DIY mastech my68 multimeter repair

When repairing electronics, you have to carry out a large number of measurements with various digital instruments. This is an oscilloscope, an ESR meter, and what is used most often and without the use of which no repair can do: of course, a digital multimeter. But sometimes it happens that help is required by the instruments themselves, and this happens not so much from the inexperience, haste or carelessness of the master, as from an annoying accident, such as happened to me recently.

DT Series Multimeter - Appearance

It was like this: after replacing the broken field-effect transistor during the repair of the LCD TV power supply, the TV did not work. An idea arose, which, however, should have come even earlier, at the diagnostic stage, but in a hurry it was not possible to check the PWM controller even for low resistance or a short circuit between the legs. It took a long time to remove the board, the microcircuit was in our DIP-8 package and it was not difficult to ring its feet on the short circuit even on top of the board.

Electrolytic capacitor 400 volt

I disconnect the TV from the mains, wait for the standard 3 minutes to discharge the capacitors in the filter, those very large barrels, electrolytic capacitors for 200-400 Volts, which everyone saw when disassembling a switching power supply.

I touch the probes of the multimeter in the mode of sound continuity of the PWM controller legs - suddenly a beep sounds, I remove the probes in order to call the rest of the legs, the signal sounds for another 2 seconds. Well, I think that's all: again 2 resistors burned out, one in the resistance measurement circuit of the 2 kOhm mode, for 900 Ohm, the second for 1.5 - 2 kOhm, which is most likely in the ADC protection circuits. I had already encountered a similar nuisance, in the past a friend hit me with a tester in the same way, so I did not get upset - I went to the radio store for two resistors in SMD cases 0805 and 0603, one ruble a piece, and soldered them.

Searches for information on the repair of multimeters on various resources, at one time, gave out several typical schemes, on the basis of which most models of cheap multimeters are built. The problem was that the reference designations on the boards did not match the designations on the found diagrams.

Burnt resistors on the multimeter board

But I was lucky, on one of the forums a person described in detail a similar situation, the failure of the multimeter when measuring with the presence of voltage in the circuit, in the mode of sound dialing. If there were no problems with the 900 Ohm resistor, several resistors on the board were connected in a chain and it was easy to find it. Moreover, for some reason it did not turn black, as is usually the case during combustion, and it was possible to read the denomination and try to measure its resistance. Since the multimeter contains precise resistors that have 4 digits in their designation, it is better, if possible, to change the resistors to exactly the same ones.

There were no precision resistors in our radio store and I took the usual one for 910 ohms. As practice has shown, the error with such a replacement will be quite insignificant, because the difference between these resistors, 900 and 910 Ohms, is only 1%. Determining the value of the second resistor was more difficult - from its terminals there were tracks to two transition contacts, with metallization, to the back of the board, to the switch.

Place for soldering thermistor

But I was lucky again: two holes were left on the board connected by tracks in parallel with the resistor leads and they were signed by RTS1, then everything was clear. The thermistor (РТС1), as we know from the pulse power supplies, is soldered in order to limit the currents through the diodes of the diode bridge when the pulsed power supply is turned on.

Since electrolytic capacitors, those very large barrels of 200-400 volts, at the moment the power supply is turned on and the first fractions of a second at the start of charging, behave almost like a short circuit - this causes large currents through the bridge diodes, as a result of which the bridge can burn out.

To put it simply, a thermistor has a low resistance in normal mode when small currents flow, corresponding to the mode of operation of the device. With a sharp multiple increase in current, the resistance of the thermistor also increases sharply, which, according to Ohm's law, as we know, causes a decrease in the current in the circuit section.

Resistor 2 Kom Ohm on the diagram

When repairing on the circuit, presumably we change to a 1.5 kΩ resistor, the resistor indicated on the circuit with a nominal value of 2 kΩ, as they wrote on the resource from which they took the information, during the first repair, its value is not critical and it was recommended to put it, nevertheless, at 1.5 kΩ.

We continue... After the capacitors are charged and the current in the circuit has decreased, the thermistor decreases its resistance and the device operates normally.

900 ohm resistor on the diagram

Why is a thermistor installed instead of this resistor in expensive multimeters? With the same purpose as in switching power supplies - to reduce large currents that can lead to the burnout of the ADC, arising in our case as a result of an error of the master conducting the measurements, and thereby protecting the analog-to-digital converter of the device.

Or, in other words, that very black drop, after the combustion of which the device usually no longer makes sense to restore, because this is a laborious task and the cost of parts will exceed at least half the cost of a new multimeter.

How can we solder these resistors - perhaps beginners who have not previously dealt with SMD radio components will think. After all, they most likely do not have a soldering hair dryer in their home workshop. There are three ways here:

1. First, you will need an EPSN soldering iron with a power of 25 watts, with a blade blade with a cut in the middle, in order to heat both terminals at once.
2. The second way, by biting off with side cutters, a drop of Rose or Wood's alloy, immediately on both contacts of the resistor, and flatten both of these terminals with a sting.
3. And the third way, when we have nothing but a 40-watt soldering iron of the EPSN type and the usual POS-61 solder - we apply it to both leads so that the solders mix and as a result, the total melting temperature of the lead-free solder decreases, and we heat both leads of the resistor alternately, while trying to move it a little.

Usually this is enough for our resistor to be sealed off and stick to the tip. Of course, do not forget to apply the flux, it is better, of course, liquid Alcohol rosin flux (GFR).

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

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

Solder Braid - Applications

The first time, it will probably turn 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 cheapness of multimeters, it makes no sense to repair them at all, they say they threw it out and went to buy a new one, others were even ready to go all the way and re-solder 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 a repair. Successful repairs to everyone! AKV.

It would be better to buy an ordinary Chinese multimeter from the M83 * series for 150-200 rubles, the main thing is not from Resanta (they lie to the arrogant).Accuracy as expected from them, at least from all that I came across on high-precision resistances gave the correct results.

Added after 13 minute (s):

at such a limit, they will not have great accuracy. these devices measure such small resistances with an error of up to 0.5-1 ohm plus contact instability of the order of 0.5 ohm.

And by the way, if the soldering looks ugly, it may be native, China is all the same.

About what the conversation. the device is not very bad and in my opinion it is not a Chinese fake, therefore I want to repair it. What do you advise, give it to the workshop or what?

Maybe I will repeat myself, but even with an unaided eye you can see where the factory soldering is and where "Uncle Petya soldered"

You probably met little factory products from China. This principle does not apply to them. There is also excellent automatic soldering, and there is also manual soldering where “Uncle Li soldered” And there is also a combined part of the components automatically, and some manually.

So far, from the measurements you have given, it follows that the device is working normally, and the error is normal, so do not rush to repair it. Look for an accurate instrument by which you can compare readings for voltages and currents and accurate resistances to test it for measuring resistance.

so we look at the speaker impedance of 4 Ohm, measure on the 326 Ohm range, the error is +/- 0.8% 326 * 0.008 = 2.608 in total, it shows your resistance 4 Ohm with an accuracy of +/- 2.608 Ohm and in addition to this there may be +/- 3 digits inaccuracy of digitization +/- 0.3 ohm. add resistance at the point of contact, it can also be up to 0.5 ohm there, depending on how the probes fall and how tightly they press.
Which one of this? such small resistances are not suitable for determining the error.

Second measurement: 1k +/- 0.8% limit 3.26k error 3.26 * 0.008 = 0.02608k your readings are 1015-1016, that is, considering that the resistor is exactly 1k your device measured it almost 2 times more accurately than passport.
inaccuracy of readings is allowed due to digitization errors +/- 1 digit in your case everything converges or +1 or -1 digit.

Hello everyone! I'll tell you a little about the repair of the Mastech MY-61 multimeter.

This device came to me a long time ago and I don’t remember how, all my hands didn’t reach it, but there was a time, I decided to pick it up. It turned out that the opamp in the capacitor measurement circuit and the ADC itself, which is made on a board without a case and filled with compound, burned out.

We could have thrown it out, but still, the old Mastech is not quite so bad China, I decided to restore it, since I had free time. Replacing the opamp is not of much interest, but I decided to share the replacement of a drop with a case ADC, suddenly someone would be interested. You must purchase an ICL7106 ADC in a TQFP-44 package.

Do not forget to look at the datasheets, different manufacturers have minor differences in conclusions, but it is not important for us, since in our case additional conclusions are not used.

We are determined by the printed circuit board and the details with the numbering of the drop pins, we make a visual layout of how the microcircuit will be located and so that you can see which tracks to remove and which ones to leave.

Next, we grind the compound with a micro drill with a cutter. The process was not filmed in detail, so as not to waste a lot of time, this is how it turned out:

The drop is removed, it remains to adjust the place so that a minimum of wires are soldered to the microcircuit.

We bend the pins of the microcircuit, adjust them to the tracks on the board.

We solder the ADC microcircuit to the prepared place.

Here is such a repair, it took about three hours. The device works, it remains to come up with something with a round socket for testing hfe transistors, as you can see in the first photo (in the lower right corner) the socket is missing for some reason unknown to me. How many I didn’t look for, I didn’t find its name to try to find it in online stores, I will be very grateful if someone tells what kind of nest it is, maybe it is used somewhere other than multimeters and what it’s called.

Mastech are quite good devices. Mastech has been serving me for more than 10 years - if only henna.

I don’t know how Mastech is doing it now, I haven’t bought multimeters for a long time, but before Mastech made really good instruments

I took it in the 2000s. With thermocouple. How many times I fell to the floor - it works.

At the very mastech my-63, for 10 years it has already served faithfully

at mnu MY-62. the thermocouple died a month later, and a month later something in the intestines died, because it did not work with the other.

and the range of capacitance measurements is too small, in my opinion.

and so cool device, although I must have been stupid by taking one for digging and mastering right away

ps For a long time I licked my lips on the unit because auto-range selection and smart indication, but even they were more expensive, much so

it is better to measure the capacity with separate devices designed for this, automatic range selection is, in my opinion, an inconvenient function, I have devices with auto range selection, I always switch them to manual mode.

Yeah, I ought to buy it. Do you take on Ali?

yes, ali. take a look at Markus's tester, if you are into electronics, there are a bunch of options and modifications for every taste and pocket.

on the automatic range selection, firstly, it measures longer, and secondly, the readings jump and it is not clear whether there is an open circuit, or whether the contact is bad, or there really is a voltage change at the lower limit. in general, I don't like

maybe in a different way, how is it set on fire? did not open, did not look inside, how well the device was made? those that I had Mastech'and about 1998-2003 were made soundly, and inside and the case itself

Familiar 🙂 It was like this (exactly 10 years ago):

Did the back cover close?

Thank you, now it became clear that this is a block for microcircuits with a round metal case, type K140UD1. Why didn't I guess right away

And the author knows a lot about perversions.

in 1999, a similar device burned out for me, it cost immensely money in those years, especially for a student with irregular earnings. I decided to change the drop for the only thing that was available, this is a large DIP-40 case. under the display, the microcircuit with the socket did not fit, I had to sculpt it from behind, cutting out a rectangular hole in the lid, since the case did not close with the soldered mikruha. then from the cut out rectangle of the case and pieces of plastic dissolved in acetone, I made a protrusion in the form of a parallelepiped, covering the microcircuit and completely restoring the integrity of the case. here it was a slight perversion, but what is shown here is so, pampering in your free time.

why did some dandy cartridges stop turning on?

I got this device in an unknown state: it turns on, but there is no indication and does not emit any signals. External examination of the board and parts did not reveal any noticeable damage to them. When connecting the battery, it turned out that the consumed current is about 40mA and does not depend on the selected range. The first step was to check all the resistors. turned out to be faulty (open circuit) R44 -10 ohm (short black black ash). Then all diodes and zener diodes, capacitors were checked (everything turned out to be in good order), then microcircuits: IC2, IC3, IC4, IC5.
All designations according to the diagram:

IC2 (NJM062D) has both op amps faulty. IC3 (ICM7555IPA) has 3.2 ohm resistance between pins 1 and 2. IC5 (ICM7555IPA) has 12.8 ohm resistance between pin 1 and pin 8. A working ICM7555IPA has a resistance of more than 200 ohms between the indicated pins. Transistors Q2 (KTC9013G) also turned out to be faulty - breakdown of the transition B-K and Q3 (KTC9015C) - breakdown of the transition E-K. To establish the cause of the failure of these microcircuits and transistors, this piece from the multimeter circuit is useful:

Obviously, the R44, Q2, Q3, IC5 chain failed due to the connection of the probes to the terminals of an uncharged capacitor or measuring its capacity directly in the circuit with the power supply of the repaired device connected.
After replacing all the faulty elements, the multimeter did not work, but the current consumption became about 6 mA, which is much closer to normal. Then IC1 (KAD7001) was checked. Positive voltage (3.4 volts) at pin 32 was present, negative voltage at pin 62 was absent.There was also no reference voltage (1.28 volts) at pin 47 and the clock generator (32.768 kHz) did not work.
Photos of faulty components:

A new KAD7001 was bought from the Chinese and, accordingly, it was sealed in place of a non-working one.
Table of voltages on the active components of the multimeter after soldering the Chinese microcircuit:

Photo of microcircuits: on the left is a native one, which was originally in the device, and on the right was bought from the Chinese.

After replacing the microcircuit, the miracle did not happen. the device did not work. Obviously, the Chinese sent a NOT WORKING microcircuit. Actually the main question: WHERE TO BUY a WORKING microcircuit. Does anyone have a real experience of buying a working microcircuit from the Chinese?

_________________
"- Use what is at hand and do not look for something else for yourself!" Philleas Fogg.
I am looking for a probe for C1-94, ES5106E ERSO microcircuit.

Last edited by Serjio on Apr 21, 2018 8:18 pm, edited 3 times in total.

Thanks for the help!
I watched the voltage between COM and battery positive, 9.4 V.
I found a trimmer resistor, 20 kOhm. There it is, the designation on the VR2 board. Adjusting it doesn't help.
I also noticed that I measured the resistance between the COM and these resistor VR2, 125 kOhm.
According to the scheme, it should be less, the 36 kOhm resistor (selected) was not found on the board.

You take the DS on the KAD7001, study it, there are also simplified modes of operation.
On the 55th leg, the V meas IN input, there is a resistor in front of it, raise one end of it
and apply the well-known 200-300 mV to the input of the ADC ms, the mode switch
in the DC voltage measurement position.
See what happens. If the readings are almost the same, then
adjust the reference voltage and figure out where what is lost
in the temporarily disconnected part of the multimeter.
Or, if the readings lie, look for what else has suffered in the ADC piping -
switchable divider (external resistors), etc.

I measured between COM and “+” power supply about +9.4, and COM and “-” power supply 0 volts

While watching the datasheet (Thanks!)

Added after 39 minutes 53 seconds:

What is your payment?
Here is mine:

According to the proposed datasheet, there is a variant of a 3-volt power supply and there is no talk of a HT7530-1 stabilizer microcircuit.

Here are examples of power supply for such ADCs, using the FS9922 as an example:

Holtek HT7530-1 100mA Low Power LDO - it's easy to check.

The board on mine is like this photo. (Version MY68-3 100895).

Measured voltage
VDD 3.4V
VSS 0 V

But my values ​​are different. 9.4V and 0V.

Now I measure a constant voltage on a 13 V battery, in automatic selection 9.8 V in manual 11.1 V

First, it was necessary from the very beginning to admit how much of what (B, A) and where
(in which measurement mode) did you "zhahnu poor man"

J176 field effect transistor - does it open and close?
To exclude "kotovasia" with power supply - connect an external
power supply of 3 volts temporarily, removing the conversion from 9 volts, as in the LH.
Check the integrity of the COM connector circuit to the ADC ground and apply again
external millivolts as before. power supply 3 volts and external mV-you should not
be galvanically coupled, that is, from two different power sources!

Voltage 0.9 V, minus 51 legs.

Found a circuit with the same microcircuit clamp 9912

And my multimeter suffered from a constant voltage slightly more than 600 V, in the mode of measuring constant voltage, but the choice of the range which was “auto” or “manual” I will not say for sure. It seems that it should not have suffered, but it happened.
On occasion, a donor turned up, almost the same payment, the performance was slightly different (I don’t know what was wrong with him, but the 7001 turned out to be intact, that's how much is also unknown), and therefore decided to repair it.
It's quite old, with an analogue scale. There are definitely 7 years, if not more.
There are repair tips, thank you very much for that!
I will try to recover.
It’s good to get it, it’s not scary to fail.
I'll take a new one. (I want to take Uni-t U61E)

And 51 legs, I asked for between 62 and 63. Moreover, 62 and 37 are COM.
Now look at the 73 leg, it should connect 63 and there should be a capacity according to the schematics from the datasheet 10-20 uF.
Negative voltage should form there.

At some point, it stopped turning on. Experimentally, it was found that it turns on only if you quickly turn the switch, passing the “Off” state. If you do the same, but do not “jump” over “Off”, then the multimeter will not turn on. Naturally, first of all I thought about the bad contacts of the switch. Disassembled, cleaned, did not help.
I found out that during a normal turn-on from the “Off” state, the controller does not start the generator (there is no 4 MHz oscillation on the quartz). Accordingly, the voltage doubler does not work and the analog ground “floats away”. In this case, power is supplied to the controller (9 V -> 3 V through the 28B2K stabilizer).

Can you tell me where to dig? The scheme is very similar to my version:

The reliability of modern measuring devices, like any other equipment itself, directly depends on the conditions of their operation. Various shocks, changes in temperature, relative humidity - all this leads to premature failure of the device. And although the manufacturer is trying to increase reliability by various means, the device may still break down sooner or later due to the banal oxidation of the contacts of the measuring range switch or protection relay. Perhaps the question posed to the owner of a digital multimeter about whether he is doing prophylaxis of 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 will no longer be possible for them to measure. And here I would like to immediately tell the reader, but do you know how to do this? If you know, then this article will not interest you. But we will continue anyway.

So let's first 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 you like instead - a knife, for example), a rubber eraser. That's all. In addition, some more chemistry is needed. Ask in Eastern Department something to clean the boards - you will be offered a lot. Perfect option - isopropyl alcohol - cheap, washes away dirt well and dissolves gumboil. In addition, you should stock up on any silicone grease... Very little of it is needed to cover the contacts with a thin film and prevent oxide. I strongly advise against using cyatim, lithol, solid oil for this business - they collect a lot of dirt on themselves, and cyatim will dry out altogether, and in the future will contribute to the breakdown of contacts. Well, don't forget some rag. Wipe your hands.

Let's think that your favorite - the digital multimeter is out of order and its segments do not display some of the information - as shown in the figure below (ugh, ugh, although this multimeter was given for repair by one friend - 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 with our fingers on the front panel just below the indicator glass - great, the indicators are 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, no segment starts to be displayed, you will have to scratch your head - the ADC of the multimeter may be faulty.

Remove the back cover of our Mastech, find the screws with which the board is attached to the front of the case. This multimeter turned out to have only two of them, but the second one simultaneously attached a board and a buzzer - that black round big thing. Carefully remove the board from the case. You can use whatever you want, the main thing is not to allow the board to bend - because of this, you can get additional problems in the form of microcracks on the tracks.

Here it is - M-832 disassembled. Check if the range switch metal balls, springs and switch contacts are missing during disassembly. Lost. In this case, you need an LED flashlight - it is much more convenient to crawl on the floor with it 🙂

Next, you need to dismantle the LCD itself from the board.This should be done carefully, alternately bending back each of the three retainers. In general, in this place you need to act extremely carefully, otherwise there is a risk of breaking off the clips themselves. They just create all the main force of pressing the LCD display to the conductive rubber band and also the rubber band to the board contacts. Break off - also okay - superglue is quite an effective tool.

When the latches are released from the board, remove the display by turning it and taking it out of the slots - oops. Oh no no no. It seems like a well-known company - Mastech, and here it is - there is a refinement of the device in the form of a wire jumper soldered directly to the contacts intended for a conductive rubber band. In addition, white streaks on the board - this indicates a violation of storage conditions (the flux was poorly washed or not washed at all, but here the device was lying somewhere, lying in the warehouse). All this is clearly visible in the lower two pictures.

Let's fix this situation. We take our pre-prepared isopropyl, and apply it with a brush to the board. If you have a bottle as big as mine, you can be generous. We try to clean off all the dirt from the board, so it's best to take a brush as hard as possible for this. I want to say that electronics are very fond of alcohol in any form and from this it starts to work very well. Well, now, it’s off to wait for the isopropyl to evaporate.

Now we take the eraser and begin to methodically rub it over the contacts. Wow, how brilliant. But I do not advise doing this with sandpaper - remove a thin layer of gold, at first everything will be fine, and then you will again climb into the device, the contacts will oxidize very quickly. Do not forget to remove the deterioration products of the wash.

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

Here are the pieces of duct tape under the display clamps on four sides:

And you can also stick strips of electrical tape on the front of the display. Will not be superfluous. I did:

Now my favorite job is - I like to lubricate and adjust everything. Apply a thin layer of silicone grease to the contacts of the measuring range switch. I hope you guessed that they could also be rubbed with an eraser. Prevention - there 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 disassembled it again to lubricate and photograph at the same time. Why? But if the multimeter did not work, you would have to look for the reason, and this will have to remove the grease. What if there is nonsense? I will not remove the grease. As a result, the whole table, hands and other places are lubricated 🙂 Therefore, we collect, check, disassemble, lubricate. We collect. I almost forgot - the range switch (yes, the same twist with small steel balls) - usually the manufacturer does not regret the lubricant there, but all the same - if not enough, do not forget to apply.

Now we collect. We check the rotation and fixation of the switch. If it does wedge, do not put extra effort. Just disassemble the multimeter and check if the switch is assembled correctly - the metal balls should be on opposite sides, each in its own hole. And don't forget the springs. It worked for me. And you?

Like any other item, the multimeter can fail during operation or have an initial, factory defect that was not noticed 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 examination of the printed circuit board, since short circuits and poor soldering are possible, as well as a defect in the leads of the elements along the edges of the board.

Factory defect in these devices is manifested mainly on the display. There can be up to ten types of them (see table). Therefore, it is better to repair digital multimeters using the instructions that come with the device.

The same breakdowns can occur after operation.The above malfunctions may also appear during operation. However, if the device operates in constant voltage measurement mode, it rarely breaks.

The reason for this is its overload protection. Also, the repair of a faulty device should begin with checking the supply voltage and the operability of the ADC: the stabilization voltage is 3 V and there is no breakdown between the power pins and the common ADC output.

Experienced users and professionals have repeatedly stated that one of the most likely causes of frequent breakdowns in the device is poor quality production. Namely, soldering contacts with acid. As a result, the contacts are simply oxidized.

However, if you are not sure what kind of breakdown caused the inoperative state of the device, you should still contact a specialist for advice or help.

It is impossible to imagine a repairman's workbench without a handy, inexpensive digital multimeter.

This article describes the device of the 830 series digital multimeters, its circuit, as well as the most common malfunctions and how to fix them.

Currently, a huge variety of digital measuring instruments of varying degrees of complexity, reliability and quality are produced. The basis of all modern digital multimeters is an integrated analog-to-digital voltage converter (ADC). One of the first such ADCs suitable for constructing inexpensive portable measuring instruments was a converter based on the ICL7106 microcircuit manufactured by MAXIM. As a result, several successful low-cost models of 830 series digital multimeters have been developed, such as M830B, M830, M832, M838. DT can be used instead of the letter M. This instrument series is currently the most widespread and most repeatable in the world. Its basic capabilities: measuring direct and alternating voltages up to 1000 V (input resistance 1 MΩ), measuring direct currents up to 10 A, measuring resistances up to 2 MΩ, testing diodes and transistors. In addition, in some models there is a mode of sound continuity of connections, temperature measurement with and without a thermocouple, generation of a meander with a frequency of 50 ... 60 Hz or 1 kHz. The main manufacturer of this series of multimeters is Precision Mastech Enterprises (Hong Kong).

The basis of the multimeter is the ADC IC1 of the 7106 type (the closest domestic analogue is the 572PV5 microcircuit). Its structural diagram is shown in Fig. 1, and the pinout for the version in the DIP-40 package is shown in Fig. 2. The 7106 core can be preceded by different prefixes depending on the manufacturer: ICL7106, ТС7106, etc. Recently, more and more often used are chipless microcircuits (DIE chips), the crystal of which is soldered directly to the printed circuit board.

Consider the circuit of the Mastech M832 multimeter (Fig. 3). Pin 1 of IC1 supplies a positive 9V battery supply voltage, and Pin 26 supplies a negative battery supply. Inside the ADC there is a 3 V stabilized voltage source, its input is connected to pin 1 of IC1, and the output is connected to pin 32. Pin 32 is connected to the common pin of the multimeter and is galvanically connected to the COM input of the device. The voltage difference between pins 1 and 32 is approximately 3 V in a wide range of supply voltages - from nominal to 6.5 V. This stabilized voltage is fed to the adjustable divider R11, VR1, R13, and from its output to the input of the microcircuit 36 ​​(in the mode measurements of currents and voltages). The divider sets the potential U at pin 36, equal to 100 mV. Resistors R12, R25 and R26 perform protective functions. Transistor Q102 and resistors R109, R110 and R111 are responsible for indicating the discharge of the battery. Capacitors C7, C8 and resistors R19, R20 are responsible for displaying the decimal points of the display.

Operating input voltage range U_max directly depends on the level of the regulated reference voltage at pins 36 and 35 and is

The stability and accuracy of the display is dependent on the stability of this reference voltage.

The display N readings depend on the input voltage U and are expressed as a number

A simplified circuit of the multimeter in the voltage measurement mode is shown in Fig. 4.

When measuring DC voltage, the input signal is fed to R1… R6, from the output of which, through a switch [according to the scheme 1-8 / 1… 1-8 / 2), it is fed to the protective resistor R17. This resistor also forms a low-pass filter when measuring AC voltage together with the capacitor C3. Then the signal goes to the direct input of the ADC microcircuit, pin 31. The potential of the common pin, generated by the 3 V stabilized voltage source, pin 32, is fed to the inverse input of the microcircuit.

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

A simplified circuit of the multimeter in the current measurement mode is shown in Fig. 5.

In the mode of measuring direct current, the latter flows through the resistors R0, R8, R7 and R6, which are switched depending on the measuring range. The voltage drop across these resistors through R17 is fed to the ADC input, and the result is displayed. ADC protection is provided by diodes D2, D3 (in some models they may not be installed) and fuse F.

A simplified circuit of the multimeter in the resistance measurement mode is shown in Fig. 6. In the resistance measurement mode, the dependence expressed by the formula (2) is used.

The diagram shows that the same current from the voltage source + U flows through the reference resistor and the measured resistor R "(the currents of inputs 35, 36, 30 and 31 are negligible) and the ratio of U and U is equal to the ratio of the resistances of the resistors R" and R ^. R1..R6 are used as reference resistors, R10 and R103 are used as current setting resistors. Protection of the ADC is provided by thermistor R18 (some cheap models use conventional 1.2 kΩ resistors), transistor Q1 in zener diode mode (not always installed) and resistors R35, R16 and R17 at inputs 36, 35 and 31 of the ADC.

Continuity mode The dialing circuit uses IC2 (LM358), which contains two operational amplifiers. A sound generator is assembled on one amplifier, and a comparator on the other. When the voltage at the input of the comparator (pin 6) is less than the threshold, a low voltage is set at its output (pin 7), which opens the switch on the transistor Q101, as a result of which a sound signal is emitted. The threshold is determined by the divider R103, R104. Protection is provided by resistor R106 at the comparator input.

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

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

The LCD display can be checked for proper operation using a 50.60 Hz AC voltage source with an amplitude of several volts. As such a source of alternating voltage, you can take the M832 multimeter, which has a meander generation mode. To check the display, put it on a flat surface with the display up, connect one probe of the M832 multimeter to the common terminal of the indicator (bottom row, left terminal), and apply the other probe of the multimeter alternately to the rest of the display. If it is possible to get the ignition of all segments of the display, then it is serviceable.

The above malfunctions may also appear during operation. It should be noted that in the DC voltage measurement mode, the device rarely fails, because well protected from input overloads. The main problems arise when measuring current or resistance.

Repair of a faulty device should begin with checking the supply voltage and the ADC's operability: the stabilization voltage is 3 V and there is no breakdown between the power pins and the common ADC output.

In the current measurement mode when using the V, Q and mA inputs, despite the presence of a fuse, there may be cases when the fuse blows out later than the safety diodes D2 or D3 have time to break through. If a fuse is installed in the multimeter that does not meet the requirements of the instructions, then in this case the resistances R5 ... R8 may burn out, and this may not appear visually on the resistances. In the first case, when only the diode breaks through, the defect appears only in the current measurement mode: the current flows through the device, but the display shows zeros. In case of burnout of resistors R5 or R6 in the voltage measurement mode, the device will overestimate the readings or show an overload. When one or both resistors are completely burned out, the device does not reset in voltage measurement mode, but when the inputs are closed, the display is set to zero. When the resistors R7 or R8 burn out on the current measuring ranges of 20 mA and 200 mA, the device will show an overload, and in the 10 A range - only zeros.

In resistance measurement mode, faults usually occur in the 200 ohm and 2000 ohm ranges. In this case, when the voltage is applied to the input, the resistors R5, R6, R10, R18, the transistor Q1 can burn out and the capacitor C6 can break through. If the transistor Q1 is completely broken, then when measuring the resistance, the device will show zeros. In case of incomplete breakdown of the transistor, the multimeter with open probes will show the resistance of this transistor. In voltage and current measurement modes, the transistor is short-circuited by a switch and does not affect the multimeter readings. With a breakdown of capacitor C6, the multimeter will not measure voltage in the ranges of 20 V, 200 V and 1000 V or significantly underestimate the readings in these ranges.

If there is no indication on the display, when there is power to the ADC, or there is a visually noticeable burnout of a large number of circuit elements, there is a high probability of damage to the ADC. The ADC's serviceability is checked by monitoring the voltage of the 3 V stabilized voltage source. In practice, the ADC burns out only when a high voltage is applied to the input, much higher than 220 V. Very often, cracks appear in the compound of the open-frame ADC, the current consumption of the microcircuit increases, which leads to its noticeable heating ...

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

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

A source of a stabilized voltage of 3 V in an ADC for cheap Chinese models can in practice give a voltage of 2.6-3.4 V, and for some devices it stops working already at a voltage of a supply battery of 8.5 V.

The DT models use low quality ADCs and are very sensitive to the C4 and R14 integrator chain ratings. High-quality ADCs in Mastech multimeters allow the use of elements of close denominations.

Often, in DT multimeters, when the probes are open in the resistance measurement mode, the device approaches the overload value for a very long time ("1" on the display) or is not set at all. It is possible to "cure" a poor-quality ADC microcircuit by reducing the value of the resistance R14 from 300 to 100 kOhm.

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

Since cheap Chinese firms use low-quality unpackaged ADCs, there are frequent cases of broken pins, and it is very difficult to determine the cause of the malfunction and it can manifest itself in different ways, depending on the broken pin. For example, one of the indicator leads is off. Since multimeters use displays with static indication, then to determine the cause of the malfunction, it is necessary to check the voltage at the corresponding pin of the ADC microcircuit, it should be about 0.5 V relative to the common pin. If it is zero, then the ADC is faulty.

There are malfunctions associated with poor-quality contacts on the biscuit switch, the device works only when the biscuit is pressed. Firms that make cheap multimeters rarely coat the tracks under the rocker switch with grease, which is why they quickly oxidize. Often the tracks are dirty. It is repaired as follows: the printed circuit board is removed from the case, and the switch tracks are wiped with alcohol. Then a thin layer of technical petroleum jelly is applied. Everything, the device is repaired.

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

It happens that manufacturers of cheap multimeters put low-quality operational amplifiers in the sound generator circuit, and then when the device is turned on, a buzzing buzzer is heard. This defect is eliminated by soldering a 5 μF electrolytic capacitor parallel to the power circuit. If this does not ensure the stable operation of the sound generator, then it is necessary to replace the operational amplifier with the LM358P.

Often there is such a nuisance as battery leakage. Small drops of electrolyte can be wiped off with alcohol, but if the board is heavily flooded, then good results can be obtained by washing it with hot water and laundry soap. After removing the indicator and unsoldering the buzzer, using a brush, for example, a toothbrush, you need to thoroughly soap the board on both sides and rinse it under running water from the tap. After repeating the wash 2.3 times, the board is dried and installed in the case.

Most of the devices produced recently use DIE chips ADCs. The crystal is mounted directly onto the PCB and is embedded in resin. Unfortunately, this significantly reduces the maintainability of the devices, because when the ADC fails, which is quite common, it is difficult to replace it. Unpackaged ADCs are sometimes sensitive to bright light. For example, if you work near a table lamp, the measurement error may increase. The fact is that the indicator and the board of the device have some transparency, and light, penetrating through them, enters the ADC crystal, causing a photoelectric effect. To eliminate this drawback, you need to remove the board and, after removing the indicator, glue the location of the ADC crystal (it is clearly visible through the board) with thick paper.

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

Sergey Bobin. "Repair of electronic equipment" No. 1, 2003