The principle of tape recorder and failure analysis of tape recorder troubleshooting method
1. The tape can not run
When overhauling the tape recorder, A standard tape should be selected, and the faults caused by the tape and the tape cassette should be eliminated first before maintenance.
If the tape does not run when pressing any of the play key’s run function keys, the motor should be checked first.
If the motor does not run and the supply voltage is not normal, troubleshoot the circuit. Measure the power supply of the recorder. If the power supply voltage is normal, but there is no DC voltage at both ends of the motor coil, or the voltage is very low, the internal and speed stabilization circuit of the motor should be mainly checked. Faults inside the motor should be checked by removing the motor.
The common motor failure is that the gaps of the commutator are filled with carbon powder from the carbon brushes, which causes the commutator to be short; the motor carbon brushes are excessively worn, causing internal short circuits. The DC voltage applied to the terminal will short-circuit to ground, resulting in no DC voltage. If the fault is minor, the commutator can be cleaned and the carbon brush can be replaced; if the damage is serious, only a new motor can be replaced.
If the motor is normal, the motor speed stabilization circuit should be checked. Using a mechanically stable speed circuit, the supply voltage is supplied by the voltage drop of the series resistance. When the motor is overloaded or there is a short circuit inside for some reason, the step-down resistor will burn out, so that the coil windings at both ends of the motor cannot get the DC power supply voltage. The breakdown of the power regulating tube of the voltage regulator circuit in the electronic speed regulating circuit will cause the electronic speed regulating circuit to have no voltage output, so that the motor cannot obtain the DC power supply voltage.
If the power supply voltage is normal and the motor cannot rotate, the main thing is to check the motor and the speed regulating circuit.
The common fault is that the spring sheet of the toggle micro-switch on the transmission mechanism is deformed and falls off, so that it cannot be in contact with the micro-switch; in addition, the micro-switch itself is damaged. It will also cause an open circuit in the power supply circuit of the motor. If the adjusting tube and sampling amplifier tube of the electronic speed stabilization circuit are damaged, the motor will have no DC voltage. The contact points of the speed-regulating centrifugal switch of the mechanically stabilized speed motor have poor contact due to long-term use; If only the contacts are not in good contact, you can use fine sandpaper to smooth the contacts and use them again. If the motor has a dead spot, only replace it.
If the motor can run, the capstan cannot run, nor can it pull the tape to run. If the drive belt falls off or is disconnected, this fault is easy to eliminate. If the drive belt is deformed and elongated, so that the driving tape is weak, you can replace the new drive belt. If there is dirt on the drive belt, causing slippage, clean the drive belt and pulley. If the capstan and the shaft sleeve are stuck due to dirt, or the use time is too long, resulting in dryness between the shaft sleeve and the shaft, and the capstan cannot run lightly, the capstan sleeve should be cleaned and lubricated. In addition, sometimes the tape running parts are stuck due to the scattering and deformation of mechanical parts or other parts, and the tape can not run normally at this time.
If the power supply voltage is normal and the motor is running normally, but the tape cannot run, in addition to excluding the quality of the tape itself, the pinch roller mechanism should be checked. When the tape is running, it mainly relies on the friction force of the pinch roller to press the capstan to drive the tape to run. The normal friction torque should be 35 ~ 70g·cm. If the load is too heavy, the transmission mechanism of the tape will not be able to drag the tape to run.
A common cause of failure is that the pressure of the pinch roller is too low, resulting in insufficient friction and the tape cannot run. The normal pressure between the pinch roller and the capstan is about 400g. When it is lower than this value, although the capstan runs normally, the tape slips when it contacts the capstan. Some of these reasons are due to the deformation of the pinch pulley mechanism, and some are due to the deformation of the pinch pulley spring, resulting in the pressure being too small. If the parts are deformed, they can be repaired. If the elastic force of the spring becomes smaller, the length of the spring can be changed and the elastic force of the spring can be increased. When adjusting the elastic force of the spring, the pressure of the pressure pulley cannot be increased too much, otherwise the capstan will be damaged and the wear between the capstan and the bearing sleeve will be accelerated.
In addition, if the position of the pause lever changes, the connecting rod of the pinch roller cannot be lifted, so that when the pause button is pressed and the normal use is resumed, the pinch roller cannot be pressed to the capstan again. , the tape will not run. When such a failure occurs, the mechanism in the suspending mechanism can be repaired, and the recorder can be restored to normal operation.
2. Tap speed too fast or too slow
①. Tap speed too fast
When the tape recorder goes out of the way too fast, you should first check the movement structure. If there is a foreign object wrapped around the capstan, such as an old tape wrapped around the capstan, make the outer diameter of the capstan thicker, which will make the tape run faster and shake when running.
Most of the faults with excessive speed are caused by faults in the speed control circuit. For example, the centrifugal contact of mechanical speed regulation is burned or oxidized due to long-term use, and the speed regulation screw is displaced, that is, it is adjusted too tightly. These faults are relatively easy to eliminate, the contact points can be re-sanded with fine sandpaper, and the adjustment screws can be re-calibrated.
②. The tape speed is too slow
The failure of the tape speed too slow is common in tape recorders, especially for tape recorders that have been used for a long time. Accompanied by the phenomenon of belt speed instability.
First look at the mechanical transmission section. Check the drive belt for deformation. Most tape recorders are driven by belts. If the belt is used for a long time, not only will the drive belt wear out, but its material will also change, making it stretched, so that the friction between it and the pulley on the capstan and the motor is reduced, and even If the belt slips, not only will the tape run weakly, but the sound will also shake. The drive belt can be replaced at this time. If the pressure of the pinch roller is reduced, the tape will run slowly and the sound will be unstable. Because the pressure of the pinch roller against the capstan is reduced, the tape will slip on the capstan, but this failure is irregular, so the playback sound is unstable. At this time, the elastic force of the pinch pulley spring can be changed to increase the pressure of the pinch pulley. It is easy to stick the magnetic powder dropped when the tape is running on the pinch roller. After a long time, a layer of magnetic powder will be formed on the outer layer of the rubber wheel of the pinch roller. Its friction coefficient is smaller than that of the rubber. The friction of the pinch roller is reduced, resulting in a reduction in the speed of the belt travel. There is dirt on the pinch roller, and the belt speed deviation is also irregular, so the playback sound is unstable. At this time, the fault can be solved only by cleaning the pinch roller.
The lack of lubricating oil on the pinch pulley shaft, capstan shaft and shaft sleeve makes them difficult to run, and also causes the failure of the belt to slow down. At this time, only a few drops of lubrication need to be added between the shaft sleeves. Oil will solve the problem. If the operation of the tape supply reel mechanism is blocked and the tape compartment door is not correct, the tape supply reel and the tape reel core cannot be matched concentrically, and the tape running speed will also be slowed down.
If the mechanical parts are working properly, then check the power supply circuit to the motor. If the piezoresistive resistor connected in series with the motor coil winding of the mechanical speed control circuit is burned out or the resistance value becomes larger, the power supply voltage of the motor will drop. When the supply voltage drops to 60% of the rated value, the motor cannot run stably. The main fault point of the electronic speed control circuit is the speed control semi-variable resistance. If the use time is too long, the adjustment end of this resistance moves, or the contact is poor, which will change the running speed of the motor. At this time, the semi-variable resistor can be replaced or the resistance value of this resistor can be readjusted.
Finally, check the motor. If the recorder is used for a long time, the powder from the wear and tear of the carbon brush and the commutator will accumulate in the gap of the commutator, causing a partial short circuit of the commutator, which will increase the motor current and reduce the power supply voltage. If it is slightly worn, it can be repaired; if it is serious, the motor needs to be replaced. In addition, due to the long-term use of the motor, its own wear and tear will increase, and various electrical properties will decline, which will also make the tape run slower. This failure cannot be repaired, only a new motor is replaced.
3. Can’t fast forward, rewind, or stop when rewinding
The main reason why the tape can’t be quickly rewound is that the idler in the rotating mechanism can’t. caused exactly. When the tape is fast-forwarding or rewinding, the inertial flywheel drives the idler wheel to rotate to rewind the tape. When rewinding, a special rewinding wheel is required. The structure of the fast reel is shown in Figure 6-2. The rewind pulley and the take-up idler are coaxial, and are controlled by a lever. When the fast-forward button is pressed, the entire movement moves upward, and the reel lever is pulled by the spring. At this time, the reel idle wheel rests on the take-up reel and the flywheel of the capstan, and the rewind reel idles to achieve fast rewinding. bring. When rewinding, the lever is also pulled by the spring. At this time, the rewinding pulley rubs together with the idler pulley, the flywheel of the capstan, and the pulley for the pulley. If the quick take-up lever does not operate normally, the take-up idler and the rewind pulley cannot reach the specified position, or the pulling spring is insufficient and falls off, the above failures will be caused.
The main reason why the tape does not rotate is that the rubber of the idler of the tape is sticky with oil, which is caused by slippage. At this time, the rubber wheel can be cleaned with alcohol to ensure its friction. The fast forward and reverse tapes of some tape recorders are rotated by gears. For example, if the gears have lost teeth, the gears are worn too much, and the gear shafts are not running smoothly, there will also be a fault phenomenon that the gear does not rotate at all times.
4. Excessive running noise or vibration
The recorder has excessive noise or vibration, mainly due to the mechanical parts and motor running caused. In addition, tape cassettes and running frictional discharges can also generate noise. The general noise of the recorder will not affect the use as long as it does not exceed the specified value. If it exceeds a certain value, it is a malfunction.
The noise generated by the motor can be solved by connecting the ground wire, cleaning the carbon brush and commutator of the motor; the vibration of the motor is too large, which will generate low-frequency noise. If the motor fastening bolts are loose, they can be re-tightened. solid. If the noise and vibration caused by the bad motor itself are too large, the motor should be replaced.
The noise generated by the rotating parts is mostly periodic, and the common causes are the noise generated by the rotation of the pinch pulley and the capstan; The noise generated by the rotating shaft can be solved by adding oil, and the wheel shaft should be removed and repaired if it is not vertical.
If a part of the machine is deformed, it will make a louder noise when the machine is running. If the tape recorder has been used for a long time, the mechanical parts will be greatly worn, so that noise will also occur during operation.
Some plastic movements will produce periodic discharge sound when the recorder is working. At this time, you can clean the movement with alcohol to eliminate the malfunction.
After using the recorder for a period of time, if the cassette door cannot be closed in the correct position, noise and other malfunctions will also occur. When the cassette door is tilted, closed loosely, and the door is deformed, the cassette will be moved to one side, and the cassette hole and the reel shaft will collide with each other, resulting in loud noise. There are two reasons for the poor closing of the cassette door. One is that the springs on both sides of the cassette door are inconsistent, causing the cassette to tilt, and the other is that the cassette door is deformed. In addition, the tape cannot be placed in the tape compartment, so that there will be running noise and excessive vibration during operation. Deformation, inelasticity or breakage of the cassette springs of the recorder movement can also affect the smooth running of the tape. Because the tape is put into the tape compartment, the tape cassette is held up by the cassette spring to prevent the tape cassette from sagging and the supply and take-up reel shafts not being concentric, which will cause the tape to run erratically and generate noise. Of course, if you use a low-quality tape, because its cassette is made of rough, there will be a lot of noise when the recorder is running.
When checking the noise of the tape recorder, be sure to accurately identify the generated part, whether it is in the mechanical transmission part or in the motor itself. When operating noise occurs, a headset can be plugged into the external speaker jack to disconnect the built-in speakers. If the speaker is disconnected, the noise is still not eliminated, at this time the noise comes from the mechanical transmission mechanism. If the speaker is disconnected and the noise disappears, the fault is from the motor. The rotational noise generated by the motor itself is only reflected by the speaker during playback; while the vibration noise generated by the motor needs to be picked up by the built-in microphone during recording, and then reflected by the playback speaker.
5. Winding when the tape is running
Winding when the tape is running is one of the common faults of tape recorders, especially when used for a long time recorder. New tape recorders can also fail with the winch if poor quality tapes are used. If the twisted belt fault is minor, the box door can still be opened; if the twisted belt has serious faults, the belt box door cannot be opened, and the only way to repair is to remove the casing first.
In addition to the poor quality of the tape itself, the cause of the twisted belt is mainly caused by the deformation and wear of the transmission machinery.
When there is a twisted tape, turn off the power first, remove the cassette door, and turn the pinch roller counterclockwise to eject the twisted tape. If the tape still cannot be ejected, the casing can be disassembled, and the inertia flywheel of the capstan can be reversed to eject the tape. For some models, the capstan flywheel cannot be rotated because the rear cover cannot be opened (before the belt compartment door is opened). If the tape is broken, after opening the case, carefully clean up the broken tape in the machine to avoid the failure again when working again.
The reason for the failure of the winding tape is that when the tape is running, the reel drive is weak, causing the reel to rotate too slowly, and the tape, so that entanglement occurs in the machine. It should be checked whether the transmission wheel is in poor contact or fails to rotate when the sound is working. If the rubber ring of the take-up wheel is deformed or damaged, so that the rotation of the take-up wheel is uneven or the rotation is difficult, the above faults will also occur. Sometimes the overrunning clutch of the take-up wheel has been used for too long, the original adjustment position has changed, and the winding belt is weak, which can also cause the belt failure.
Another reason for the twist is that the pinch roller is not parallel to the capstan, so that when the tape passes between the two axes, the magnetic head moves up and down, so that the tape is easily wound on the capstan. on the shaft and pinch rollers. If the cause of this is excessive wear on the capstan, it cannot be repaired. If the two axial directions are not parallel due to the failure of the pinch roller, you can carefully adjust the pinch roller to make it parallel to the capstan.
If no problem is found in the above inspection, the supply pulley mechanism should be checked. If the brake action of the supply pulley is wrong, or the rotating part of the supply pulley is blocked, the tape sent out by the supply pulley will be uneven, sometimes more and sometimes less, which will easily cause the tape to wind up in the machine.
When the tape recorder is faulty, be sure not to pull the tape out forcibly, this will not only damage the tape, but also damage the capstan and other components, causing more failures.
6. Failure of automatic shutdown
The main reasons for the failure of semi-automatic shutdown are insufficient pressure of the pinch roller, inflexible return of the magnetic head plate, and inertia flywheel of the capstan The brake pin is worn out, the connecting rod of the automatic detection head does not move in place, and the self-stopping detection head is stuck, etc. The above faults can generally be ruled out after reconditioning the mechanical parts. However, if the brake pin of the inertia flywheel of the capstan is worn, it is generally not easy to repair. If the connecting rod of the self-stop probe does not move flexibly, add some oil to make it move correctly.
The reason for the failure of the mechanical fully automatic self-stop mechanism is that the parts of the mechanism are deformed, which makes the movement of the self-stop mechanism difficult; the aging and deformation of the transmission belt makes the transmission force insufficient; the friction force of the rubber wheel in the rotating mechanism insufficient. These faults can generally be repaired, if the belt is worn out, it can be replaced.
The reason for the failure of the electronic automatic self-stop mechanism is that in addition to the deformation and jamming of the components in the actuator, if the circuit fails, it will also cause the self-stop failure. At this time, the self-stop control amplifier should be detected. If there is a problem with the amplifier, it cannot output the control signal, and the automatic stop mechanism will not work. In addition, if the automatic detection circuit fails, there will be no automatic shutdown pulse signal output, and the amplifier cannot get the input signal, so it cannot control the signal output.
Sometimes the sensitivity of the automatic stop detection mechanism is too high. When the play key is pressed, the self-stop mechanism enters the working state and the play key is released. At this time, the position of the parts in the self-stop mechanism can be adjusted, or a section of the plastic rod of the self-stop detector can be removed to reduce the sensitivity, and the fault can be eliminated.
7. Excessive belt speed deviation
The reason for the excessive belt speed deviation mentioned here is that the motor speed stabilization circuit is not approved, and The transport speed is not too fast due to mechanical failure. For the reason of the excessively fast tape transport speed caused by the electronic mechanical failure, please refer to the above-mentioned troubleshooting method for troubleshooting.
The standard tape transport speed of the cassette recorder is 4.76cm/s, and the deviation between fast and slow has a specific range. If it exceeds too much, it is a fault, and the tape speed should be re-calibrated at this time. The allowable deviation range of the standard tape transport speed is different for different grades of models. Table 6-1 lists the allowable values of the tape transport error for different grades of tape recorders.
A simple way to measure the speed of the tape is to find a good quality blank tape, choose a high-quality tape recorder, tune a radio station with clear radio, and record the flute of the time signal of the radio station. Voice. When recording, use a mechanical stopwatch or an electronic stopwatch with a display, then put the tape into the repaired tape recorder to play the tape, starting from the first strike of the chime, and watch the stopwatch run until the last strike The total time to distinguish the speed of the tape running time, and then adjust the electronic speed stabilization device to make the tape increase as close to the standard tape speed as possible. In addition, you can also make a calibration tape, find a tape with good quality tape and cassette, cut one, usually 3 to 5 minutes, and then use a high-end tape recorder to play the tape back. Start from the starting position, use the stopwatch to record, and then put it into the overhauled tape recorder to play the tape, observe the length of its running time, and make corrections based on the degree of the difference in tape speed error.
The method of self-made simple tape to correct the tape speed is still simple and feasible in general tape recorders. When it is necessary to use a standard test tape and a digital frequency meter or shake meter to measure.
The quality of the test tape itself is high, and a 1000Hz or 3000Hz sine wave signal is recorded on the tape. Generally, for high test accuracy, the middle part of the tape should be used, and the leading or trailing part should not be used. The output of the playback signal can use line output, speaker output, and headphone output. The impedance of the line output is 47~100kQ, the impedance of the speaker output is 4~8Q, and the impedance of the headphone output is 8~32Q. During the test, different load resistors should be connected according to the output of different impedances.
When adjusting, first find the position of the speed-adjusting semi-variable resistance of each motor. In the case of a recorder with two tape speeds, the normal tape speed should be adjusted first, and then the high speed tape speed should be adjusted. When adjusting the ordinary tape transport speed, the standard tape of 3000Hz should be used, so that the playback frequency of the recording and playback card is 2970-3000Hz. If it is a dual-card recorder, after adjusting the recording and playback card, calibrate the other playback card so that its frequency is between 2985 and 3015Hz, that is, the frequency of the playback card is about 15Hz higher than the frequency of the recording and playback card, that is, the adjustment is completed. To adjust the transport speed of the double-speed recording, the standard tape of 1000Hz should be used, so that the playback rate of the recording and playback card is 1980~2000Hz; then adjust the tape speed of the playback card to make the playback rate 1990~2010Hz, that is, the playback card The frequency is about 10Hz higher than that of the recording and playback card, that is, the adjustment is completed. After the adjustment is completed, make each card of the recorder work for about 30 minutes, and then debug it again to reduce the debugging error.
In addition to the above professional methods of adjusting the tape speed, you can also use a signal generator and a general oscilloscope, such as SB-10, to observe the state of the Lissajous figure output by the recorder to accurately calibrate the recorder the transport speed.
First use a high-end recorder to record the sine wave single-tone oscillating signal generated by the signal generator, then put it into the debugged recorder, output the signal from its headphone or speaker jack, and send it to the oscilloscope y-axis input. Then use the audio signal generator to output a single audio signal and send it to the X-axis input end of the oscilloscope, set the X-axis to the external scanning gear, and increase the x-axis amplification potentiometer to display the Lissajous waveform on the screen of the oscilloscope. At this time, if the tape recording speed of the detected recorder is normal, the Lissajous figure displayed on the screen will present a static elliptical circle. If a static elliptical circle cannot be displayed, it means that there is a deviation in the transport speed. At this point, the adjustment potentiometer can be adjusted so that the screen can present a stable Lissajous figure, and the belt speed can be adjusted accurately at this time. The still image displayed on the screen is temporary, because the tape recorder inevitably shakes, so the Lissajous figure will roll over, but as long as the Lissajous figure is basically normal, it means that the tape recording speed of the tested recorder is basically normal.
8. Oil filling method for rotating parts
When the tape recorder is used for a long time, the malfunction of the tape running smoothly often occurs. Generally speaking, if the mechanical parts are not found to be damaged or deformed, the main reason for the failure is that the lubricating oil of the shaft sleeve of each mechanical rotating part is dry. At this time, the method of troubleshooting should be to clean the shaft sleeve first, and then properly Fill with lubricating oil.
The bearing of the recorder is mainly made of oil-impregnated brass, and in most cases it is not necessary to add lubricating oil. However, if oiling is required for maintenance, be careful not to add more oil, and must not add lubricating oil to the leading shaft and each friction rubber wheel, so as to avoid the slipping phenomenon of insufficient friction when the tape is running. Strictly speaking, each mechanical transmission part of the recorder should be filled with lubricating oil of different properties. For the cooperation between the shaft sleeve and the shaft, watch or precision instrument oil should be injected; for the running parts of the machine parts, the grease lubricating oil of molybdenum disulfide should be injected. In a word, the lubricating oil filled in the recorder should have anti-oxidation, anti-corrosion, anti-friction, non-volatile, and non-volatile lubricating oil under high temperature and low temperature use environment.
①. Oil-filled motor and capstan bearing
The motor bearing and capstan bearing are made of oil-impregnated brass bushings, oil-impregnated brass The bushing has been immersed in an appropriate amount of lubricating oil during production. The oil-containing brass material has many tiny holes. When the shaft rotates in the middle of the bearing sleeve, the lubricating oil will be sucked out from the bearing sleeve, and an oil film will be formed on its surface; when the shaft stops rotating, the shaft sleeve will remove the oil film. The oil is sucked back into the bushing so that these filled oils can be reused. But when the recorder is used for a long time, the lubricating oil will gradually dry up. Generally speaking, when the use time exceeds 500h, an appropriate amount of lubricating oil should be added to these bearing sleeves.
When the bearing of the motor is short of oil, due to poor lubrication, the rotation noise of the motor will increase, which will reduce the torque of the motor and cause various failures. When filling the motor with oil, its position should be accurate, and the amount of oil filling should be appropriate. If the oil filling position is inaccurate or the oil filling is too much, the excess lubricating oil will overflow. When the oil flows on the carbon brush and the commutator, the contact resistance between the carbon brush and the commutator will increase, which will affect the normal operation of the motor. . When oiling the motor, mainly add oil to the upper part of the motor shaft (that is, the end cover), use a thin iron wire to dip one or two drops of oil, and inject it into the bearing sleeve along the axial direction. After oiling, wipe the spilled oil on the outside. If the fault is eliminated after oil filling, the work will be completed. The lower part of the motor shaft sleeve (the end part) is generally not filled with lubricating oil, because this part is close to the carbon brush and commutator. If oil is filled, the overflowing lubricating oil can easily flow to the carbon brush and commutator. . If the fault is still not eliminated after oiling the upper part of the electric motor, the oil immersion method can be used when oiling the lower part of the electric motor is necessary. When immersed in oil, the lower bearing sleeve of the motor must be removed. At this time, after cleaning the lower oil-containing brass bushing that was removed, soak it in lubricating oil heated to 60-70°C for more than half an hour, then take out the bushing, wipe its surface clean, and re-install it. After assembling the motor, it can be installed and used.
When adding lubricating oil to the capstan bearing, the capstan should be removed, the capstan and bearing sleeve should be cleaned first, and the lubricating oil should be evenly dripped onto the inner peripheral wall of the bearing sleeve with iron wire dipped in lubricating oil. Make it slowly immersed in many tiny holes in the inner wall of the oil-impregnated brass bushing, stay for more than 15 minutes, so that it can be immersed in the inner holes, then wipe off the excess oil that seeps out, and then put the capstan close to the plastic washer of the inertia flywheel. Drop a drop of lubricating oil to reduce the frictional resistance between the bushing and the washer, wipe it clean, and then reassemble. After all the work is over, wipe the capstan shaft again, so as to avoid lubricating oil on it, which will cause the tape to slip when running.
②. Oil filling of each revolving wheel
The supply pulley, take-up pulley, pulley and intermediate pulley of the tape recorder are mainly made of