Welding is a technique that a repairman must master. If there is no correct welding method, it will often lead to false welding, missing welding and wrong welding, resulting in new faults.

Selection of (1) solder

Solder is an alloy of tin and lead. Different proportions of tin and lead have different melting points and welding temperatures, so different weldments need different solders. See table 1-4 for the composition and use of common solders.

(2) The flux can be distributed on the weld in thin layers at room temperature or when the temperature of the weldment rises, forming a protective layer to prevent it from reacting with oxygen in the air and improve the welding performance. Different weldments need different fluxes.

(3) Selection of soldering iron

Electric soldering irons should be classified according to the shape (straight head and elbow), power and working voltage of welding head. The voltage of high-voltage soldering iron is 220V, and the power is usually 25W, 45W, 75W, 100W, 150W, 300w; Low voltage is 12V and 24V, and power is 15W and 20W. Compared with high-voltage soldering iron, low-voltage soldering iron has the following two advantages.

First, the electric soldering iron with safe low-voltage power supply has no danger of electric shock and no personal accident.

Second, the influence of induced voltage is small. The shell induced voltage of low-voltage electric soldering iron is smaller than that of ordinary electric soldering iron, which greatly reduces the possibility of breakdown of integrated circuits and other components during welding.

(4) the use of electric soldering iron

Electric soldering iron is the main tool for welding, and the correct use of electric soldering iron is the key to welding technology. Pay attention to the following points when using an electric soldering iron.

A. Before using the new soldering iron, clean the surface coating with a file to expose the copper head, so that the soldering iron can easily "eat" the solder.

B the control of welding time mainly depends on the welding temperature and the geometric size of the welding part. Usually, the higher the welding temperature, the smaller the solder joint size and the shorter the welding time; On the contrary, the welding time will increase.

C. When using the soldering iron, do not touch it, so as not to break the heating wire due to vibration and damage the soldering iron.

D. Always check whether there is electric leakage with a test pen to prevent electric shock accidents.

E. If the soldering iron mouth is grayish white, it can be brushed with a wire brush. If the soldering iron is brown, the soldering iron is not easy to stick to solder, and the deteriorated part must be taken out or the welding head replaced.

(5) Welding of steel parts

Large steel parts should be welded with high-power soldering iron, and small parts should be welded with low-power soldering iron. The specific welding method is divided into the following three steps.

A. Before surface treatment and tin dipping welding, the parts to be welded must be surface treated. The treatment method is as follows: scrape its surface with waste saw blade and knife; (2) The surface was oxidized and corroded by hydrochloric acid solution, but then dehydrated. After surface treatment, the welding parts should be rubbed with flux, and then dipped in solder with an electric soldering iron for tin dipping.

B, welding and fixing the solder-dipped weldment. When welding, the weldment should be kept still until the weldment is completely solidified, otherwise the solder will crystallize and cause false welding. Some weldments can be welded directly in vice or other fixtures. It is best to put asbestos or wood between the jaws and the workpiece, otherwise the solder will not melt easily when using a small soldering iron, which will affect the welding quality.

When welding, dip the welding head into a small amount of flux and then a little solder, and quickly approach the welding place of the weldment with a little force. When the solder flows from the welding head to the welding place, it is necessary to lift the welding head quickly, so that the left welding spot will be round and bright.

When welding a long seam, first drop a drop of solder on the weld at regular intervals, and then connect the tin points with a soldering iron in one hand to form a complete weld.

When welding a larger workpiece with a smaller soldering iron, the workpiece must be preheated to a certain extent in the furnace or blowtorch before welding.

C after welding and cleaning the workpiece, the flux should be wiped clean, because the acid in the flux is corrosive to the metal and will damage the metal. When using solder paste or rosin, when the workpiece is still hot after welding, wipe it hard with a rag or soak it in alcohol.

(6) Welding of aluminum parts

Aluminum is easily oxidized, and its surface is usually covered with an alumina film. Even if this film is scraped off before welding, due to the high temperature of the soldering iron during welding, an oxide film will be quickly formed on the welding surface to prevent the solder from adhering. If we can scrape off the oxide film on the surface of aluminum, tin can adhere to aluminum. Here are two simple ways.

A. First, polish the welding surface of aluminum parts with sandpaper, put some rosin and iron powder, and after the molten iron is fully heated, dip enough solder on the welding surface and rub hard. When iron powder grinds off the oxide layer, tin adheres to the aluminum surface. Then, under the condition that the tin is not solidified, the iron powder on the welding surface and soldering iron can be wiped off with a cloth, and welding can be carried out in an ordinary way.

B, put a small piece of aluminum into a pliers pot to melt, and add 2-5 times of tin. After aluminum and tin are completely melted and mixed, the ribbon covered electrode is made. When welding, put the welded aluminum sheet into a fire or alcohol lamp and heat it to a proper temperature, and then rub it repeatedly at the welding place with covered electrode until the oxide film on the aluminum sheet is wiped off, so that tin can be firmly plated on the aluminum. When welding small aluminum parts (such as thin aluminum wires), solder with high tin content and electric soldering iron with high temperature and power can be used for direct welding.

(7) Welding of electronic components

Due to the characteristics of electronic components, it is decided that the welding of electronic components should be carried out in the following ways. Otherwise, it will cause virtual welding, even damage components or leave hidden dangers.

A. Bend the legs of the component Before installing the component, the pins of the component must be bent and arranged in an appropriate shape. This step is often referred to as "bending legs" or "nesting legs". Because the connection part (or root) between the component and the pin is fragile, it can't bear too much mechanical stress. When bending the leg, clamp the part near the root of the pin with tweezers (to protect the root), and use the fingers of the other hand to bend the pin. The distance between the bending point and the root shall not be less than 3mm; Do not bend at right angles, and the bending radius of the pin shall not be less than 2MM. Figure 1-8 shows the correct shaping method.

B. Surface treatment and surface treatment of electronic components. Pay special attention to the operation of pins and printed boards of pipes or integrated circuits when conducting tin dipping and the operation of tin dipping of the board to be brushed.

Pin of pipeline or integrated circuit: Because the outer layer of the pin of pipeline or integrated circuit has only a very thin gold layer or silver layer, it is difficult to tin-plate the replaceable wire inside, and the pin is rusty, so it is not easy to tin-plate. Therefore, the method of processing printed boards must not be used. So when their pins rust, they can only use fine sandpaper to gently hit them twice. Even if the rust is not completely removed, the knife should not be perforated again. At this time, you should dip a soldering iron into a big tin ball to "rub" the pin and let it "bury" in the melted tin ball. If the wire is welded after friction, it can still be used. If the pins are tinned in only a few places, the device can no longer be used.

Printed board: If there is protective wax or paint on the printed board, or the surface is too dirty, knock it off gently with fine sand or steel wool until the exposed copper foil or tin is bright, then wipe it off gently with alcohol cloth, and then tin plating.

When tin is plated on the through hole, it should be ensured that the pin of the component can still pass through the through hole. If the hole is blocked, you can suck the tin in the hole with a tin absorber, or poke the hole with a tin-free iron wire or aluminum wire.

C. welding is divided into spot welding and drag welding.

Spot welding: there are two kinds of welding with tin wire and welding without tin wire.

Wire-assisted welding is to hold the wire in the left hand and the soldering iron in the right hand to spot weld. The welding process is as follows: insert the component pin to be welded into the welding hole of the circuit board, and adjust the height of the component after checking the correct position (for example, the transformer cannot stick the bottom surface to the motherboard). Weld one foot first, and then spot weld the other foot. When welding, let solder wire touch the solder joint, weld the solder wire with a soldering iron, and then electroplate on the solder joint. At this time, the left hand moves forward according to the size of the welding point. After canning, the left hand immediately recovers and stops canning. At this time, the soldering iron of the right hand stays on the solder joint, and the residence time is generally about 0.8 seconds, so that the spot welding can be completed. In addition, the soldering iron should move away from the solder joint quickly and be pulled out horizontally.

There is no solder wire welding, which means that there is enough solder on the solder joint or a certain amount of solder on the welding head, and then the solder joint is spot-welded with an electric soldering iron. For example, the transistor is directly soldered to the copper foil circuit board. First, put enough solder on the solder joint on the copper foil analysis board, and then weld the triode on the solder joint. The process is to melt the solder with a soldering iron first, and when it is in full contact with the solder, the soldering iron quickly leaves the solder joint.

Drag welding is generally a welding method used in welding multi-needle assemblies. When operating, try to fix the components in a good position (which can be fixed by soldering), and then lean the welding wire on the welding feet. When the C-type welding head of large area contact melts solder wire, tin will continue to enter the solder joint; When the solder joint (molten solder) is large enough, the welding wire of the left hand and the soldering iron of the right hand move to the left synchronously, that is, the position of the solder joint moves to the left. When moving, the welding head will not touch the pin of the component. Instead, high-temperature molten solder is filled between the lead and the solder joint of the circuit board, which is the special feature of drag welding. In order to ensure the welding quality of drag welding, the moving speed and tin feeding speed of soldering iron should be specially controlled during operation.

D, using cleaning flux to wipe off the coking rosin on the pins of the printed circuit board and around the through holes. Available fluxes are: ethanol (anhydrous alcohol) and isopropanol.

(8) Stripping of electronic components

When soldering printed circuit board components, special attention should be paid not to damage the circuit board, and the components must be protected.

A. The welding head of the soldering iron used in the desoldering method of components with less than three pins should be filed very sharply, so that welding head will not touch other parts of the printed circuit board when touching the solder joint. When removing the solder joint, touch the solder joint on the back of the printed board with the welding head. At the same time, tweezers should be used to clamp the component pins on the front of the printed board to prevent excessive heat from being conducted into the components. When the solder begins to melt, use tweezers to slowly pull the pin out of the through hole. If the pin pulling process is not smooth, the welding head should be temporarily removed from the solder joint.

B. Welding method of multi-pin components For example, for integrated circuits, tin absorbents are usually needed. If there is no tin absorbing board, the following alternative methods can be adopted: the back of the printed board is tilted downwards, the soldering point on the back is contacted with the soldering head of the electric soldering iron, and the soldering point is contacted with the aluminum wire. The other end of the aluminum wire faces down. After the solder melts, insert the aluminum wire into the through hole. At this time, the molten solder will fall down along the aluminum wire. This method is also very useful.

Pay special attention to the contact time between the welding head and the solder joint, which cannot exceed 10 second. Overheating will separate the copper-clad layer from the printed board or the printed board will be burned. Do it after the solder joint is a little cold.

Although there are many kinds of teaching instruments and equipment with different principles, their maintenance procedures and basic methods are similar.

(1) General maintenance procedure

The general maintenance process of teaching instruments is divided into five stages. It is necessary to emphasize the formulation of maintenance plan and maintenance according to procedures to reduce its blindness.

1, preparation stage

There are many preparations for the maintenance of teaching instruments. The most important thing is to understand the fault phenomenon of the instrument, collect technical data, and prepare inspection tools (or inspection instruments) for disassembling tools.

(1) Understand the fault phenomenon of the instrument.

When you get the equipment to be repaired, you should carefully check all kinds of information about the equipment and make proper records. If the instrument is damaged for a long time, consult the instrument damage registration form; If the instrument has been repaired, you should also consult the instrument maintenance registration form or the relevant records on the instrument card for reference during maintenance.

(2) Preparation of technical data

Complete technical data, including instructions, schematic diagrams and assembly drawings, maintenance registration forms, instrument cards, etc. If the ready-made technical data is incomplete, you can also draw it, or find another instrument of the same type that can work normally, so that the faulty part can be found faster through comparison.

(3) Prepare maintenance tools and test instruments.

Correct selection of maintenance teaching instruments and tools can not only avoid damaging instruments, ensure maintenance quality, but also improve work efficiency. There are many kinds of teaching instruments, so there are also various tools or calibration instruments commonly used to maintain teaching instruments, so the maintenance tools and calibration instruments must be prepared according to the faults of teaching instruments.

2. Inspection stage

According to the fault phenomenon, the maintenance personnel should carry out tests to confirm whether the fault is true. If the fault is not fully understood, check the equipment as usual. There are many ways to check equipment, and some equipment needs to be disassembled to check. After inspection, a maintenance plan should be made, and maintenance tools and materials should be prepared or made by themselves.

(1) fault identification method

Different instruments have different methods to distinguish faults. The following are five common troubleshooting methods.

A. Comprehensive method: it is to perform various standardized operations on the instrument and check whether the display, indication, function and operation of the instrument are normal. At this time, if you find anything unusual, you can start an in-depth inspection from there.

B, segmentation method: divide the instrument into several units, first roughly check each unit, and then further check the problematic unit through some eyebrows. Divide the problematic unit into several blocks, and check it again ... check it again, divide it again, and check it again until the fault is found.

C. intuitive method: instrument faults can be found through human senses. For example, we should pay attention to whether the electronic instrument is out of welding, whether the wire resistance is burnt out, whether there are foreign bodies in the machine, whether the fuse is broken, whether the filament is bright, whether the electrolytic capacitor leaks liquid, whether the discharge is sonic boom, whether there is burning smell and so on. These visual inspections can usually find faults quickly. Pay special attention to safety when conducting visual inspection.

Comparison method: compare the faulty instrument with the non-faulty instrument. In particular, problems can be found by measuring and checking the faulty parts of faulty instruments and the corresponding parts of non-faulty instruments.

E replacement method: replace good spare parts, components and plug-ins with faulty instrument components, or replace the same components and plug-ins on the same instrument, which will make it easier to find out the fault of the instrument.

(2) Disassembly method

In the process of disassembly, if it is not considered well and the method is wrong, it is often easy to cause damage or deformation of the disassembled parts, which will seriously cause irreparable losses and make the whole instrument scrapped.

When disassembling, it is often from the outside to the inside to avoid disassembling the whole machine as soon as the instrument is repaired. For unfamiliar instruments, carefully observe and analyze their structural characteristics before disassembling parts, so as to understand and remember them; And carefully consider the disassembly sequence, determine the appropriate disassembly method and select the appropriate disassembly tool. Mark and record the disassembly when disassembling, and draw a sketch in the notebook to help you remember when necessary. If it is difficult to disassemble the parts, it is necessary to carefully figure out the assembly method and then try to disassemble them. Do not pry or twist hard to avoid damaging the original intact parts.

When disassembling teaching instruments and equipment, it is generally carried out in accordance with the principle of first disassembling into parts or groups of parts from outside to inside and from top to bottom, and then disassembling into parts. For maintenance personnel, you must also pay attention to the following types of disassembly methods.

A. Disassembly of shell parts At present, the shells and covers of too many teaching instruments are connected with the skeleton or main parts by screws or nuts and screws. Obviously, these connectors can be disassembled using the general tools mentioned above.

B. Disassembly of mechanical parts: Three situations should be paid special attention to when disassembling mechanical parts. First, when disassembling threaded parts, the loosening direction of the parts must be clear. Second, there is a certain cooperation between the machine parts that are generally assembled. Although the tightness of the fit varies according to the nature of the fit, it is usually necessary to use a hammer to impact the disassembly. When hammering demolition, protective measures must be taken for the impacted part. Generally, copper bars, bakelite bars, wooden bars or wooden boards are used to protect the shaft end, shaft sleeve or other parts on the shaft. Third, for parts with high precision requirements, hammering is not allowed to be used for disassembly. When pulling and unloading, the hooks of the puller should be parallel to each other, and the hooks and parts should be flat. At the same time, a suitable gasket should be placed between the screw and the shaft end to prevent the shaft end or the central hole from being damaged. The pad should also be inserted between the parts and the retractor to avoid damaging the parts due to concentrated tension.

Dismantling mechanical parts must also be done: the removed parts must be placed in a wooden frame, wooden box or parts tray in order to avoid disorderly stacking or even dragging and losing; Precision parts (such as screws and long shaft parts) should be carefully placed and wrapped to prevent bending deformation and collision; Dismantled parts (such as shafts, gears, screws, nuts, keys, washers, locating pins, etc.). ) as far as possible according to the original structure together, to prevent loss; For parts that are processed in complete sets or cannot be interchanged, they should be marked before disassembly and strung together with ropes to avoid confusion and mistakes during assembly and affect their original matching performance.

C. Dismantling of electronic equipment In order to check faults, it is sometimes necessary to measure electronic equipment. Components must be removed from the circuit board before measurement. In order to prevent mistakes, maintenance personnel should follow the following operating principles: First, it is best to disassemble only one component at a time. When disassembling parts, pay attention to the welding position of each foot. When there are a large number of pins or you are not familiar with the pin arrangement of the component, you should stick tape, write marks or adopt other marking methods on each pin. Second, if you have to remove more components, you must label each component separately. Thirdly, if the measurement results show that there is no fault in the component, it should be soldered to the circuit board in time. Fourthly, we should take the opportunity of removing components to check whether there is any problem with the printed circuit board.

In the process of maintenance, no matter which method is adopted, as long as it is a part that needs to be disassembled, you should remember the above operating rules. Never disassemble many components casually, and finally you can't figure out how to put them back.

3. Repair phase

The repair stage of teaching instruments includes several steps: disassembling instruments, cleaning parts, repairing parts (including replacement or substitution), re-inspection, assembly and adjustment. The disassembly method is the same as that described above, and will not be repeated here.

(1) Cleaning of parts and components

In order to check the damage of parts and determine whether to replace them, the removed parts should be cleaned according to different objects.

A, metal parts cleaning in the maintenance of teaching instruments are common in the following four kinds. 1. Cleaning of oily parts: Generally, oily parts can be cleaned with kerosene or diesel oil. Besides aluminum alloy parts and precision parts, they can also be soaked in hot alkaline solution. Second, the cleaning of general metal parts: cleaning metal parts, generally wipe with 80 # gasoline. In order to save gasoline and improve cleaning quality, gasoline can be divided into two cylinders. Wash your hair for the first time and the second cylinder for the second time. When the gasoline in the second cylinder is dirty, it is used as the first cylinder, and the original gasoline in the first cylinder is replaced by new gasoline, and the second cylinder is replaced. When cleaning, soak the metal parts in gasoline one by one, and then brush them with a suitable brush (in a test tube or soft brush, etc.). ). For threaded parts, care should be taken not to collide with each other to avoid damaging the threads. Small screws should be cleaned in Petri dishes to prevent them from being lost. After cleaning the parts, dry them with silk cloth or blow dry them with a hair dryer, waiting for installation. Third, the cleaning of the metal dial: metal dials are mostly made of copper, aluminum or other alloys, which are generally soft and have fine lines. If it is not wiped well, it will directly affect the reading accuracy, so you must be very careful when cleaning. If the dial is bright and rust-free, just dip a little gasoline in absorbent cotton or clean deerskin and wipe it along the tangent direction of the dial. If the dial is dirty and rusty, clean the floating dust and sand on the dial surface with absorbent cotton or deerskin before wiping, then mix the ground carbon fine powder and common instrument oil into paste and wipe it along the tangent direction of the dial. Look side by side until the stains and rust on the dial are wiped clean. The carbon paste and oil stains left on the dial should be wiped with absorbent cotton. 4. Shafting cleaning: Do not clean the vertical shaft, horizontal shaft and shaft sleeve together with general metal parts, just soak the silk cloth in clean solvent gasoline and wipe it repeatedly until it is clean. Oil stains on the shaft angle should be pulled out with sharpened wicker to wash your hair. For the oil stain in the shaft sleeve, cut a wooden stick slightly smaller than the shaft diameter, wrap it with silk cloth soaked in solvent gasoline (gauze is not suitable), extend it into the shaft sleeve, and then wipe it carefully. Steel balls should be washed in clean Petri dishes to avoid loss.

B. Cleaning of optical parts: Generally, anhydrous alcohol, anhydrous ether or a mixture of alcohol and ether (1: 3- 1: 5) is used as the cleaning solution for optical parts. The absorbent cotton used for scrubbing should be ultra-long fiber cotton, or the original medical long fiber absorbent cotton should be wrapped in gauze, put into a cylinder, soaked in ether for 6-8 hours, and taken out to dry before use. In the maintenance of optical instruments, there are two common cleaning methods. 1. General cleaning of prisms and lenses: The surfaces of prisms and lenses are usually coated with magnesium fluoride antireflection films, some lenses are glued and some lenses are silver-plated. Be careful not to use solution when cleaning, so as not to degum the lens, or wipe off the antireflection film on the mirror surface and the silver surface of the prism. If there is dust and oil, the prism and lens generally do not need to be removed from the frame or the frame. Just cut the head of the willow branches smooth, roll them with absorbent cotton, dip them in a little mixed liquid, rub them gently on the mirror a few times, and turn the cotton balls while rubbing, but don't rub them back on the mirror. When wiping, it is also necessary to prevent the cleaning liquid from infiltrating around the mirror surface and the inner surface of the pressure ring, so as not to corrode the metal. If the mirror surface is mildewed, it can be cleaned with 3204 solution (a mildewproof and antifogging agent). After wiping, if there are still spots, you can put No.309 red powder (iron oxide) in water, take its sediment, apply it to deerskin, and wipe it with it in a circular motion from the center to the edge of the mirror, or dip absorbent cotton in the red powder sediment and wipe it in the same way. In order to avoid the change of mirror curvature, it is generally only necessary to wipe the mirror surface without mildew spots, and finally clean the red powder with mixed liquid until it is clean. Second, the cleaning of optical parts with lines: some optical parts with lines are directly marked and colored; Some are made of photographic negatives; Some of them are bonded with protective glass, so they must be treated differently when cleaning. Check the dirt on the surface before wiping the directly scored optical parts. If it is engraving, you can wipe it with a dry cotton swab first. When it can't be wiped off, dip a cotton swab in a small amount of mixed liquid and gently wipe it along the vertical direction of the notch to avoid erasing the color of the notch. When wiping, check with a magnifying glass until it is clean. If the scratched back is dirty, put it on Cobb paper and wipe it thoroughly (Cobb paper should be slightly moistened with mixed solution first). Put it back after wiping. After reassembly, if local stains are found, local cleaning can be carried out again until the requirements are met. When wiping optical parts with scribing, cotton swabs should be dipped in a small amount of mixed liquid. If there is protective glass on the scribing surface, the cleaning solution should be prevented from penetrating into the adhesive layer to avoid degumming.

(2) Maintenance, replacement and substitution of spare parts

A. Compared with replacement, when maintaining any instrument and parts, we should carefully analyze and check the causes and extent of parts damage, and consider the maintenance scheme according to the damage situation, required repair cycle and maintenance economy. If new parts are replaced, in terms of speed, the maintenance cycle can be shortened under the condition of sufficient spare parts and accessories. However, its disadvantage is uneconomical. Because many worn or damaged original factory parts can be repaired in different ways, generally speaking, repairing parts is much more beneficial than manufacturing new parts. Therefore, it is more economical to adopt the repair method when the structure or working performance of the parts allows.

There are many ways to repair parts, and there are three commonly used at present (see "Basic Repair Skills" in this chapter for specific repair skills). 1. Welding method: It is used to repair the worn or local fracture of parts. Second, epoxy resin bonding method: used to compensate the wear and defects of parts and repair the fracture of shafts, bushings and other parts. Third, the fitter repair method: it is only used for repairing parts that need fitter processing.

B. Replacement of parts When the original parts are unavailable, replace them with similar parts. Pay attention to the following three points when replacing. First, the performance of the newly replaced parts should reach or exceed that of the original parts. Second, it is necessary to consider whether the replaced accessories will affect the performance of other parts of the instrument. If yes, other accessories should be adjusted to keep the performance of the whole instrument basically unchanged. Third, for special parts, especially those specially registered by manufacturers, substitutes cannot be used in principle.

(3) Re-inspection

After the faulty component is found, repaired or replaced, it needs to be checked again to see whether the original fault phenomenon has completely disappeared and whether there is a new fault phenomenon. If there is still a fault, you must start over. If there is no fault, the contents of the instrument should also be carefully visually inspected to avoid leaving hidden dangers. Different types of instruments have different inspection contents. Now take electronic instruments as an example to illustrate. The inspection items of electronic instruments include: whether there are foreign objects such as welding slag, thread ends, screws or nuts in the machine; Whether the auxiliary connecting wire or flying wire used in the maintenance process is removed; Whether the moving solder joints still meet the quality requirements during maintenance, and whether the moving parts in the machine are fixed; Whether the routing positions of cables and wires in the machine are restored to their original positions, and whether the places that need to be fixed have been fixed; Whether the wiring is scalded and worn, and whether the wiring is sandwiched between the base and the shell; Whether the thread of the connection is complete; Whether all shields are installed; Whether there are any parts that are in an incorrect position and may touch the shell or cover plate, etc.

4. Test phase

Inspection is an important basis to determine whether the instrument is qualified after repair. Generally, it should include testing (safety testing, performance testing) and testing.

(1) safety test

In order to ensure that the operator is not in danger of electric shock, there are several safety performance tests in teaching, such as insulation resistance measurement, electrical strength measurement, leakage current measurement, leakage distance measurement, grounding resistance measurement, temperature rise and damp heat test. If these items pass the test, people can generally avoid electric shock accidents. The following are the test methods of several projects.

I. Measurement of Insulation Resistance Measure the insulation resistance between live parts and very live metal parts, or between live parts with different polarities with a megohm measuring instrument. The insulation resistance of general teaching instruments should be at least 2m Ω. When measuring insulation resistance, first disconnect the power supply of the instrument under test, and then lead two wires from the "circuit" and "ground" terminals on the megger to the measured part of the instrument. After the handle is stably applied at a speed of 2 revolutions per second and rotated for 65438 0 minutes, when the pointer is stationary, the insulation resistance value can be read. & lt

B, electrical strength test (withstand voltage test) Electrical strength test is to measure the insulation performance of electrical appliances under the action of electric field.

When the electrical strength of teaching instruments is generally above 500V, 1 min will not break down. There are many kinds of equipment diagrams for measuring electrical strength tests. Figure 1-7 is the general circuit diagram.

C. Temperature rise measurement To determine whether the instrument works reliably, it is also necessary to measure the temperature rise or temperature of coils, bearings and handles in electrical appliances or the whole instrument. The concepts of temperature and temperature rise are different. The temperature is equal to the temperature of the measuring instrument minus the ambient temperature. The temperature rise of teaching instruments is generally around 60℃, and the maximum temperature rise shall not exceed 80℃ ..

(2) Performance test

Use testing tools or instruments to test the repaired instrument to see whether it meets the performance and technical parameters or technical indicators in the manual (if there are slight changes, as long as the phenomenon is obvious during the experiment, it is considered qualified).

(3) Testing

According to the instrument manual or relevant data, only the test is successful can the test be considered as qualified.

5. End phase

After the maintenance and testing of teaching instruments are qualified, they should be modified, assembled and recorded.

(1) modification

After the maintenance of teaching instruments, after re-inspection or testing, the appearance should be reformed. The purpose of decoration is to prevent the surface of the instrument from rusting and make the appearance of the instrument more beautiful. The revision should be carried out according to the revision method of the original teaching aid.

(2) Assembly

A. Inspection of parts before assembly Before assembly, the parts to be installed and the main parts to be installed must be inspected separately. Taking the transistor as an example, it is explained that the new transistor should be tested with a transistor side tester or multimeter before being soldered to the circuit board to avoid the transistor being disassembled after being soldered. Check the main components installed in the transistor separately. Taking triode as an example, it is explained that before welding the new triode to the circuit board, the transistor side tester or multimeter should be used for testing to avoid the triode being disassembled after welding. After the triode is soldered, turn it on to check the voltage between the base solder joint and the emitter solder joint on the circuit board, and check whether the base solder joint is short-circuited to the power supply; Secondly, check whether the collector solder joint is short-circuited to the power supply; If the load on the collector is not a resistor, but an inductor, check whether the damping diode or current limiting diode is intact. These circuit failures mentioned above will damage the newly replaced transistors.

B. Assembly technology In the maintenance of teaching instruments, assembly technology often includes welding technology, bonding technology and mechanical assembly technology. Because welding technology and bonding technology will be introduced later in this chapter, only a few mechanical assembly technologies will be introduced below.

First, the assembly of nuts and screws When assembling nuts and screws, it is very important to ensure that they are firmly connected and will not loosen. Assembly requirements of screws and nuts: the surface of screws or nuts attached to parts should be smooth and flat, and the surface attached to parts should be machined, otherwise the screws are easy to loosen or bend; Contact surface should be clean, screws and nuts should be cleaned with engine oil, and dirt in screw holes should be blown clean with compressed air; When there is vibration in the work, in order to prevent the screws and nuts from loosening, safety devices must be adopted. There are several safety devices, such as cotter pin, stop screw, spring washer and safety washer.

Second, there are two kinds of pins commonly used for pin shaft assembly and connection: cylindrical pin and conical pin. Cylindrical pin depends entirely on the interference during assembly. When assembling, oil the pin first, pad the end face of the pin with soft metals such as copper and aluminum, and drive the pin into the hole, or press the pin into the hole with a vise. The cone is generally a locating pin, which is convenient to assemble and disassemble, and can be assembled and disassembled many times in one hole without damaging the connection quality. The assembly pin is generally a locating pin, which is convenient to assemble and disassemble, and can be assembled and disassembled many times in one hole without damaging the connection quality. After assembly, the big end of the pin should be slightly exposed or flush with the surface of the part; The small head should be flat or serrated with the surface.

Third, riveting assembly rivets include solid rivets and hollow rivets. When connecting with a hollow rivet, firstly punch the uncapped end of the rivet with a punch or nail, and then hammer it tightly. When connecting with solid rivets, the pressure can be directly applied to both sides of the rivets, and the rivets can