Sewerage and drainpipes portal / Trays / Typical malfunctions of a digital multimeter. Maintenance and repair of radio electronic equipment, measuring instruments and laboratory equipment

Typical malfunctions of a digital multimeter. Maintenance and repair of radio electronic equipment, measuring instruments and laboratory equipment

08.10.2021

SKF flux

In any case, no matter how you dismantle this resistor from the board, bumps of the old solder will remain on the board, we need to remove it using the 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.

Dismantling 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 25-watt soldering iron tip, the pads and leads located at the edges of the resistor, 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 on such repairs were divided, some argued that due to the cheapness of multimeters, it makes no sense to repair them at all, they say they threw it out and went to buy a new one, others were even ready to go to the end 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 handle such repairs. Everyone! AKV.

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 inspection 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.

Defect	Cause	Solution
When the device starts up, the screen lights up and slowly goes out	Talking about a breakdown of the master oscillator, from where the signal is fed to the screen substrate	You need to check two elements: C1 and R15
During startup, the screen lights up and slowly goes out, but if the back cover is missing, the problem does not occur	With the cover closed, the contact helical spring presses on the resistor R15 and closes the master oscillator circuit	You can bend or shorten the spring itself
Screen readings change from 0 to 1 when the instrument 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 put down new ones
Device resets to zero for too long	The reason is a poor-quality capacitor SZ at the input
The readings on the screen are set for a long time during the change of 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 is due to the short-circuiting of the long pins of the transistor test connector.	You just need to open the conclusions
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 the terminals, or the absence of the capacitor itself	It is necessary to replace a working capacitor with a low absorption coefficient
During switching on, the device either beeps or is silent during the dialing	Occurs due to poor-quality soldering of the pins of the IC2 microcircuit	For the solution, you need to solder the conclusions
Disappearing segments on the screen	Bad contact of the screen with the contacts of the board through conductive inserts	It is necessary to fix the conductive rubber inserts, clean the contacts with alcohol and irradiate the contacts on the board

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 terminals and the common output 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 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.

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 the verification and adjustment of various measuring instruments. They are used in both laboratory and field conditions. Among the most popular and well-known devices of this kind are equipment manufactured by Fluke. It has proven itself to be extremely reliable and safe to operate.

However, like any equipment, Fluke universal calibrators can break down, fail and demonstrate inaccuracies in work. In particular, their individual electronic components are damaged, the main elements become unusable. As a result, the device starts to "junk", it turns out to be unable to quickly and accurately 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 depletion of the resource of parts to mechanical and other influences.

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 a question asked 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 of things. Perfect option - isopropyl alcohol- cheap, washes away dirt well and dissolves flux. 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, solidol 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.

We remove the back cover of our Mastech, we 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 the board and the buzzer - that black round big thing. Carefully remove the board from the case. 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 very 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 pulling it out of the slots - oops. Oh no no no. It seems like a well-known company - 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 not regret it. 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. It 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 cause, 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 in grease :) Therefore, we collect, check, disassemble, grease. 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 that 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?

Additionally: