The MASTECH MS8209 multimeter has been lying around for a long time. I received it in a non-working condition. I don't know his background. I decided to restore it. It seems that the parameters and capabilities are not bad.
The multimeter does not turn on. Those. When you turn it on in any mode, there is silence on the display, the consumption only jumps from 0 to about 200 µA. But if you press on the board (it seems that it is not pressure that plays a role, but the resistance of your fingers) and twist the limit switch, you can turn on the multimeter and it even measures something, while consuming about 20 mA. But the numbers seem to be incorrect; something seems to be around minus two thousand. Although the numbers change. The image seems to be faded, and the contrast floats. It responds to buttons and switches modes. The backlight does not work. When you press the backlight button, the current consumption increases slightly and that’s it.
An external examination of the board under a microscope did not reveal anything suspicious.
I'm sinning with the power on/off circuit. Maybe someone has a diagram of this multimeter or knows where I can look at it? Or, at least, what kind of ADC is used there? 
Like any other item, a multimeter can fail during operation or have an initial, factory defect that went undetected during production. In order to find out how to repair a multimeter, you should first understand the nature of the damage.
Experts advise starting the search for the cause of the malfunction with a thorough inspection of the printed circuit board, since short circuits and poor soldering are possible, as well as defective pins of elements at the edges of the board.
Manufacturing defects in these devices appear mainly on the display. There can be up to ten species (see table). Therefore, it is better to repair digital multimeters using the instructions that come with the device.
| Defect | Cause | Solution |
| When the device starts up, the screen lights up and slowly goes out. | Indicates a breakdown of the master oscillator, from where the signal is supplied to the screen substrate | It is necessary to check two elements: C1 and R15 |
| During startup, the screen lights up and slowly goes out, but without the back cover the problem does not occur | When the lid is closed, the contact coil spring presses on resistor R15 and closes the master oscillator circuit | You can bend or shorten the spring itself |
| The screen indicators change from 0 to 1 when the device is turned on in voltage measurement mode | The reason may be poorly soldered, defective capacitors: C4, C5 and C2 and resistor R14 | You need to solder them or install new ones |
| The device takes too long to reset to zero | Reason: poor quality capacitor SZ at the input | |
| The readings on the screen take a long time to set when changing resistances | This happens due to a poor-quality capacitor C5 | It is worth replacing it with another one with a lower absorption coefficient |
| The device does not work correctly when each mode is turned on. IC1 is overheating | This occurs due to the shorting of the long pins of the transistor test connector | You just need to open the leads |
| The readings jump when the voltage changes: instead of 220 volts they show from 200 to 240 volts | The reason is the loss of capacitance of the SZ capacitor due to its poor soldering, soldering of leads, or the absence of the capacitor itself | It is necessary to replace a working capacitor with a low absorption coefficient |
| When turned on, the device either beeps or is silent during dialing. | Occurs due to poor-quality soldering of the pins of the IC2 chip | To solve, you need to solder the pins |
| Disappearing segments on the screen | Poor contact of the screen with the board contacts through conductive inserts | It is necessary to correct the conductive rubber inserts, clean the contacts with alcohol and tin the contacts on the board |
The same breakdowns can occur after use. The malfunctions described above may also appear during operation. However, if the device operates in constant voltage measurement mode, it rarely breaks down.
The reason for this is its overload protection. Also, repairing 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 pin of the ADC.
Experienced users and professionals have repeatedly stated that one of the most likely causes of frequent breakdowns in the device is poor quality manufacturing. Namely, soldering contacts using acid. As a result, the contacts simply oxidize.
However, if you are not sure what kind of breakdown caused the device to not work, you should still contact a specialist for advice or help.
Multifunctional calibrators are an integral part of the toolkit of specialists employed in production and research laboratories, repair shops and service centers. These are compact reference devices used for checking and adjusting various measuring instruments. They are used both in laboratory and field conditions. Among the most popular and well-known devices of this kind is equipment manufactured by Fluke. It has proven itself to be extremely reliable and safe to use.
However, like any equipment, Fluke universal calibrators may break, malfunction and exhibit operational inaccuracies. In particular, their individual electronic components are damaged and the main elements become unusable. As a result, the device begins to act up, it is still unable to quickly and clearly solve the tasks assigned to it, and in rare cases it stops working altogether. The causes of breakdowns can be very different - from improper handling, banal exhaustion of parts, to mechanical and other influences.
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.
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 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 the ADC IC1 type 7106 (the closest domestic analogue is the 572PV5 microcircuit). 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. Recently, DIE chips have been increasingly used, the crystal of which is soldered directly onto the 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 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 fed 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 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 of the M832 multimeter
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 the DC current measurement mode, 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, resulting in a 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 a visual inspection of the printed circuit board. The most common factory defects of M832 multimeters are shown in the table.
Factory defects of M832 multimeters
| Defect manifestation | Possible reason | Troubleshooting |
|---|---|---|
| When you turn on the device, the display lights up and then goes out smoothly | Malfunction of the master oscillator of the ADC chip, the signal from which is supplied to the LCD display substrate | Check elements C1 and R15 |
| When you turn on the device, the display lights up and then goes out smoothly. The device works normally when the back cover is removed. | When the back cover of the device is closed, the contact helical spring rests on resistor R15 and closes the master oscillator circuit | Bend or shorten the spring slightly |
| When the device is turned on in voltage measurement mode, the display readings change from 0 to 1 | The 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 reset the readings to zero | Low quality capacitor SZ at the ADC input (pin 31) | Replace the SZ with a capacitor with a low absorption coefficient |
| When measuring resistances, the display readings take a long time to settle | Poor quality of capacitor C5 (automatic zero correction circuit) | Replace C5 with a capacitor with a low absorption coefficient |
| The device does not work correctly in all modes, the IC1 chip overheats. | The long pins of the connector for testing transistors are shorted together | Open the connector pins |
| When measuring alternating voltage, the instrument readings “float”, for example, instead of 220 V they change from 200 V to 240 V | Loss of capacitance of the capacitor SZ. Possible bad soldering of its terminals or simply the absence of this capacitor | Replace the SZ with a working capacitor with a low absorption coefficient |
| When turned on, the multimeter either constantly beeps, or, conversely, remains silent in connection testing mode | Poor soldering of IC2 pins | Solder the pins of IC2 |
| Segments on the display disappear and appear | Poor contact of the LCD display and the contacts of the multimeter board through the conductive rubber inserts | To restore reliable contact you need: adjust the conductive rubber bands; wipe the corresponding contact pads on the printed circuit board with alcohol; tin the 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 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 where 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 a high voltage is applied to the input, much higher than 220 V. Very often, in this case, cracks appear in the compound of the unpackaged 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 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.
The stabilized voltage source of 3 V in the ADC of cheap Chinese models can in practice produce a voltage of 2.6...3.4 V, and for some devices it stops working even at a supply voltage of 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.
An effective way to find the cause of a malfunction is to test 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 alternating voltage is measured with a minus sign. This indicates that D1 has been installed incorrectly, usually due to incorrect markings on the diode body.
It happens that manufacturers of cheap multimeters install low-quality operational amplifiers in the sound generator circuit, and then when the device is turned on, a buzzer is heard. 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 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 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.
So, a couple of weeks ago I received several faulty laboratory DC power supplies: Mastech HY3005D-3
HY3003M-2
and HY3002D-3
.
Let me explain the markings: HY-series; the first two digits are the maximum voltage (30 V); the second are the maximum current (5.3 and 2, respectively). The letter indicates the type: M-push-button, D-twist handle.The last digit means the number of channels (3rd channel fixed: +5V, 3A).
So, although the symptoms varied slightly, the essence was the same for all - one channel does not work for one reason or another. One of them also had no current regulation on the other channel.
I started by opening BP 3005:
This is what the board itself looks like. Master and Slave are identical boards. The arrows show the terminals of the windings from the transformer.There are three trimming resistors on the board: Left and right are responsible for max. current and max. voltage respectively. The upper left one is responsible for the voltage at the terminals when the current regulator is set to zero (the voltage should be set within 1-5 V).
So, you need to act:
1) Check the fuse (they turn on for me, I missed this step).
2) Conduct a visual inspection of the boards, wires and everything else for scorch, etc. On one of the 3005 boards, the resistor became a marsh color (instead of blue) and one of the electrolytes swelled. After replacing the IP worked :)
3) Check the power elements (the 3003 has two of them per radiator, the 3002 has one at a time): unhook it from the board and connect it to the second one and vice versa. Practice has shown that in all cases the power elements were intact.
4) Check the windings of the transformer(s): in the case of 3002, the transformer turned out to be half broken and remains there... For the remaining 3003, nothing has changed.
As you can see, boards for power supplies with lower current have fewer elements accordingly. All differences come down to the number of 2N3055 power elements and resistors for them. The boards of all three power supplies are similar and differ only slightly in the connection to the power supply of the maximum current regulator.
Thus, it was determined that the only thing that could cause a problem in this case is the indicator and adjustment control board:
And here lies the pitfall... It turned out that the microcircuit had failed (there is one in the photo on the left, only a connector on the right). And everything would be fine, butit is worn out and it is impossible to find a suitable one. Most likely, this is some kind of Atmega or PIC MK, but it was not possible to read the firmware. As a result, out of three power supplies, two were made fully functional after moving the transformer. And the remaining power supply unit still stands and gathers dust to this day, because... without the mikruhi it's a bunch of junk. In the future I plan to convert the control system to a resistor one.
