have such defects.

The DT models use low quality ADCs and are very sensitive to the C4 and R14 integrator circuit values. In Mastech multimeters, high-quality ADCs make it possible to use elements of close ratings.

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. You can "cure" a low-quality ADC chip 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 "fills up" 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 microfarads.

Since cheap Chinese firms use low-quality frameless ADCs, there are often cases of broken outputs, while it is very difficult to determine the cause of the malfunction and it can manifest itself in different ways, depending on the broken output. For example, one of the indicator outputs is not lit. Since multimeters use displays with static indication, in order to determine the cause of the malfunction, it is necessary to check the voltage at the corresponding output of the ADC chip, it should be about 0.5 V relative to the common output. If it is zero, then the ADC is faulty.

An effective way to find the cause of a malfunction is to check the outputs of the analog-to-digital converter chip as follows. Another, of course, serviceable, digital multimeter is used. It enters the diode test mode. The black probe, as usual, is installed in the COM jack, and the red one in the VQmA jack. The red probe of the device is connected to pin 26 [minus power), and the black one touches each leg of the ADC chip in turn. Since protective diodes in reverse connection are installed at the inputs of the analog-to-digital converter, with this connection they should open, which will be reflected on the display as a voltage drop across the open diode. The actual value of this voltage on the display will be slightly higher, because. resistors are included in the circuit. In the same way, all the ADC outputs are checked when the black probe is connected to pin 1 [to the ADC power plus) and alternately touching the remaining outputs of the microcircuit. The instrument readings should be similar. But if you change the polarity of the inclusion during these checks to the opposite, then the device should always show an open circuit, because. the input impedance of a good chip is very high. Thus, outputs that show finite resistance for any polarity of connection to the microcircuit can be considered faulty. If the device shows a break with any connection of the output under study, then this ninety percent indicates an internal break. Specified method verification is quite versatile and can be used when testing various digital and analog microcircuits.

There are malfunctions associated with poor-quality contacts on the biscuit switch, the device only works when the biscuit is pressed. Companies that produce cheap multimeters rarely cover the tracks under the biscuit switch with grease, which is why they quickly oxidize. Often the paths are dirty with something. 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 devices of the DT series, it sometimes happens that the alternating voltage is measured with a minus sign. This indicates that D1 has been installed incorrectly, usually due to incorrect markings on the body of the diode.

In most of the devices produced recently, unpackaged (DIE chips) ADCs are used. The crystal is mounted directly on the printed circuit board and filled with resin. Unfortunately, this significantly reduces the maintainability of devices, because. when the ADC fails, which occurs quite often, it is difficult to replace it. Devices with unpackaged ADCs are sometimes sensitive to bright light. For example, when working 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 the light, penetrating through them, falls on the ADC crystal, causing a photoelectric effect. To eliminate this shortcoming, you need to remove the board and, having removed the indicator, glue the location of the ADC crystal (it can be clearly seen through the board) with thick paper.

When buying DT multimeters, you should pay attention to the quality of the mechanics of the switch, you should definitely turn the multimeter's flip switch several times to make sure that the switching occurs clearly and without jamming: plastic defects cannot be repaired.

Publication: www.cxem.net

See other articles section.

A multimeter is one of the inexpensive measuring instruments that is used by both professionals and amateurs repairing home wiring and electrical appliances. Without it, any electrician feels like without hands. Previously, three different instruments were required to measure voltage, current, resistance. Now all this can be measured using one universal device. Using a digital multimeter is very easy.

There are two main rules to remember:

⚡ where to connect the measuring probes
⚡ in what position to set the switch for measuring different values

Multimeter appearance and connectors

On the front of the tester, all the inscriptions are made on English language, and even with the use of abbreviations.

What do these inscriptions mean:

OFF - the device is turned off (so that the batteries of the device do not run out, set the switch to this position after measurements)
ACV - variable U measurement
DCV - constant U measurement
DCA - DC current measurement
Ω - resistance measurement
hFE - measurement of transistor characteristics
diode icon - continuity or diode test

Switching modes occurs using the central rotary switch. At the very beginning of using the DMM, it is recommended that you immediately mark the pointer mark on the switch with contrasting paint. For example like this:

Most of the failures of the device are just due to the wrong choice of the switch position.

Power is supplied from a battery type krone. By the way, by the connector for connecting the crown, you can indirectly judge whether the tester was assembled in the factory or somewhere in Chinese "cooperatives". With a high-quality assembly, the connection occurs through special connectors designed for the crown. In lower quality versions, conventional springs are used.

The multimeter has several connectors for connecting probes and only two probes. Therefore, it is important to correctly connect the probes to measure certain quantities, otherwise you can easily burn the device.

The probes are usually of different colors - red and black. The black probe is connected to the connector labeled COM (translated as “common”). Red probe into the other two connectors. The 10ADC connector is used when it is necessary to measure the current strength from 200mA to 10A. The VΩmA connector is used for all other measurements - voltage, current up to 200mA, resistance, continuity.

The main criticism is caused precisely by the factory probes that come with the device. Almost every second multimeter owner recommends replacing them with better ones. True, at the same time, their cost can be comparable to the cost of the tester itself. In the extreme case, they can be improved by reinforcing the bends in the wires and insulating the probe tips.

If you want high-quality silicone probes with a bunch of tips, then you can order them with free shipping on AliExpress.

Previously, pointer testers were also widely used. Some electricians even prefer them, considering them more reliable. However, it is less convenient for ordinary consumers to use them because of the large error in the measurement scale. In addition, when working with a dial multimeter, it is imperative to guess the polarity of the contacts. For digital correct connection to the poles, the reading will simply be displayed with a minus sign. This is the normal mode of operation, which will not damage the multimeter.