Repair of the electrical part of magnetoelectric ammeters and voltmeters

Such repairs mean making adjustments, mainly in electrical circuits measuring instrument, as a result of which its readings are within the specified accuracy class.

If necessary, adjustment is carried out in one or more ways:

• · change in active resistance in serial and parallel electrical circuits of the measuring device;
• · changing the working magnetic flux through the frame by rearranging the magnetic shunt or magnetizing (demagnetizing) a permanent magnet;
• · changing the counteracting moment.

In the general case, the first step is to install the pointer in a position corresponding to the upper measurement limit at the nominal value of the measured value. When such compliance is achieved, check the measuring device on the numerical marks and record the measurement error on these marks.

If the error exceeds the permissible, then find out whether, by means of adjustment, it is possible to deliberately introduce a permissible error at the end mark of the measurement range, so that the errors at other numerical marks “fit” within the permissible limits.

In cases where such an operation does not give the desired results, the instrument is re-calibrated and the scale is redrawn. This usually occurs after a major overhaul of the measuring instrument.

Adjustment of magnetoelectric devices is carried out when powered DC, and the nature of the adjustments is determined depending on the design and purpose of the device.

According to their purpose and design, magnetoelectric devices are divided into the following main groups:

• · voltmeters with the nominal internal resistance indicated on the dial,
• · voltmeters whose internal resistance is not indicated on the dial;
• · single-limit ammeters with internal shunt;
• · multi-range ammeters with a universal shunt;
• · millivoltmeters without temperature compensation device;
• · millivoltmeters with temperature compensation device.

Adjusting voltmeters that have the nominal internal resistance indicated on the dial

The voltmeter is connected in a series circuit according to the connection circuit of a milliammeter and is adjusted so as to obtain, at the rated current, a deviation of the pointer to the final numerical mark of the measurement range. The rated current is calculated as the quotient of the rated voltage divided by the rated internal resistance.

In this case, adjustment of the deviation of the pointer to the final numerical mark is performed either by changing the position of the magnetic shunt, or by replacing the spiral springs, or by changing the resistance of the shunt parallel to the frame, if any.

A magnetic shunt generally diverts through itself up to 10% of the magnetic flux flowing through the interferon space, and the movement of this shunt towards the overlap of the pole pieces leads to a decrease in the magnetic flux in the interferon space and, accordingly, to a decrease in the angle of deflection of the pointer.

Spiral springs (stretch marks) in electrical measuring instruments serve, firstly, to supply and remove current from the frame and, secondly, to create a moment that counteracts the rotation of the frame. When the frame is rotated, one of the springs is twisted, and the second is untwisted, and therefore a total counteracting moment of the springs is created.

If it is necessary to reduce the angle of deflection of the pointer, then the spiral springs (extensions) present in the device should be replaced with stronger ones, i.e., install springs with an increased counteracting moment.

This type of adjustment is often considered undesirable, since it is associated with painstaking work of replacing springs. However, repairmen who have extensive experience in resoldering spiral springs (stretch marks) prefer this method. The fact is that when adjusting by changing the position of the magnetic shunt plate, in any case, it ends up being shifted to the edge and it is no longer possible to further correct the instrument readings, which are disturbed by the aging of the magnet, by moving the magnetic shunt.

Changing the resistance of the resistor shunting the frame circuit with additional resistance can only be allowed as a last resort, since such current branching is usually used in temperature compensation devices. Naturally, any change in the specified resistance will violate temperature compensation and, in extreme cases, can only be tolerated within small limits. We must also not forget that a change in the resistance of this resistor, associated with the removal or addition of turns of wire, must be accompanied by a long but mandatory operation of aging the manganin wire.

In order to maintain the nominal internal resistance of the voltmeter, any changes in the resistance of the shunt resistor must be accompanied by a change in the additional resistance, which makes adjustment even more difficult and makes the use of this method undesirable.

Adjusting voltmeters whose internal resistance is not indicated on the dial

The voltmeter is turned on, as usual, in parallel with the electrical circuit being measured and adjusted to obtain the deviation of the pointer to the end numerical mark of the measurement range at rated voltage for a given measurement limit. The adjustment is made by changing the position of the plate when moving the magnetic shunt, or by changing the additional resistance, or by replacing the spiral springs (stretch marks). All the comments made above are also valid in this case.

Often the entire electrical circuit inside the voltmeter - the frame and wire resistors - turns out to be burned out. When repairing such a voltmeter, first remove all burnt parts, then thoroughly clean all remaining unburnt parts, install a new moving part, short-circuit the frame, balance the moving part, open the frame and, turning on the device according to the milliammeter circuit, i.e. in series with the standard milliammeter, The total deflection current of the moving part is determined, a resistor with additional resistance is made, the magnet is magnetized if necessary, and finally the device is assembled.

Adjustment of single-limit ammeters with internal shunt

In this case, there may be two cases of repair operations:

• 1) there is an intact internal shunt, and it is necessary, by replacing the resistor with the same frame, to switch to a new measurement limit, i.e., re-calibrate the ampere meter;
• 2) when major renovation The frame of the ammeter was replaced, and therefore the parameters of the moving part changed; it is necessary to calculate, make a new one and replace the old resistor with additional resistance.

In both cases, first determine the total deflection current of the device frame, for which the resistor is replaced with a resistance store and, using a laboratory or portable potentiometer, the resistance and current of the total deflection of the frame are measured using a compensation method. The shunt resistance is measured in the same way.

Adjustment of multi-range ammeters with internal shunt

In this case, a so-called universal shunt is installed in the ammeter, i.e. a shunt, which, depending on the selected upper measurement limit, is connected in parallel to the frame and a resistor with additional resistance in whole or part of the total resistance.

For example, the shunt in a three-limit ammeter consists of three series-connected resistors Rb R2 and R3. Let's say an ammeter can have any of three measurement limits - 5, 10 or 15 A. The shunt is connected in series to the measuring electrical circuit. The device has a common terminal “+”, to which the input of resistor R3 is connected, which is a shunt at the measurement limit of 15 A; resistors R2 and Rx are connected in series to the output of resistor R3.

When an electrical circuit is connected to the terminals marked “+” and “5 A”, the voltage is removed from the series-connected resistors Rx, R2 and R3 to the frame through resistor Rext, i.e. completely from the entire shunt. When the electrical circuit is connected to the “+” and “10 A” terminals, the voltage is removed from the series-connected resistors R2 and R3, and at the same time, the Rx resistor is connected in series to the circuit of the resistor Rext; when connected to the “+” and “15 A” terminals, the voltage is the frame circuit is removed from resistor R3, and resistors R2 and Rx are included in the Rext circuit.

When repairing such an ammeter, two cases are possible:

• 1) the measurement limits and shunt resistance do not change, but in connection with replacing the frame or defective resistor, it is necessary to calculate, manufacture and install a new resistor;
• 2) the ammeter is calibrated, i.e. its measurement limits change, and therefore it is necessary to calculate, manufacture and install new resistors, and then adjust the device.

In case of extreme necessity, which happens in the presence of high-resistance frames, when temperature compensation is needed, a circuit with temperature compensation through a resistor or thermistor is used. The device is verified at all limits, and with the correct adjustment of the first measurement limit and correct production The shunt usually does not require additional adjustments.

Adjustment of millivoltmeters that do not have special temperature compensation devices

The magnetoelectric device contains a frame wound from copper wire, and spiral springs made of tin-zinc bronze or phosphor bronze, the electrical resistance of which depends on the air temperature inside the device body: the higher the temperature, the greater the resistance.

Considering that the temperature coefficient of tin-zinc bronze is quite small (0.01), and the manganin wire from which it is made additional resistor, is close to zero, the temperature coefficient of the magnetoelectric device is approximately assumed:

X pr = Xp (Rр / Rр + R ext)

ammeter voltmeter measuring

where X p is the temperature coefficient of the copper wire frame, equal to 0.04 (4%). It follows from the equation that in order to reduce the influence on the instrument readings of deviations of the air temperature inside the case from its nominal value, the additional resistance must be several times greater than the resistance of the frame. The dependence of the ratio of additional resistance to frame resistance on the accuracy class of the device has the form

R ext /R r = (4 - K / K)

where K is the accuracy class of the measuring device.

From this equation it follows that, for example, for devices of accuracy class 1.0, the additional resistance should be three times greater than the resistance of the frame, and for accuracy class 0.5 it should be seven times greater. This leads to a decrease in the usable voltage on the frame, and in ammeters with shunts - to an increase in the voltage on the shunts. The first causes deterioration in the characteristics of the device, and the second causes an increase in shunt power consumption. Obviously, the use of millivoltmeters that do not have special temperature compensation devices is advisable only for panel devices of accuracy classes 1.5 and 2.5.

The readings of the measuring device are adjusted by selecting additional resistance, as well as by changing the position of the magnetic shunt. Experienced repairmen also use magnetization of the permanent magnet of the device. When adjusting, turn on the connecting wires included with the measuring device or take into account their resistance by connecting a resistance magazine with the corresponding resistance value to the millivoltmeter. When repairing, they sometimes resort to replacing spiral springs.

Adjustment of millivoltmeters with temperature compensation device

The temperature compensation device allows you to increase the voltage drop across the frame without significantly increasing the additional resistance and power consumption of the shunt, which dramatically improves the quality characteristics of single-limit and multi-limit millivoltmeters of accuracy classes 0.2 and 0.5, used, for example, as ammeters with a shunt . At a constant voltage at the millivoltmeter terminals, the measurement error of the device due to changes in air temperature inside the case can practically approach zero, i.e., be so small that it can be ignored and ignored.

If, when repairing a millivoltmeter, it is discovered that it does not have a temperature compensation device, then such a device can be installed in the device to improve the characteristics of the device.

Previously, I had seen this device only in color photos on the Internet, but now I saw it on the market; the glass is broken, some ancient batteries are attached to the body and all this is covered with a layer of, to put it mildly, dust. And I remember the ampere-voltmeter - transistor tester TL-4M because, unlike many others, it can check, in addition to the gain, other characteristics of transistors:

• reverse current of the collector-base (Ik.o.) and emitter-base transitions (Ie.o.)
• initial collector current (Ic.p.) from 0 to 100 μA;

At home I disassembled the case - the measuring head burst in half, five wire-wound resistors burned almost to the state of embers, the balls fixing the position of the dial switch are no longer round, and only scraps stick out from the connection block for the transistors being tested. I didn’t take any photos, but now I regret it. A comparison would also provide clear confirmation of the commonly held opinion that the devices of that time were practically indestructible.

Of all the restoration work, the longest and most painstaking was the general cleaning of the device. I didn’t wind the resistors, but installed the usual OMLTs (clearly visible - the left row, all “sawed”), finely adjusted to the desired value with a “velvet” file. Everything else from electronic components was intact.

Finding a new original connector for the transistors being tested, as well as restoring the old one, was not realistic, so I picked up something more or less suitable and cut something off, glued something on, and in the end, in a functional sense, the replacement was a great success. I didn’t like turning the dial switch to “zero” (turn off the power) every time after finishing measurements - I installed a slide switch on the power compartment. Luckily a place was found. The measuring head turned out to be in good working order, I just glued the body together. The switch balls were made of plastic (“bullets” from a children’s pistol).

To connect transistors with short legs, I made extension cords with alligator clips, and for ease of use, two pairs of connecting wires (with probes and with alligator clips). That's all. After power was applied, the device started working in full. If there are any errors in the measurements, they are clearly insignificant. A comparison of current, voltage and resistance measurements with a Chinese multimeter did not reveal any significant differences.

I categorically disagreed with looking for standard batteries for the power compartment in stores every time. Therefore, I came up with the following: I removed all the contact plates, in order for two “AA” batteries to fit into the compartment along the width, I made a cut measuring 9 x 60 mm in the side wall from the side of the device compartment, and “removed” the excess free space along the length thanks to the manufactured inserts with contact springs.

If anyone happens to “repeat”, then using this sketch it will not be difficult to do so.

How to repair a V7-40 voltmeter? Typical faults.

Equipment required for repair and calibration(the equipment used is written in brackets):

tester (MY64); oscilloscope (GDS-820); calibrator (H4-6); resistance magazine (P3026).

Abbreviations used:

1.cr. – red probe of the tester (polarity +), i.e. signal probe

2.black - black probe of the tester (polarity -), i.e. body probe

3. four-digit number of the form – readings from the MY64 tester in dialing mode

4. designations of a field-effect transistor: i – source, c – drain, z – gate, j – body

Some tips before renovation.

If you are repairing a voltmeter for the first time or are experiencing some difficulties during repair, then I advise you to look through technical description. It quite clearly describes the principle of operation of the device and its functional units. I will give just a couple of additional aspects.

Logic of conversion boards (boards 1 and 2): “0” = -13V, “1” = 0V.

Continuity of the field-effect transistor (using a tester): i-s → ≈; cr. z – black and → ≈; black.z - cr. and → ∞

Where to begin?

So, in front of you stands a non-working V7-40 voltmeter and you are full of enthusiasm and determination to make an excellent working device from a pile of scrap metal. First of all, it is necessary to determine which functional unit is faulty. In a simplified form, there are 4 of them: power supply, input devices (protection, voltage dividers, V~, I, R to V= converters), ADC (elements that convert V= into a time interval), control unit (elements responsible for the operating mode , limit selection, indication).

We will determine by external signs where to climb first.

The device does not turn on, the indicators do not light up - check for the presence of +5V supply voltage.

After switching on, the indicators show frozen readings - see control unit (FS “Hold”) → power supply.

The device turned on, but the operating mode and limits are not set correctly - power supply → control unit.

The device is turned on, operating modes and limits are switched properly, but the readings at the limits of 0.2V= and 2V= differ from the input voltage values ​​- power supply → ADC → input devices → control unit.

The voltmeter does not measure (zero readings, distorted readings, overload) in modes V~, I, R, V= >2V – input devices→ ADC→ control unit→ power supply.

Power supply malfunction.

Malfunctions of the digital stabilizer.

1) When the device is turned on, the indicators do not light up and the stabilizer does not squeak.

The +5V power supply has shorted to the housing on the interface unit or COP/CPU board. Most often due to deformation of the covers or poor fastening of the board.

2) There is no +5V power supply.

Capacitor C8 is faulty;

Poor contact of inductance L1;

The D1 142EP1 chip is faulty (without load the power supply is +4V, with load - +0.7V).

3) Large ripples ≈1V.

Capacitor C8 is faulty.

Malfunctions of the analog stabilizer.

The R→V= converter is faulty: the zener diode VD10 and transistor VT3 on board 6.692.040 are broken.

2) Voltages increased -15V to -13V, -13V to -11V.

Transistor VT16 on board 6.692.050 is faulty.

3) The power supply is connected to -13V (transistor VT16 is intact).

The digital chip (several/all) in the analog part is faulty.

Method for finding a faulty microcircuit:

1. Solder the pins of the microcircuits connecting -13V and common ┴.

2. Call for food: kr. – -13V, black. - ┴ →; black – -13V, cr. - ┴→∞.

3. We call the pins of the microcircuits -13V - ┴, the faulty one will not have ∞.

The faulty microcircuit can be soldered back and make sure that it supplies power.

General information on troubleshooting ADCs.

In the V7-40 voltmeter, the ADC is assembled using a double integration circuit and operates in 3 steps. Step 1 – stored on capacitor C22 input voltage. Step 2 – capacitor C22 is discharged by the reference voltage. Step 3 – correction of the ADC zero. Accordingly, it is necessary to determine at what step the failure occurs. For this purpose, Appendix 6, Part 2 of Maintenance provides voltage diagrams at control points.

First, let's make sure that it is the ADC that is not working. To do this, short-circuit the input/feed constant pressure and look at pin 23 “in V=” to see what input voltage is supplied to the ADC. If 0/applied voltage, and the display shows other numbers, it means the ADC is faulty. Otherwise, the fault lies in the input circuits. If in doubt, you can solder pin 23 to the common wire.

It was determined that the fault was in the ADC. Now let’s see if there is a direct integration pulse on pin 8 “T0”. If it is missing, then it is necessary to analyze the passage of this signal through the microcircuits.

Everything is fine with the T0 pulse, so let’s check reference voltage: KT2 – -1V, KT4 – -0.1V, KT3 – +10V. Voltages -1V and/or -0.1V may differ slightly from the nominal voltage due to faulty field-effect transistors. If all 3 voltages are incorrect (and significantly), then this is a clear sign of a faulty reference voltage source.

The support is normal, but the device still “does not breathe.” I suggest putting off brainstorming for now and calling field effect transistors on board 6.692.040. It is not necessary to solder them - we are looking for obviously dead ones. To do this, we call i-s (to the break) and z-i, s, k (to the short). This, of course, is not a 100% option, but sometimes it helps to detect a faulty element without a thorough analysis of the breakdown.

Still not working? Apparently, the stars in the sky have aligned in an unfavorable way and according to your horoscope, today is a bad day for you. You will have to thoroughly delve into the device and analyze the operation of digital microcircuits. To do this, we look at the input and output of the microcircuit and analyze the results obtained. If in doubt, you can give up on the working microcircuit. I advise you to first read ADC malfunctions and control unit malfunctions.

ADC malfunctions.

1) With warming up, the error +V= increases sharply.

Defective element D14.1 564LA9 on sq. 6.692.040.

2) Very large measurement error -V=.

Transistors VT10, VT19 KP303G on the square are faulty. 6.692.040.

3) The readings of the last discharge flicker within 200 mV= and 20 V=.

ADC excitation associated with interference from pulse block supply +5V → replacement of C8.

The analog block contains boards from 1987 with R47, which is not present in newer devices → short-circuit R47.

4) Incorrect reference voltage.

Replacement of microcircuits D1, D3, transistors VT1, VT20 on the square. 6.692.040.

5) There are no T0 pulses.

The D14 564LA9 microcircuit on the square is faulty. 6.692.040.

6) No 0 when the input is short-circuited, distorted readings during measurements.

The power supply is faulty.

7) The device starts working if you attach the oscilloscope probe to the CT scanner.

The D7 564LN2 microcircuit on the square is faulty. 6.692.050 (broken 2 legs in the microcircuit).

8) It is not possible to set 0 with a short-circuited input (readings float ±5 e.m.r.).

Transistor VT23 is faulty.

A little about management.

The operation of the digital part of the voltmeter is described in some detail in the technical documentation. In addition, the breakdown of the control part did not have to be repaired often. Therefore, if the device does not switch operating modes, the commas do not light up, etc., then we find the element responsible for the function we are interested in and analyze the passage of the control signal. The only thing I would like to pay attention to is the “hold” signal generator. The thing is unnecessary, but creates problems. If the device readings are frozen and do not respond to manipulations with the device, then check the operation of the “Hold” FS.

Control related problems.

1) Blocking measurements at input AC voltage≥ 400V.

Using an oscilloscope, we observe on R61 (pl. 6.692.050) pulses of the corresponding frequency of the applied voltage as the input voltage increases. Add capacitance (≥22nF) to the connection point between K13.2 and R61.

2) When the device is turned on, readings other than 0 are displayed on the display and do not change with further manipulations with the device.

The MKA-10501 reed switch is stuck in relay K13 on board 6.692.050.

3) When you press the limit switching button “→”, the ohmmeter mode is activated.

The R mode switch input is poorly connected to +5V power and 5V power with ripples greater than normal.

4) Periodically (5-10 times a day) the relay spontaneously clicks and the overload is displayed.

Relay K10 clicks → chip D11 564TM3 on board 6.692.050 is faulty.

5) Limits and operating mode are not switched.

Replacement of D18 133LN1 in the connecting block.

6) Commas do not appear.

Replacement of D32 134ID6 in the connecting block.

7) Relays do not click when switching modes

No 6V power

There is 6V power supply. Transformer T3 is broken → the control signal from the digital part did not enter the analog part.

Input converters.

The principle of operation here is quite simple. The input physical quantity (V~, I=, I~, R) is converted to V=. The maximum input voltage of the ADC is 2V, so dividers + protection are used in the input circuits. So, we have determined which mode does not work. We are looking for the element on which the converter is assembled. We applied V~,/ I=,/ I~,/ R to the input (can be short-circuited) and analyze how the conversion occurs.

Malfunctions of input converters.

1) Measures V= after applying voltage 2 times.

VT5, VT8 KP303G pl. are faulty. 6,692,050 (died).

2) No 0 when the input is closed.

At pin 23 “in U=” a voltage of -17 mV is observed → VT5, VT8 KP303G pl. are faulty. 6.692.050.

3) At the limit of 20V= there is no 0 with a short-circuited input (readings -4-10 e.m.r.).

1. Poor contact of pin 4 of the voltage divider board.

4) Does not measure R - overload.

The D4 544UD1A chip is faulty. It is checked as follows: the zener diode VD7 rings in the return line, if the tester readings are different from [∞], then the microcircuit is faulty. Usually more than one microcircuit burns, so you should check VD7, VD10, VT2, VT3, R35 pl. 6.692.040 and VT9, VT11, VD29, VD30 on the square. 6.692.050.

5) Distorted readings when measuring R 1 kOhm at the input = 0.6 kOhm on the indicator.

1kOhm is applied to the input, look at the converted voltage on R6 (pl. 6.692.050) → voltage -1V, therefore, the ohmmeter is working. At pin 23 “in U=” the voltage is -0.6V → the ADC protection is faulty. In this case, the zener diode is VD8.

6) Chaotic readings in R mode.

Poor contact in relay K1.2 between contacts 2 and 4. It is detected as follows: remove the cover from the RV-5A relay and carefully press the closing contact.

7) Long time to establish zero R readings.

After setting 0, we make a break, short-circuit the input again and observe a long installation of zero values: the protection transistors VT9, VT11 (dead and -c) on board 6.692.050 are faulty.

8) No zero reading with shorted input.

VT13 pl. faulty 6.692.040.

9) Error at the limits of 2 and 20 MOhm > tolerance.

1. Leakage of transistor VT11

2. Half-dead capacitor C14

3. If, after checking the ohmmeter elements, no faulty elements are found, then try drying the 6.692.040 plate. To do this we install table lamp above the board, so that the elements warm up well and leave for 3 hours. If this does not help, then you need to look for a faulty element and moisture has nothing to do with it.

10) Large error at the limit of 20 MΩ (readings are greatly underestimated)

The error at the limit of 2 MΩ is normal. If the device is left for some time (~1-2 hours) at the limit of 20 MOhm, the error is leveled out. When switching to the 2MΩ limit and back, the voltmeter returns to the inoperative state. Therefore, we look at what changes when switching limits. I had to unsolder all the elements responsible for 2MΩ to determine that the D21 chip on board 6.692.050 was faulty.

11) There is not enough adjustment at the 20 kOhm limit.

The reference resistor R78 988 kOhm±0.1% (usually >0.1%) is faulty.

12) Does not measure I.

1. The current fuse has blown/poor contact between the fuse and the terminal.

2. Check the shunt.

Conclusion.

Of course, I understand that the V7-40 voltmeter is an outdated device and now you can buy better equipment. But I hope that my efforts in writing this article will not be in vain and will be useful to someone ;)/> . End of connection.