How to repair a V7-40 voltmeter? characteristic faults.
Necessary equipment for repair and calibration(in brackets the equipment used is written):
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 - the readings of the MY64 tester in the dialing mode
4. FET designations: i - source, c - drain, h - gate, k - case
Some tips before renovation.
If you are repairing a voltmeter for the first time or experiencing some difficulties during repairs, 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 only a couple of additional aspects.
Logic of conversion boards (boards 1 and 2): "0" = -13V, "1" = 0V.
FET continuity (using a tester): i-s → ≈; cr. h - cher. and → ≈; cher.z - cr. and → ∞
Where to begin?
So, you have a non-working B7-40 voltmeter in front of you and you are full of enthusiasm and determination to make an excellent working device out of 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 in the first place.
The device does not turn on, the indicators do not light up - we look for the presence of a supply voltage of + 5V.
After switching on, frozen readings on the indicators - see the 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 turned on, the operating modes and limits switch properly, but the readings at the limits of 0.2V= and 2V= differ from the values of the input voltage - power supply → ADC → input devices → control unit.
The voltmeter does not measure (zero readings, distorted readings, overload) in V~, I, R, V= >2V modes - input devices → ADC → control unit → power supply.
Power supply failure.
Malfunctions of the digital stabilizer.
1) When the device is turned on, the indicators do not light up, the squeak of the stabilizer is not heard.
The +5V supply has shorted to the case on the board of the interface unit or CPC / CPU. Most often due to deformation of the covers or poor-quality fastening of the board.
2) No +5V power supply.
Faulty capacitor C8;
Bad contact inductor L1;
The D1 142EP1 chip is faulty (with no load, the power supply is + 4V, with a load - + 0.7V).
3) Large ripple ≈1V.
Faulty capacitor C8.
Analog stabilizer malfunctions.
Converter R→V= is faulty: zener diode VD10 and transistor VT3 are broken on board 6.692.040.
2) The voltages are -15V to -13V, -13V to -11V.
Faulty transistor VT16 on board 6.692.050.
3) Power supply -13V is connected (transistor VT16 is intact).
Faulty digital chip (several / all) in the analog part.
Method for finding a faulty chip:
1. Solder the microcircuit legs connecting -13V and common ┴.
2. We call food: kr. - -13V, cher. - ┴ →; black - -13V, cr. - ┴→∞.
3. We call the legs of microcircuits -13V - ┴, the faulty one will not have ∞.
A faulty chip can be soldered back and make sure that it adds power.
General information on ADC troubleshooting.
In the V7-40 voltmeter, the ADC is assembled according to the double integration scheme and operates in 3 steps. Step 1 - the input voltage is stored on the capacitor C22. Step 2 - capacitor C22 is discharged by the reference voltage. Step 3 - ADC zero correction. Accordingly, it is necessary to determine at which step the failure occurs. To do this, in Appendix 6, Part 2 TO, voltage diagrams at control points are given.
First, let's make sure that it is the ADC that is not working. To do this, we short-circuit the input / apply a constant voltage and look at pin 23 “in V =”, what input voltage is supplied to the ADC. If 0 / applied voltage, and other numbers on the display, then the ADC is faulty. Otherwise, the fault lies in the input circuits. If in doubt, contact 23 can be soldered to a common wire.
Determined that the malfunction in the ADC. Now we look to see if there is a direct integration pulse on pin 8 "T0". If it is absent, then it is necessary to analyze the passage of this signal through the microcircuits.
Everything is fine with the T0 pulse, which means we check the reference voltage: KT2 - -1V, KT4 - -0.1V, KT3 - + 10V. Voltages of -1V and/or -0.1V may vary slightly from nominal due to faulty FETs. If all 3 voltages are incorrect (and significantly), then this is a clear sign of a malfunction of the reference voltage source.
The reference is normal, but the device still “does not breathe”. I suggest brainstorming for the time being to postpone and ring FETs on board 6.692.040. It is not necessary to solder - we are looking for obviously dead ones. To do this, we call i-s (to a break) and s - i, s, k (to kz). This, of course, is not a 100% option, but sometimes it helps to detect a faulty element without a thorough breakdown analysis.
Still not working? Apparently, the stars in the sky converged in an unfavorable way and according to the horoscope you have an unlucky day today. You will have to thoroughly dig into the device and analyze the work digital microcircuits. To do this, we look at the input and output of the microcircuit and analyze the results. If in doubt, you can give up on a working microcircuit. I advise you to start by reading ADC malfunctions and control unit malfunctions.
ADC malfunctions.
1) With warming up, the error +V= increases sharply.
Defective element D14.1 564LA9 on the square. 6.692.040.
2) Very large measurement error -V=.
Faulty transistors VT10, VT19 KP303G on the square. 6.692.040.
3) The readings of the last discharge flicker within 200 mV= and 20 V=.
ADC excitation associated with pickup from impulse block supply + 5V → replacement C8.
The analog block has 1987 boards with R47, which is not in newer devices → short R47.
4) Wrong reference voltage.
Replacement of microcircuits D1, D3, transistors VT1, VT20 on the square. 6.692.040.
5) No T0 pulses.
Faulty chip D14 564LA9 on the square. 6.692.040.
6) No 0 with shorted input, distorted measurement readings.
Faulty power supply.
7) The device starts working if you connect the oscilloscope probe to the CT.
Faulty chip D7 564LN2 on the square. 6.692.050 (breakage of 2 legs in the microcircuit).
8) It is not possible to set 0 when the input is shorted (readings float ±5 digits).
Faulty transistor VT23.
A little about management.
The operation of the digital part of the voltmeter is described in some detail in TO. In addition, the breakdown of the control part had to be repaired infrequently. Therefore, if the device does not switch operating modes, commas do not light, and the like, then we find the element responsible for the function of interest to us, and analyze the passage of the control signal. The only thing I want to pay attention to is the “hold” signal generator. The thing is not needed, but it creates problems. If the readings of the device are frozen and do not respond to manipulations with the device, then check the operation of the FS "Hold".
Control related faults.
1) Blocking of measurements at the input AC voltage≥ 400V.
Using an oscilloscope, we observe on R61 (pl. 6.692.050) pulses of the corresponding frequency of the applied voltage with increasing input voltage. Add capacitance (≥22nF) to the connection point of K13.2 and R61.
2) When the device is turned on, the display shows indications other than 0 and does not change during further manipulations with the device.
Stuck reed switch MKA-10501 in relay K13 on board 6.692.050.
3) Pressing the limit switching button "→" turns on the ohmmeter mode.
The mode switch input R is poorly connected to the +5V supply and the 5V supply with more than normal ripple.
4) Periodically (5-10 times a day) spontaneously clicks the relay and overload is displayed.
Relay K10 clicks → the D11 564TM3 chip on the board 6.692.050 is faulty.
5) Do not switch limits and operation mode.
Replacement of D18 133LN1 in the connecting block.
6) Commas do not burn.
Replacement of D32 134ID6 in the connecting block.
7) Do not click the relay when switching modes
No 6V power
There is a 6V supply. Open transformer T3 → the control signal from the digital part did not go to the analog.
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 determined which of the modes 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 takes place.
Malfunctions of input converters.
1) Measures V= after applying voltage for 2 times.
Faulty VT5, VT8 KP303G pl. 6.692.050 (died and-s).
2) No 0 when the input is closed.
On pin 23 "in U =" voltage -17mV is observed → faulty VT5, VT8 KP303G pl. 6.692.050.
3) At the 20V= limit, there is no 0 when the input is shorted (reading -4-10 num).
1. Poor contact of output 4 of the voltage divider board.
4) Does not measure R - overload.
Faulty chip D4 544UD1A. It is checked as follows: the VD7 zener diode is called in the return line, if the tester readings are different from [∞], then the microcircuit is faulty. Usually, the microcircuit does not burn alone, 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.
1kΩ is applied to the input, we look at the converted voltage at R6 (pl. 6.692.050) → voltage -1V, therefore, the ohmmeter is working. On pin 23 “in U=” voltage -0.6V → ADC protection is faulty. In this case, the zener diode VD8.
6) Chaotic readings in R mode.
Bad contact in relay K1.2 between 2 and 4 contacts. It is detected as follows: the cover is removed from the RV-5A relay and the closing contact is carefully pressed.
7) Long establishment of zero readings R.
After setting 0, we make a break, short the input again and observe a long installation zero values: faulty protective transistors VT9, VT11 (dead and-s) on the board 6.692.050.
8) No zero reading with shorted input.
Faulty VT13 square. 6.692.040.
9) Error within 2 and 20 MΩ > tolerance.
1. Leakage transistor VT11
2. Half-dead capacitor C14
3. If, after checking the elements of the ohmmeter, no faulty elements are found, then try to dry pl.6.692.040. For this we set table lamp above the board, so that the elements warm up well and leave for 3 hours. If it doesn’t help, then you need to look for a faulty element and moisture has nothing to do with it.
10) Big error at the limit of 20MΩ (readings are greatly underestimated)
The error at the limit of 2MΩ is normal. If the device is left for some time (~1-2 hours) at the limit of 20 MΩ, then the error levels out. When switching to the 2MΩ limit and back, the voltmeter returns to Out of Service. Therefore, we look at what changes when switching limits. I had to solder all the elements responsible for 2MΩ to determine if the D21 chip on the 6.692.050 board was faulty.
11) There is not enough adjustment at the limit of 20 kOhm.
Defective reference resistor R78 988 kOhm ± 0.1% (typically> 0.1%).
12) Does not measure I.
1. Current fuse blown / poor contact of the fuse with the terminal.
2. Check the shunt.
Conclusion.
Of course, I understand that the B7-40 voltmeter is an outdated device and now you can buy better equipment. But I hope that my work on writing this article will not be wasted and will be useful to someone;) /> . End of connection .
Such repair refers to the implementation of adjustments, to a greater extent in electronic circuits. measuring instrument, as a result of which its readings are within the boundaries of a given accuracy class.
As needed, adjustment is made by one or more methods:
configuration of active resistance in serial and parallel electronic circuits of the measuring device;
configuration of the working magnetic flux through the frame by means of rearranging the magnetic shunt or magnetization (demagnetization) of the permanent magnet;
torque configuration.
In the general case, the pointer is first set to the position corresponding to the upper measurement limit at the nominal value of the measured quantity. When such agreement is reached, check the measuring instrument at the numerical marks and record the measurement error at these marks.
If the error exceeds the allowable one, then they will find out whether it is possible to deliberately introduce the allowable error at the final mark of the measurement spectrum by the adjustment method, so that the errors at other numerical marks “fit” within the allowable limits.
In cases where such an operation does not give suitable results, a new calibration of the instrument is created with the scale redrawn. As a rule, this occurs after a long repair of the measuring device.
The adjustment of magnetoelectric devices is done when powered by a constant current, and the nature of the adjustments is set depending on the design and purpose of the device.
By purpose and design, magnetoelectric devices are divided into the following main groups:
- voltmeters with nominal internal resistance marked on the dial,
- voltmeters, in which the internal resistance is not indicated on the dial;
- single limit ammeters with internal shunt;
- multirange ammeters with a universal shunt;
- millivoltmeters without temperature compensation device;
- millivoltmeters with temperature compensation device.
Adjustment of voltmeters, in which the dial is indicated
nominal internal resistance
The voltmeter is connected in turn to the circuit according to the milliammeter switching circuit and adjusted so as to obtain at the rated current the deviation of the pointer to the final numerical mark of the measurement spectrum. The rated current is calculated as a personal dividing of the rated voltage by the rated internal resistance.
With all this, the adjustment of the difference of the pointer to the final numerical mark is done either by configuring the position of the magnetic shunt, or by replacing the coil springs, or
shunt resistance configuration parallel to the loop, if any.
The magnetic shunt generally removes through itself up to 10% of the magnetic flux flowing through the inter-iron space, while moving this shunt towards the overlapping of the pole pieces leads to a decrease in the magnetic flux in the inter-iron space and, accordingly, to a decrease in the pointer difference angle.
Spiral springs (stretch marks) in electrical measuring devices serve, firstly, to supply and drain 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 one is untwisted, in connection with which a total counteracting moment of the springs is created.
If you need to reduce the angle of difference of the pointer, then you should change the coil springs (stretch marks) available in the device to stronger ones, that is, install springs with an increased counteracting moment.
This type of adjustment is often referred to as unnecessary, because it is associated with careful work to replace the springs. But repairmen who have extensive experience in resoldering coil springs (stretch marks) prefer this particular method. The fact is that when adjusting the position of the magnetic shunt plate by the configuration, in any case, it eventually turns out to be shifted to the edge and there is no possibility to correct the instrument readings disturbed by the aging of the magnet in the forthcoming movement of the magnetic shunt.
Changing the resistance of the resistor shunting the loop circuit with additional resistance can only be allowed as a last resort, because such a branching of the current is usually used in temperature compensation devices. Naturally, any change in the indicated resistance will violate the temperature compensation and in the latter case can only be allowed 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-term, but inexorable aging operation of the manganin wire.
In order to maintain the nominal internal resistance of the voltmeter, any configuration of the shunt resistor resistance must be accompanied by an additional resistance configuration, which is even more
makes adjustment difficult and makes the use of this method unnecessary.
Adjustment of voltmeters, which have an internal
resistance is not indicated on the dial
The voltmeter is connected, as usual, in parallel with the measured electronic circuit and adjusted to obtain the deviation of the pointer to the final numerical mark of the measurement spectrum at rated voltage for a given measurement limit. Adjustment is made by changing the position of the plate when moving the magnetic shunt, or by means of configuring additional resistance, or by replacing coil springs (stretch marks). All the remarks made above are valid in this case as well.
Often the entire electronic circuit inside the voltmeter - the frame and wire resistors - is burned. When repairing such a voltmeter, all burnt parts are first removed, then all remaining unburned parts are painstakingly cleaned, the newest moving part is installed, the frame is short-circuited, the moving part is balanced, the frame is opened and, turning on the device according to the milliammeter circuit, i.e. alternately with an approximate milliammeter, determine the current of the total difference of the moving part, make a resistor with additional resistance, magnetize the magnet as necessary, and finally assemble the device.
Adjustment of single-limit ammeters with internal shunt
With all this, there can be two options for repair operations:
1) there is an intact internal shunt, and it is required by replacing the resistor at the same frame to go to
new measurement limit, i.e. re-calibrate the ampere
meter;
2) when the ammeter was completely repaired, it was replaced
frame, in connection with which the characteristics of the movable
parts, you need to calculate, make a new one and change
an old resistor with additional resistance.
In both cases, the full difference current is first determined
the frame of the device, why replace the resistor with a resistance store and, using a laboratory or portable potentiometer, determine the resistance and current of the total difference in a compensation way
framework. The same method determines the resistance of the shunt.
Adjustment of multi-range ammeters with internal
shunt
In this case, the so-called universal shunt is installed in the ammeter, i.e., a shunt that, in
depending on the selected upper limit of measurements, they are connected in parallel with the frame and the resistor with additional resistance in full or in part from the total resistance.
For example, a shunt in a three-limit ammeter consists of 3 alternately connected resistors Rb R2 and R3. Suppose an ammeter can have any of the 3 measurement limits - 5, 10 or 15 A. The shunt is cut in turn into the measuring electronic circuit. The device has a common terminal "+", to which the input of the resistor R3 is connected, which is a shunt at the measurement limit of 15 A; Resistors R2 and Rx are alternately connected to the output of resistor R3.
When connecting an electronic circuit to the terminals marked “+” and “5 A”, to the frame through a resistor
Radd removes voltage from the alternately connected resistors Rx, R2 and R3, i.e., completely from the entire shunt. When the electronic circuit is connected to the “+” and “10 A” terminals, the voltage is removed from the alternately connected resistors R2 and R3, and with all this, the resistor Rx turns out to be connected in turn to the resistor circuit
R ext, when connected to the “+” and “15 A” terminals, the voltage in the frame circuit is removed from the resistor R3, and the resistors R2 and Rx are included in the circuit
R ext.
When repairing such an ammeter, two options are likely:
1) the measurement limits and the resistance of the shunt do not change, but due to the replacement of the frame or defective
resistor must be calculated, made and installed
new resistor;
2) the ammeter is calibrated, i.e. its measurement limits change, and therefore it is necessary to
count, make and install new resistors,
after that, adjust the device.
In the case of the latter necessity, which happens in the presence of high-resistance frames, when temperature compensation is needed, a circuit with temperature compensation by means of a resistor or thermistor is used.
The device is verified at all boundaries, while with the correct fit of the first measurement limit and the correct manufacture of the shunt, additional adjustments are usually not required.
Adjustment of millivoltmeters without devices
special temperature compensation
The magnetoelectric device has a frame wound from copper wire and helical springs made of tin-zinc bronze or phosphor bronze, the electronic resistance of which depends on the air temperature inside the device case: the higher the temperature, the greater the resistance.
Taking into account that the temperature coefficient of tin-zinc
bronze is small enough (0.01), and the manganin wire from which it is made
additional resistor, close to zero, approximately consider the temperature
coefficient of magnetoelectric device:
Xpr \u003d Xp ( R p / R p
+ R ext)
where Xp 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 impact on the instrument readings of deviations in the air temperature inside the housing from its nominal value, an additional
resistance should be several times greater than the resistance of the frame.
The dependence of the case of additional resistance to the resistance of the frame on the accuracy class of the device has the form
Radd / Rp = (4 - K / K)
where K is the accuracy class of the measuring device.
It follows from this equation that, for example, for devices of accuracy class 1.0, the additional resistance should be three times the resistance of the frame, and for accuracy class 0.5, seven times more. This leads to a decrease in the useful voltage across the frame, and in ammeters with shunts - to an increase in the voltage across the shunts. The first causes a deterioration in the features of the device, and the second - an increase in the power consumption of the shunt. Of course, the introduction of millivoltmeters that do not have special temperature compensation devices is purposeful only for panel devices of accuracy classes 1.5 and 2.5.
The adjustment of the readings of the measuring device is done by selecting an additional resistance, as well as by configuring the position of the magnetic shunt. Experienced repairmen also use magnetization of the permanent magnet of the device. When adjusting, the connecting wires included in the measuring instrument kit are included or their resistance is taken into account by connecting a resistance store with a suitable resistance value to the millivoltmeter. When repairing, from time to time they resort to replacing the coil springs.
Adjustment of millivoltmeters with a device
temperature compensation
The temperature compensation device allows you to increase the voltage drop across the frame without resorting to a significant increase in additional resistance and power consumption of the shunt, which dramatically improves the high-quality properties of single-limit and multi-limit millivoltmeters of accuracy classes 0.2 and 0.5, used, for example, as ammeters with shunt. With a constant voltage at the terminals of the millivoltmeter, the measurement error of the device from the configuration of the air temperature inside the case can actually approach zero, i.e., be so small that it can be ignored and ignored.
If, during the repair of a millivoltmeter, it turns out that it contains
there is no temperature compensation device, then to improve the features
device such a device can be installed in the device.
Previously, I had to see this device only on color photos on the Internet, but then I saw it on the market; the glass is broken, some ancient batteries are attached to the case, and all this is covered with a layer, to put it mildly, of dust. And I remember the ampervoltmeter - the tester of TL-4M transistors in that, unlike many others, they can check, in addition to the gain, other characteristics of transistors:
- reverse current collector-base (Ik.o.) and emitter-base (Ie.o.)
- initial collector current (Ik.p.) from 0 to 100 μA;
At home, I dismantled the case - the measuring head burst in half, five wire resistors burned almost to the state of coals, the balls fixing the position of the disk switch are far from round, only lumps stick out from the connection block of the tested transistors. I didn't take pictures, but now I'm sorry. The comparison would also give a visual confirmation of the justly prevailing opinion that the devices of that time were practically unkillable.
Of all the restoration work, the longest and most painstaking was the general cleaning of the device. I didn’t wind the resistors, but put the usual OMLTs (it’s clearly visible - the left row, all “sawn”), finely tuned to the desired value with a “velvet” needle file. Everything else from electronic components it was whole.
It was not realistic to find a new original block for connecting the transistors to be checked, as well as to restore the old one, so I picked up something more or less suitable and cut something off, glued something and as a result, in a functional sense, the replacement was a success. I did not like to turn the disk switch every time after the end of the measurements to "zero" (turn off the power) - I put a slide switch on the power compartment. Fortunately, the place was found. The measuring head turned out to be serviceable, only the case was glued together. I put plastic switch balls (“bullets” from a children's gun).
To connect transistors with short "legs", I made extension cords with crocodile clips, and for ease of use, two pairs of connecting wires (with probes and with "crocodiles"). And that's it. After the power was turned on, the device started working in full. If there are any errors in the measurements, then they are clearly insignificant. Comparison of measuring current, voltage and resistance with a Chinese multimeter did not reveal significant differences.
I categorically disagreed to look for regular batteries for the power compartment every time I went shopping. Therefore, I came up with the following: I removed all the contact plates, in order to fit two “finger-type” batteries into the compartment in width, I made a cut 9 x 60 mm in size in the side wall from the side of the device compartment, and the excess free space“removed” 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 this.
Such a repair is understood as the implementation of adjustments, mainly in the electrical circuits of the measuring device, as a result of which its readings are within the specified range.
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;
change in the opposing moment.
In the general case, the pointer is first set to the position corresponding to the upper limit of measurements at the nominal value of the measured quantity. When such agreement is reached, check the measuring instrument at the numerical marks and record the measurement error at these marks.
If the error exceeds the allowable one, then it is found out whether it is possible to deliberately introduce the allowable error at the final mark of the measurement range by adjusting it so that the errors at other numerical marks “fit” within the allowable limits.
In cases where such an operation does not give the desired results, the instrument is calibrated again with the scale redrawn. This usually occurs after a major overhaul of the meter.
Adjustment of magnetoelectric devices is carried out with power direct current, and the nature of the adjustments is set depending on the design and purpose of the device.
By purpose and design, magnetoelectric devices are divided into the following main groups:
- voltmeters with nominal internal resistance indicated on the dial,
- voltmeters, in which the internal resistance is not indicated on the dial;
- single limit ammeters with internal shunt;
- multirange ammeters with a universal shunt;
- millivoltmeters without temperature compensation device;
- millivoltmeters with temperature compensation device.
Adjustment of voltmeters, which have a nominal internal resistance indicated on the dial
The voltmeter is connected in a serial circuit according to the milliammeter switching circuit and adjusted so as to obtain at the rated current the 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 .
In this case, the deviation of the pointer to the final numerical mark is adjusted either by changing the position of the magnetic shunt, or by replacing the coil springs, or by changing the resistance of the shunt parallel to the frame, if any.
The magnetic shunt generally removes through itself up to 10% of the magnetic flux flowing through the inter-iron space, and the movement of this shunt towards the overlapping of the pole pieces leads to a decrease in the magnetic flux in the inter-iron space and, accordingly, to a decrease in the deflection angle of the pointer.
Spiral springs (stretch marks) in electrical measuring instruments serve, firstly, to supply and drain 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, in connection with which a total counteracting moment of the springs is created.
If it is necessary to reduce the deflection angle of the pointer, then it is necessary to change the spiral springs (stretch marks) available in the device for stronger ones, i.e., install springs with an increased counteracting moment.
This type of adjustment is often considered undesirable, as it involves painstaking work to replace the springs. However, repairers who have extensive experience in soldering coil springs (stretch marks) prefer this method. The fact is that when adjusting by changing the position of the plate of the magnetic shunt, in any case, as a result, it turns out to be shifted to the edge and there is no possibility in the future by moving the magnetic shunt to correct the readings of the device, violated by the aging of the magnet.
Changing the resistance of the resistor shunting the loop circuit with additional resistance can only be allowed as an extreme measure, since such current branching is usually used in temperature compensation devices. Naturally, any change in the specified resistance will violate the temperature compensation and, in extreme cases, can only be allowed 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 aging operation of the manganin wire.
In order to maintain the nominal internal resistance of the voltmeter, any change in the resistance of the shunt resistor must be accompanied by a change in the additional resistance, which further complicates the adjustment and makes the use of this method undesirable.
Adjustment of voltmeters in which the internal resistance is not indicated on the dial
The voltmeter is switched on, as usual, in parallel with the measured electrical circuit and adjust to obtain the deviation of the pointer to the end numerical mark of the measuring range at the rated voltage for the given measuring range. The adjustment is performed by changing the position of the plate when moving the magnetic shunt, or by changing the additional resistance, or by replacing the helical springs (stretch marks). All the remarks made above are valid in this case as well.
Often the entire electrical circuit inside the voltmeter - the frame and wire resistors - is burned out. When repairing such a voltmeter, all burnt parts are first removed, then all remaining unburned parts are thoroughly cleaned, a new movable part is installed, the frame is short-circuited, the movable part is balanced, the frame is opened and, turning on the device according to the milliammeter circuit, i.e. in series with the exemplary milliammeter, determine the current of the total deflection of the moving part, make a resistor with additional resistance, if necessary, magnetize the magnet, and finally assemble the device.
Adjustment of single-limit ammeters with internal shunt
In this case, there can be two cases of repair operations:
1) there is an intact internal shunt, and it is required, by replacing the resistor with the same frame, to switch to a new measurement limit, i.e. re-calibrate the ampere meter;
2) during the overhaul of the ammeter, the frame was replaced, in connection with which the parameters of the moving part changed, it is necessary to calculate, manufacture a new one and replace the old resistor with additional resistance.
In both cases, the current of the total deflection of the frame of the device is first determined, for which the resistor is replaced with a resistance box and, using the compensation method, the resistance and current of the total deflection of the frame are measured. The resistance of the shunt 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, that is, a shunt, which, depending on the selected upper measurement limit, is connected in parallel with the frame and the resistor with additional resistance in whole or in part from the total resistance.
For example, the shunt in a three-limit ammeter consists of three resistors Rb R2 and R3 connected in series. Suppose an ammeter can have any of the 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 the 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”, voltage is removed from the series-connected resistors Rx, R2 and R3 to the frame through resistor R ext, i.e. completely from the entire shunt. When an 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 resistor Rx is connected in series to the circuit of the resistor R ext, when connected to the “+” and “15 A” terminals, the voltage in the frame circuit is removed from the resistor R3, and the resistors R2 and Rx are included in the circuit R ext.
When repairing such an ammeter, two cases are possible:
1) the measurement limits and the resistance of the shunt do not change, but in connection with the replacement of the frame or the defective resistor, it is necessary to calculate, manufacture and install a new resistor;
2) the ammeter is calibrated, i.e. its measurement limits change, in connection with which it is necessary to calculate, manufacture and install new resistors, and then adjust the device.
In case of emergency, which happens in the presence of high-resistance frames, when temperature compensation is needed, a temperature compensation circuit is used using a resistor or thermistor. The device is verified on all limits, and with the correct fit of the first measurement limit and the correct manufacture of the shunt, additional adjustments are usually not required.
Adjustment of millivoltmeters that do not have special temperature compensation devices
The magnetoelectric device has a frame wound from copper wire and helical springs made of tin-zinc bronze or phosphor bronze, which depend on the air temperature inside the device case: the higher the temperature, the greater the resistance.
Given that the temperature coefficient of tin-zinc bronze is rather small (0.01), and the manganin wire from which the additional resistor is made is close to zero, the temperature coefficient of the magnetoelectric device is approximately assumed:
Xpr \u003d Xp ( R p / R p + R ext)
where Xp 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 of deviations of the air temperature inside the case from its nominal value on the readings of the device, the additional resistance must be several times greater than the resistance of the frame. The dependence of the ratio of the additional resistance to the resistance of the frame on the accuracy class of the device has the form
Radd / Rp = (4 - K / K)
where K is the accuracy class of the measuring device.
It follows from this equation that, for example, for instruments of accuracy class 1.0, the additional resistance should be three times 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 loop, and in ammeters with shunts - to an increase in the voltage on the shunts. The first causes a deterioration in the performance of the device, and the second - an increase in the power consumption of the shunt. Obviously, the use of millivoltmeters that do not have special temperature compensation devices is advisable only for switchboard instruments of accuracy classes 1.5 and 2.5.
The readings of the measuring device are adjusted by selecting the 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, the connecting wires included in the measuring instrument are included or their resistance is taken into account by connecting a resistance store with the appropriate resistance value to the millivoltmeter. When repairing, sometimes they resort to replacing the coil springs.
Adjustment of millivoltmeters with a temperature compensation device
The temperature compensation device allows you to increase the voltage drop across the loop without resorting to a significant increase in 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 . With a constant voltage at the terminals of the millivoltmeter, the measurement error of the device due to changes in the air temperature inside the case can practically approach zero, that is, be so small that it can be ignored and ignored.
If, during the repair of the millivoltmeter, it is found 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.
The ammeter installed on a number of cars of the Soviet automobile industry (Volga, Moskvich, UAZ, LuAZ) often fails. How to restore its performance?
Sometimes the ammeter overheats and distorts the readings. It happens from high temperature even the plastic case of the instrument panel is melted at the place where the instrument is attached, which causes its scale to warp. This phenomenon is a consequence of the oxidation of the screws of the device where they are in contact with the magnetic circuit. Made from different materials, these parts corrode over time at the point of pressing screws with a slotted seat, which is accompanied by an increase electrical resistance and heating. Soldering does not always help, since the magnetic circuit can be made of a "solderless" zinc alloy. In this case, contact can be made with a washer and nut of low height (see photo below right). Bypassing the oxidized surfaces, the current will go through the washer and nut.
Sometimes the ammeter jams or, conversely, the arrow begins to oscillate continuously. This means that you need to pay attention to the supports of its axis. Clogged bushings must be cleaned, and thickened PMS-type damping grease must be replaced. It can be looked for in laboratories of control and measuring instruments (CIP) at industrial enterprises. As an alternative, Litol is suitable for axle lubrication.
If, after turning off the ignition, the arrow does not return to zero, then it or the armature of the device has turned on the axis. In this case, the arrow must be bent so that it returns to its original position.
Constant deviations of the instrument readings in one direction (overestimation or underestimation) indicate a change in the characteristics of the permanent magnet. In this case, the device must be replaced. By the way, when starting to repair the ammeter, you should make sure that the steel tools used are not magnetized. Most of the malfunctions of other control devices can be eliminated in the described way.