Multimeters MASTECH. Typical malfunctions and their causes.
First of all, make sure the battery is working. Replace the battery if necessary.

Never leave the probe in socket “10A” after finishing measurements! A short circuit will burn the circuit board traces under the switch. This is not recoverable!

Malfunction	Probable Cause	Repair
the display at all limits shows random numbers much larger than zero	multimeter ADC is faulty	replace ADC
the device overestimates the readings	battery is low	replace the battery
temperature (M838, M890C+,G, MY62, 64) is measured only with a thermocouple	200mA fuse blown	replace fuse
Individual display segments do not light up	in older models of testers there were cases of poor clamping of the LCD display to conductive rubber	glue a strip of electrical tape to the glass of the LCD display (under the clamping frame)
M830 series: 1. when measuring voltage, the device overestimates the readings or goes off scale, and may not reset to zero	1. R6 burned out (100 Ohm ± 0.5%), most often; 2. R5 burned out (900 Ohm ± 0.5%), this happens less often. Visually, the resistors may appear intact.	replace. check C6 and Q for breakdown.
2. when measuring voltages at the upper limits, the readings are greatly underestimated	broken (leak) in C6 - 0.1mF	check by replacement
3. when measuring resistance (ranges 200Ohm, 2KOhm) slow counting, gradual decrease in readings	defect in C3 - 0.1mF	check by replacement
4. when measuring resistance (ranges 200Ohm, 2KOhm), count slowly, gradually increase readings	defect in C5 - 0.1mF	check by replacement
5. when measuring alternating voltages, the readings float (20 - 40 units)	Capacitance loss C3 - 0.1mF	check by replacement
6. When measuring resistance, the display shows zeros	transistor Q1 (9014), switched on by a diode, is broken	replace
7. glitches when measuring resistance, other modes work	transistor Q1 (9014), turned on by the diode, is faulty	check by replacement
9. The device takes a long time to set the readings	defect in C3 - 0.1mF	check by replacement
10. when measuring current it goes off scale	Resistors R7 (9 Ohm), R8 (1 Ohm) are faulty	check by replacement
11. For all measurements, “1” is displayed	ADC faulty, bad soldering or short circuits	in a working ADC, the voltage between pins 1 and 32 is 3V *)
M890 series: 1. does not reset at frequency, may lie in other modes	chip IC8 - 7555 is faulty	check by replacement
characteristic malfunctions of devices on the ADC 7106: 1. when measuring DC voltage, if you change the polarity of connecting the probes, the device readings differ from the original	1. The capacitor connected to pin 27 of the ADC is faulty. 2. The capacitor connected to pins 33 and 34 is faulty.	check by replacement
2. when the probes are short-circuited in the DC voltage measurement mode, the display readings differ from zero in several digits	The capacitor connected to pins 33 and 34 is faulty (high leakage current)	check

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Organizing and repairing a multimeter independently is within the capabilities of every user who is well acquainted with the basics of electronics and electrical engineering. But before you begin such repairs, you need to try to understand the nature of the damage that has occurred.

Visually detectable defects (manufacturing defects)

It is most convenient to check the serviceability of the device at the initial stage of repair by examining its electronic circuit. For this case, the following troubleshooting rules have been developed:

If the multimeter gives incorrect readings in all modes and the IC1 chip heats up, then you need to inspect the connectors to check the transistors. If the long leads are shorted, then the repair will simply consist of opening them.

In total, there can be a sufficient number of visually detectable faults. You can familiarize yourself with some of them in the table and then eliminate them yourself. (at: http://myfta.ru/articles/remont-multimetrov.) Before repairs, you need to study, which is usually given in the passport.

Checking the display

If they want to check the serviceability and repair the multimeter indicator, they usually resort to the help of an additional device that produces a signal of a suitable frequency and amplitude (50-60 Hz and units of volts). If it is not available, you can use a multimeter type M832 with the function of generating rectangular pulses (meander).

To diagnose and repair the multimeter display, you need to remove the working board from the device body and select a position convenient for checking the indicator contacts (screen up). After this, you should connect the end of one probe to the common terminal of the indicator under study (it is located in the bottom row, far left), and with the other end, alternately touch the signal terminals of the display. In this case, all its segments should light up one after another according to the wiring of the signal buses, which should be read separately. Normal “activation” of the tested segments in all modes indicates that the display is working properly.

Additional Information. This malfunction most often manifests itself during the operation of a digital multimeter, in which its measuring part fails and requires repair extremely rarely (provided that the requirements of the instructions are met).

The last remark concerns only constant quantities, when measuring which the multimeter is well protected against overloads. Serious difficulties in identifying the causes of device failure most often occur when determining the resistance of a circuit section and in testing mode.

Problems related to resistance testing

In this mode, characteristic faults, as a rule, appear in measuring ranges up to 200 and up to 2000 Ohms. When extraneous voltage comes into contact with the input, as a rule, resistors designated R5, R6, R10, R18, as well as transistor Q1, burn out. In addition, capacitor C6 often breaks through. The consequences of exposure to extraneous potential are manifested as follows:

Note! In other measurement modes, this transistor is short-circuited and therefore does not affect the display readings.

In the event of a breakdown of C6, the multimeter will not work at the measuring limits of 20, 200 and 1000 Volts (the possibility of a strong underestimation of readings is not excluded).

If the multimeter constantly beeps when dialing or is silent, then the cause may be poor-quality soldering of the pins of the IC2 microcircuit. Repair involves careful soldering.

Problems with the ADC

It is recommended to begin the inspection and repair of a non-working multimeter, the malfunction of which is not related to the cases already considered, by checking the 3 Volt voltage on the ADC supply bus. In this case, first of all, you need to make sure that there is no breakdown between the supply terminal and the common terminal of the converter.

The disappearance of indication elements on the display screen in the presence of a supply voltage to the converter most likely indicates damage to its circuit. The same conclusion can be drawn if a significant number of circuit elements located near the ADC burn out.

Important! In practice, this unit “burns out” only when a sufficiently high voltage (more than 220 Volts) is applied to its input, which manifests itself visually in the form of cracks in the module’s compound.

ADC testing

Before we talk about repairs, it is necessary to carry out an inspection. A simple way to test an ADC for suitability for further operation is to test its outputs using a known-good multimeter of the same class. Note that the case when the second multimeter incorrectly displays measurement results is not suitable for such a test.

When preparing for operation, the device is switched to the diode “testing” mode, and the measuring end of the wire in red insulation is connected to the “minus power” terminal of the microcircuit. Following this, each of its signal legs is successively touched with a black probe. Since the circuit inputs have protective diodes connected in the reverse direction, they should open after applying forward voltage from a third-party multimeter.

The fact of their opening is recorded on the display in the form of a voltage drop across the junction of the semiconductor element. The circuit is checked in the same way by connecting a probe in black insulation to pin 1 (+ ADC power supply) and then touching all other pins. In this case, the readings on the display screen should be the same as in the first case.

When changing the polarity of the connection of the second measuring device, its indicator always shows a break, since the input resistance of the working microcircuit is quite high. In this case, the terminals that in both cases show the final resistance value will be considered faulty. If, with any of the described connection options, the multimeter shows a break, this most likely indicates an internal break in the circuit.

Is repair possible in this case?

Since modern ADCs are most often produced in an integrated version (without a housing), rarely can anyone replace them. So if the converter burns out, then it will not be possible to repair the multimeter; it cannot be repaired.

Problems with the rotary switch

Repair will be required if problems arise due to loss of contact in the circular biscuit switch. This manifests itself not only in the fact that the multimeter does not turn on, but also in the inability to obtain a normal connection without pressing hard on the biscuit. This is explained by the fact that the contact tracks are rarely covered with high-quality lubricant, which leads to their rapid oxidation.

When used in dusty conditions, for example, after some time they become dirty and lose contact with the switch bar. To repair this multimeter assembly, it is enough to remove the printed circuit board from its body and wipe the contact tracks with a cotton swab dipped in alcohol. Then a thin layer of high-quality technical Vaseline should be applied to them.

In conclusion, we note that if factory “missing solders” or contact shorts are detected in the multimeter, these defects should be eliminated by using a low-voltage soldering iron with a well-sharpened tip. If you are not completely sure of the cause of the device failure, you should contact a specialist in the repair of measuring equipment.

Flux SKF

In any case, no matter how you remove this resistor from the board, there will be tubercles of old solder on the board; we need to remove it using a dismantling braid, dipping it in alcohol-rosin flux. Place the tip of the braid directly on the solder and press it in, heating it with the tip of the soldering iron 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, which we freed from solder, press it down with a screwdriver from above and touching the tip of a 25-watt soldering iron, the pads and leads located at the edges of the resistor, solder it in place.

Solder braid - application

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 about such repairs were divided; some argued that due to the low cost of multimeters, it makes no sense to repair them at all, they say they threw them away and went and bought a new one, others were even ready to go all the way and resolder the ADC). But as this case shows, sometimes repairing a multimeter is quite simple and cost-effective, and any home craftsman can easily handle such repairs. Everyone! AKV.

The reliability of modern measuring instruments, like any other equipment, directly depends on their operating conditions. Various shocks, changes in temperature, relative humidity - all this leads to premature failure of the device. And although the manufacturer tries to increase reliability by various means, the device can still break down sooner or later due to banal oxidation of the contacts of the measuring range switch or protection relay. Perhaps a question asked to the owner of a digital multimeter about whether he carries out preventative maintenance on his device will confuse him, or most likely make him laugh - no matter what they say, we begin to disassemble the device only when it is no longer possible to measure with it. And here I would like to immediately tell the reader, do you know how to do this? If you do, then this article will not be interesting to you. But we will continue anyway.

So, first, let's select the tools. Of course, a Phillips screwdriver with a long and thin blade, tweezers, a flat thin medical spatula (optional, you can use anything instead - a knife, for example), a rubber eraser. That's all. Besides this, some more chemistry is needed. Ask at Eastern Department something for cleaning circuit boards - they will offer you a lot of things. Perfect option - isopropyl alcohol- cheap, removes dirt well and dissolves flux. In addition, you should stock up on any silicone grease. You need very little of it - to cover the contacts with a thin film and prevent oxidation. I categorically do not recommend using cyatim, litol, grease for this purpose - they collect a lot of dirt on themselves, and cyatim will dry out altogether and will contribute to the breakdown of contacts in the future. Well, don’t forget some rag. Wipe your hands.

Let's think that your favorite digital multimeter is out of order and its segments do not display some information - as shown in the figure below (ugh, ugh, although one friend gave this multimeter for repair - this is not yours :) We will repair it and at the same time carry out preventive maintenance .

Let's get started. To begin with, without disassembling the device, we try to press our fingers on the front panel just below the indicator glass - great, the indicators are now displayed, which means that the device can be repaired 100% if nothing is accidentally broken during the repair process. Now, if with this method of checking not a single segment begins to be displayed, you will have to scratch your head - the ADC of the multimeter may be faulty.

We remove the back cover of our Mastech, find the screws that secure the board to the front of the case. This multimeter turned out to have only two of them, but the second one simultaneously attached the board and the buzzer - that big black round thing. Carefully remove the board from the case. You can use whatever you want, the main thing is to avoid bending the board - because of this, you can get additional problems in the form of microcracks on the tracks.

Here he is - M-832 in disassembled form. Check if the metal balls of the range switch, springs and switch contacts were lost during disassembly. Lost????? In this case, you need an LED flashlight - it’s much more convenient to crawl on the floor with it :)

Next, you need to remove the LCD display itself from the board. This should be done carefully, bending each of the three clamps one by one. In general, you need to act extremely carefully in this place, otherwise there is a risk of breaking off the fasteners themselves. It is they who create all the main force of pressing the LCD display to the conductive rubber band, as well as the rubber band to the contacts of the board. If you break it off, that's okay too - superglue is a pretty effective remedy.

When the latches are released from the board, remove the display by turning it and removing it from the grooves - oops. Oh no no no. It seems to be a well-known company, but here it is - there are improvements to the device in the form of a wire jumper soldered directly to the contacts intended for conductive rubber. In addition, white stains on the board indicate violations of storage conditions (the flux was poorly washed or not washed at all, but the device was lying somewhere, lying in a warehouse). All this is clearly visible in the bottom two pictures.

Let's fix this situation. We take our pre-prepared isopropyl and apply it to the board with a brush. If you have a big bottle like me, you won't regret it. We are trying to clean all the dirt from the board, so it is better to use a brush as hard as possible. I want to say that electronics really love alcohol in any form and this makes them work very well. Well, now it’s okay to wait for the isopropyl to evaporate.

Now we take the eraser and begin to methodically rub it along the contacts. Wow, how they sparkled. But I don’t recommend doing this with sandpaper - if you remove a thin layer of gold, at first everything will be fine, but then you get into the device again, the contacts will oxidize very quickly. You also need to remember to remove wear products from the eraser.

Now you can install the display back. You can put pieces of electrical tape under the clamps to slightly increase the force of pressing the display to the contacts.

Here are pieces of electrical tape under the display latches on four sides:

You can also stick strips of electrical tape on the front of the display. It won't be redundant. I did:

Now my favorite job is to lubricate and adjust everything. Apply a thin layer of silicone grease to the contacts of the measuring range switch. I hope they realized that they could also be rubbed with an eraser. Prevention is prevention :) By the way, I cheated a little here. The fact is that I lubricate everything when the multimeter is already working properly. Of course, I assembled the multimeter, checked it, and then took it apart again to lubricate it and take a picture at the same time. Why? But if the multimeter did not work, you would have to look for the reason, and this would mean removing the grease. What if it's nonsense? I won't remove the grease. As a result, the entire table, hands and other places are covered in grease :) Therefore, we assemble, check, disassemble, and lubricate. We collect. I almost forgot - the range switch (yes, that same knob with small steel balls) - usually the manufacturer doesn’t spare any grease there, but still - if it’s not enough, don’t forget to apply it.

Now let's collect. We check the rotation and fixation of the switch. If it gets stuck, don't put in extra effort. Just disassemble the multimeter and check that the switch is assembled correctly - the metal balls should be on different sides, each in its own hole. And don't forget about the springs. It worked for me. And you?

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