Block diagram of is 7106 drop description. Analog-to-digital converters VT7106 and VT7107

It is impossible to imagine a repairman's workbench without a convenient, inexpensive digital multimeter. This article discusses the design of digital multimeters of the 830 series, the most common faults and methods for eliminating them.

There is currently 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 a converter based on the ICL71O6 chip, produced by MAXIM. As a result, several successful low-cost models of digital multimeters of the 830 series were developed, such as M830B, M830, M832, M838. Instead of the letter M there may be DT. Currently, this series of devices is the most widespread and most repeated in the world. Its basic capabilities: measuring direct and alternating voltages up to 1000 V (input resistance 1 MOhm), measuring direct currents up to 10 A, measuring resistances up to 2 MOhm, testing diodes and transistors. In addition, some models have a mode for audibly testing connections, measuring temperature with and without a thermocouple, and generating a meander with a frequency of 50...60 Hz or 1 kHz. The main manufacturer of multimeters in this series is Precision Mastech Enterprises (Hong Kong).

Scheme and operation of the device

Rice. 1. Block diagram of ADC 7106

The basis of the multimeter is ADC IC1 type 7106 (the closest domestic analogue- microcircuit 572ПВ5). Its block diagram is shown in Fig. 1, and the pinout for execution in the DIP-40 housing is shown in Fig. 2. The 7106 core may have different prefixes depending on the manufacturer: ICL7106, TC7106, etc. IN Lately Increasingly, packageless chips (DIE chips) are being used, the crystal of which is soldered directly onto printed circuit board.

Rice. 2. Pinout of ADC 7106 in DIP-40 package

Let's consider the circuit of the company's M832 multimeter (Fig. 3). Pin 1 of IC1 is supplied with a positive 9 V battery supply voltage, and pin 26 is supplied with a negative voltage. Inside the ADC there is a source of stabilized voltage 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 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 supplied to the adjustable divider R11, VR1, R13, and its output. -at the entrance microcircuits 36 (in current and voltage measurement mode). 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 indicating low battery power. Capacitors C7, C8 and resistors R19, R20 are responsible for displaying the decimal points of the display.

Rice. 3. Schematic diagram multimeter M832

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

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

Let's consider the operation of the device in the main modes.

Voltage measurement

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

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

When measuring alternating voltage, it is rectified by a half-wave rectifier using 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 divider R1...R6 and resistor R17.

Current measurement

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

A simplified circuit of a multimeter in current measurement mode is shown in Fig. 5. In measuring mode direct current the latter flows through resistors RO, R8, R7 and R6, switched depending on the measurement range. The voltage drop across these resistors is fed through R17 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

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

A simplified diagram of a multimeter in resistance measurement mode is shown in Fig. 6. In the resistance measurement mode, the dependence expressed by 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 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.

Dialing mode

The dialing circuit uses IC2 (LM358), which contains two operational amplifiers. An audio 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 transistor Q101, as a result of which the sound signal. The threshold is determined by the divider R103, R104. Protection is provided by resistor R106 at the comparator input.

Defects of multimeters

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

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

Factory defects of M832 multimeters

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 alternating voltage source, you can take the M832 multimeter, which has a meander generation mode. To check the display, place it on a flat surface with the display facing 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 remaining terminals of the display. If you can get all segments of the display to light up, it means it is working.

The malfunctions described above 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 functionality of the ADC: stabilization voltage of 3 V and the absence of breakdown between the power pins and the common terminal of the ADC.

In 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 be visually visible on the resistances. In the first case, when only the diode breaks down, the defect appears only in the current measurement mode: current flows through the device, but the display shows zeros. If resistors R5 or R6 burn out in voltage measurement mode, the device will overestimate the readings or show an overload. If one or both resistors burn completely, the device does not reset to zero in voltage measurement mode, but when the inputs are shorted, the display resets to zero. If resistors R7 or R8 burn out, the device will show an overload in the current measurement ranges of 20 mA and 200 mA, and only zeros in the 10 A range.

In resistance measurement mode, damage typically occurs 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 Sb can break through. If transistor Q1 is completely broken, then when measuring resistance the device will show zeros. If the breakdown of the transistor is incomplete, a multimeter with open probes will show the resistance of this transistor. In voltage and current measurement modes, the transistor is short-circuited with a switch and does not affect the multimeter readings. If capacitor C6 breaks down, the multimeter will not measure voltage in the ranges of 20 V, 200 V and 1000 V or significantly underestimate readings in these ranges.

If there is no indication on the display when there is power to the ADC or visually noticeable burnout of a large number of circuit elements, 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 applied to the input high voltage, much higher than 220 V. Very often, cracks appear in the compound of a packageless ADC, and 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 voltage measurement mode, a breakdown may occur in the elements (resistors) and on the printed circuit board; in the case of voltage measurement mode, the circuit is protected by a divider across resistances R1 ... R6.

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

Source of stabilized voltage 3 V in ADCs for cheap Chinese models can in practice produce a voltage of 2.6...3.4 V, and for some devices it stops working even when the supply battery voltage is 8.5 V.

DT models use low quality ADCs and are very sensitive to the values ​​of the integrator chain C4 and R14. In Mastech multimeters, high-quality ADCs allow the use of elements of similar values.

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

When measuring resistances in the upper part of the range, the device “overwhelms” the readings, for example, when measuring a resistor with a resistance of 19.8 kOhm, it shows 19.3 kOhm. It is “cured” by replacing capacitor C4 with a capacitor of 0.22...0.27 µF.

Since cheap Chinese companies use low-quality unpackaged ADCs, there are frequent cases of broken pins, while 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 pins does not light up. Since multimeters use displays with static indication, to determine the cause of the malfunction it is necessary to check the voltage at the corresponding pin of the ADC chip; it should be about 0.5 V relative to the common pin. If it is zero, then the ADC is faulty.

Effective way Finding the cause of the malfunction is to check the pins of the analog-to-digital converter microcircuit as follows. Another, of course, working, digital multimeter is used. It goes into diode test mode. The black probe, as usual, is installed in the COM socket, and the red one in the VQmA socket. 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 are installed at the inputs of the analog-to-digital converter in reverse connection, 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. All ADC pins are checked in the same way by connecting the black probe to pin 1 [plus the ADC power supply] and alternately touching the remaining pins of the microcircuit. The device readings should be similar. But if you change the switching polarity during these tests to the opposite one, then the device should always show a break, because The input resistance of a working microcircuit is very high. Thus, pins that show finite resistance at any polarity of connection to the microcircuit can be considered faulty. If the device shows a break with any connection of the terminal under test, then this is ninety percent an indication of an internal break. This testing method is quite universal 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 switch is pressed. Companies that produce cheap multimeters rarely coat the tracks under the switch with lubricant, 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 Vaseline is applied. That's it, the device is fixed.

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

It happens that manufacturers of cheap multimeters put low-quality operational amplifiers in the circuit sound generator, and then when the device is turned on, a buzzer sounds. This defect is eliminated by soldering an electrolytic capacitor with a nominal value of 5 μF in parallel with the power circuit. If this does not ensure stable operation of the sound generator, then it is necessary to replace operational amplifier on 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 tweeter, using a brush, such as a toothbrush, you need to thoroughly soap the board on both sides and rinse it under running tap water. After repeating the wash 2...3 times, the board is dried and installed in the case.

Most devices produced recently use DIE chips ADCs. The crystal is installed directly on the printed circuit board and filled with resin. Unfortunately, this significantly reduces the maintainability of the devices, because... When an ADC fails, which happens quite often, it is difficult to replace it. Devices with bulk 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 device board have some transparency, and light, penetrating through them, hits the ADC crystal, causing a photoelectric effect. To eliminate this drawback, you need to remove the board and, having removed the indicator, cover the location of the ADC crystal (it is clearly visible through the board) with thick paper.

When purchasing DT multimeters, you should pay attention to the quality of the switch mechanics; be sure to rotate the multimeter switch several times to make sure that the switching occurs clearly and without jamming: plastic defects cannot be repaired.

This digital voltmeter is ideal for use in DC power supply. It includes a 3.5-digit common-cathode LED display. It measures DC voltage from 0 to 199.9V with 0.1V resolution. The voltmeter is based on a single ICL7107 chip and can be mounted on a small 3cm x 7cm PCB. The circuit must be supplied with a 5V power supply and consumes a current of only about 25mA.

The brightness of the LED segments of the display can be changed by adding or removing the number of 1N4148 diodes that are connected in series.

The voltmeter can also be configured to measure voltage for different ranges. Replacing the 1M resistor to 100K will allow you to measure voltage 0 - 19.99V \ 0.01V (10mV) - accuracy.

Calibration
Adjust the 10K potentiometer to set the reference voltage between pins 35 and 36 of the ICL7107 chip, the voltage between these pins should be equal to - 1V.

It is possible to use other indicators.
Source - http://electronics-diy.com/ICL7107_volt_meter.php

This microcircuit is widely used in measuring technology. Almost all multimeters (made in the 90s and 2000s) used it as a “brain”. It was ordered to restore almost lost devices. I will be repairing the well-known (or almost everyone) MASTECH M890F device. This review is exclusively for those who are familiar with soldering irons.
I ordered these chips in mid-August. It's been a little over a month.


Unfortunately, this product is this moment not available. I bought it spontaneously. Price played a decisive role. At one time, our company ordered these MS from a well-known Moscow company. The price has changed slightly in accordance with the dollar exchange rate.


The price is about 33 rubles per piece on Ali - it’s almost nothing. But that's not the point. I'll tell you why I took it and what I did.
First, let’s look at how it was packaged and in what form everything arrived. This information is sometimes important.


A standard paper bag, “pimpled” on the inside.


The microcircuits with their legs were inserted into foamed polyethylene (I tried to explain as best I could), so none of them were damaged.


These microcircuits are found in one of the most popular multimeters from MASTECH M890F. But not only in them. They are also used in other devices of this company (and not only). The most common: M830, M832, M838.
The basis of this device (M890F), like most inexpensive multimeters, is the ICL706 analog-to-digital converter, which operates on the principle of double integration. This is a complete analogue of the well-known domestic IC K572PV5. It can also be used as a repair kit. But it's more expensive.
The main operating errors leading to malfunction of the device are taking measurements with input overload and choosing the wrong measurement mode as a result of inattention or haste. This leads to breakdown of the ADC, burnout of tracks, and failure of other microcircuits. No less dangerous is switching limits and measurement modes without disconnecting from the circuit being measured. In this case, the conductive tracks of the switch often burn out. As a result, the device can no longer be repaired. This is a disadvantage of all devices with this type of switches.
I don’t know what exactly caused the damage to this multimeter.


The tracks at the limits: 20 kOhm, 200 kOhm and 200 mV evaporated. Theoretically, they can be restored. But this is already the art of appliqué. In the meantime, I’ll test my strength in the art of repair :)
I have several of them (multimeters). I personally haven’t burned a single one yet. I collected faulty ones from friends. About ten years ago, repairs were impractical due to the cost of microcircuits (already wrote). And such devices can only be restored taking into account their future disability. Some functions will be lost forever, even after restoration. The tracks cannot be glued back. :(
This is the most common multimeter.

His appearance is certainly shabby. But he has a lot of years.
With frequent disassembly, one or more cable wires come off, well, very tough.


There are only two options: either not to climb, or to resolder.

As you can see, I re-soldered. The procedure is tedious.


In addition to the processor, the printed circuit conductors of this device also burned out. I restored them. Several exemplary resistances burned down. They must be selected very precisely. The error of the entire device depends on them. These resistance markings have one more strip.
There are also such instances.


This is a slightly different device, although from the same company. But it's good as an example. It is clearly visible that the board burned out in resistance measurement mode. This is where you have to put it in order for such a hole to form in the board!
I understood that. But not everyone knows that network voltage is measured in Volts, not Ohms :)
It is also possible to restore, but some measurement limits will have to be sacrificed. But that will be another story...
And this is M832, which can no longer be restored.


In such multimeters, you must first remove the “blot”, then solder the microcircuit to the printed contacts. They are kindly provided.
I'll return to the M890.
First of all, when the board burns out and the printed conductors burn out, the IC1 processor, the IC8 7555 integrated timer and two LM358 capacitance meter MCs turn out to be faulty. Faulty MSs often drain the supply voltage. IC8 7555 is located on the top board.
The current consumption of a working multimeter is about 4mA. Specifically, the processor consumes a little less than 2mA. And nothing else. This needs to be remembered. Increased current consumption indicates some kind of malfunction.
I am attaching an edited diagram of the multimeter. It is very convenient to repair and calibrate the device. The diagram was originally downloaded from the Internet and edited over several years. There may be shortcomings in the scheme. Perhaps I didn’t have time to correct everything.

IC8 7555 can simply be removed from the circuit, which is what I did. The multimeter will not be able to measure frequency. For me this is not critical.
There is also a diagram on the Internet with a later modification of this device.

This is (one might say) a completely different device. In my opinion, more miserable. There are simplifications in the diagram.
All elements of the circuit are collected on one board. It is very difficult to distinguish purely externally (without opening it), except that it is lighter in weight. And it was sold several years later and cheaper.
I'll move directly to the repair.
To determine what has burned out, you need to remove the top board. To do this, you need to unscrew four small screws and remember how the slats are located on the switch. They have a tendency to jump off at the most inopportune moment. It’s best to take them off right away so you don’t have to look for them on the floor later.

The device works well without the top board. You only need to bridge pins 2 and 6 of the connector (I marked them in the figure). 9V power passes through them. In this case, the points and measured values ​​on the display will disappear. During repairs this is not very important.
The protection transistor Q4 (9014) almost always burns out.

I've already soldered it. The multimeter can work without it. But it's better to replace it. No matter what, but still protection.
Now you need to measure the voltage between pins 1 and 32 of the processor. In this case, the switch of the REPAIRED multimeter must be in any mode except resistance measurement.


It should be approximately within the specified limits (2.8-3.0V). If the values ​​are exceeded (usually more than 6V), there is a 99% probability that the processor is dead.
The percentage itself is located on the other side of the board under the indicator. To get to it, you need to unscrew four screws and remove the module with the indicator.
These are the microcircuits found in MASTECH M890F multimeters. “Blots” were more common.


In both cases, the faulty microcircuit is soldered off. Instead, a regular MS from China is installed. Which I have successfully done.


You can also solder our analog KR572PV5. At one time it was soldered into another faulty device. It's been working for ten years now.

It’s just that the distance between the legs is slightly different. You'll have to bend it a little.
After the procedures were completed, the multimeter came to life. I measured the voltage on the battery.


Almost true. All that remains is to set up the multimeter using standard devices. But not everyone has them. Alternatively, you can adjust the readings by comparison with another device in which you have confidence.
You need to start with calibration constant voltages(VR1). And only then variables (VR2). The sequence of other adjustments does not affect the “speed” :)
The accuracy of resistance measurements is determined by the accuracy of the reference resistances inside the device and is not regulated by any potentiometers.
That's all.
And one more thing at the end.
I tried to talk about the use of ICL706 microcircuits as a repair kit. It is impossible to describe all the malfunctions in multimeters that require their replacement. If anything is unclear about microcircuits, ask questions. For advice on repairs, please contact us in PM.
I hope it helped at least someone.
Good luck everyone!