Troubleshooting of Common Faults of JX-5 Book Monitor

With the development of science and technology, most domestic libraries have gradually realized a modernized book management model with computers as the core, among which the open-shelf borrowing service of the circulation department is the most popular service item among readers. In the work of carrying out the open-shelf borrowing service, due to the difference in the moral level of readers, the loss of books in many libraries is quite serious. In order to effectively prevent the loss of books and documents and ensure the service quality of open-shelf borrowing, the JX-5 book monitor has begun to be widely used in various libraries.

1 Basic principles, installation and debugging of book monitoring instrument

The JX-5 book monitor is an electromagnetic induction monitoring system EMSS (Electronical Magnetic SurVeillance System) device. It is designed and manufactured based on the principle of electromagnetic induction. A stable and minimally distorted sine fundamental frequency is introduced into a fixed monitoring bracket, and an electromagnetic field of f0=2KH is established by the transmitting coil in the bracket; the signal with a center frequency of f40=16KH is monitored by the receiving coil in another bracket. When books and documents with magnetic strips pass through the monitoring bracket, according to the principle of electromagnetic induction, the magnetic strips will generate eddy currents, which will be saturated in the magnetic field of f0 and stimulate 16KH harmonics. After the circuit in the receiving coil selects the frequency, identifies and amplifies it, the monitor will automatically alarm with sound and light.

Installation of library monitor: The monitor should be installed away from interference sources and a standard grounding wire should be installed, otherwise the instrument will work unstably. It is best to install the monitoring bracket in the east-west direction. When detecting the north-south direction, the transmitting bracket should be placed on the east side to reduce the influence of geomagnetism. The photoelectric counter should avoid strong light exposure to prevent counting errors. The monitoring brackets should be aligned in parallel, and the width between the brackets should be within the range of 70~80cm. The distance between the monitor and the monitoring bracket should be more than two meters. The transmitting and receiving lines should be separated as much as possible to avoid interference.

Debugging of the book monitor: first set the working selection knob to the test gear, turn on the power and preheat for 5 minutes, then adjust the zero adjustment knob to make the pointer reach the zero position. At this time, the working current of the instrument should be around 0.8~1.3A. Take a book with a magnetic strip installed and pass it through the detection bracket. After the alarm can be normal, set the working selection to low, medium and high gears for monitoring experiments. The time interval between each switch start of the monitor must be more than 2 minutes, and the monitoring bracket must not be damp, otherwise it will reduce the Q value of the transmitting coil and the receiving coil, affecting the detection accuracy. The photoelectric head of the photoelectric counter must be aligned, and there must be no stains on the photoelectric head to avoid inaccurate counting. [page]

2. Common troubleshooting and analysis of book monitors

Based on the schematic diagram of the JX-5 book monitor, the following analysis explains how to troubleshoot common faults of the monitor.

Fault 1: The monitor does not alarm, the emission current meter indicates zero, check the +40V power supply BX2 fuse is blown, and there is still no current after replacing the 1A fuse. Open the chassis and use a multimeter to measure the +40V voltage to zero. Remove the +40V power supply adjustment tube CD77-1B (actually MJE2955) and measure and find that it is damaged. Replace it with the same model (the magnification is 60) and re-fine-tune 3W1 to make the voltage output at +40V. Analysis: The power supply design of the JX-5 book monitor is relatively simple, and it heats up severely when working for a long time, which is a frequent point of failure. Therefore, when replacing it, you should choose a high-quality high-power tube with Vce≥300V, Icm≥15A, 80≥β≥50.

Fault 2: The monitor does not alarm, the emission current meter indicates zero, the emission fuse BX4 is blown, and there is still no current after replacing the 2A fuse. The ±40V voltage measured with a multimeter is normal, and the 2L3 voltage is only about 2V (normal value is AC 30V). The voltage of each foot of 2G2 and 2G3 is measured, and the voltage between the base and emitter of 2G2 is zero (normal value 0.3V). After disassembly, it was found to be broken down and damaged, and it is normal after replacement.

Analysis: The design of the transmitter part of the monitor is quite unique. The manufacturer uses a specially provided module GF to ensure working efficiency, and the GF module has high requirements for two high-power tubes. That is: Vce ≥ 500V, Icm ≥ 15A, fT ≥ 10M, the β value is matched between 20 and 40, and the saturation voltage drop should be small. Generally speaking, the working efficiency of the transmitter circuit is not too high (≤ 70%), so the power consumption of the circuit is large when working, and the tube heats up severely. It is recommended to install a fan of more than 25W in the chassis to dissipate heat and reduce failures caused by heating.

Fault 3: The emission current of the monitor is too small, only about 0.6A. The measured power supply voltage and 2L3 voltage are normal, but the voltage of 2C14 is only 40V (the normal value is about AC 90~110V). After replacing the capacitor of the same model, it is normal.

Analysis: 2C14 is an oil-immersed paper dielectric capacitor, which is prone to capacity reduction after long-term use. When replacing, please note that you must replace it with the original model, and cannot replace it with other types of capacitors with the same withstand voltage and capacity, because capacitors are divided into power sizes in addition to the commonly used withstand voltage and capacity.

Fault 4: The monitor's emission current is normal, the light alarm is normal, but there is no sound alarm. The multimeter is used to measure 1BG7 and J1, and they are working normally. The single junction tube 1T2 is seriously hot. It is damaged after being removed and measured. It works normally after being replaced.

Analysis: In order to meet the alarm volume, the working current of 1T2 is relatively large. In order to extend the life, the resistance value of 1R39 can be appropriately increased, but the alarm volume will be reduced.

Fault 5: The transmitter current of the monitor is normal, the zero adjustment knob does not work, and the instrument always alarms. The power supply voltage and the voltages of 2L3, 2C14 and the receiving coil Q3 are normal. When adjusting 1W1, it was found that the voltage of 1R19 did not change. After replacing the same model of 1W1, it was normal. Analysis: The multi-turn potentiometer 1W1 is wound with very fine resistance wire. It is easy to break and damage after repeated adjustment. It is a consumable part. Fault 6: The transmitter current of the monitor is normal, the zero adjustment knob can be adjusted, but it cannot be adjusted to zero, and false alarms are often given. The voltages of other points are normal.

Analysis: The JX-5 book monitor uses a large number of resistors, capacitors, diodes, transistors and integrated operational amplifiers. Due to the aging of components and temperature changes over time, the original circuit zero adjustment will drift, so the book monitor can be re-zeroed according to the following steps after being used for a period of time.

Debugging of the receiving part: unplug the transmitting board, turn on the power, and there is an indication on the zero-adjusting μA meter (B1). Adjust the zero-adjusting knob to 35μA when the alarm sounds. If not, first check the ±12V power supply of F1, fine-tune 1W2, 1W3 or change the resistance value of 1R17 to meet the requirements. Debugging of the transmitting part: plug the transmitting board that was just unplugged back in, restart the machine, and the zero-adjusting μA meter is biased to the left. At this time, first adjust the zero-adjusting knob to the position where the μA meter can rotate (that is, not to the left), then adjust the ±40V power supply voltage (power supply voltage range ±38V~±48V) to make the μA meter as far as possible to the right, and then adjust the coil in the wooden frame or the 1L1 magnetic core gap of the receiving front panel. Repeat several times until the μA meter pointer remains basically unchanged when the transmitting board is inserted and unplugged. At this point, the adjustment is completed and the machine returns to normal.

Fault 7: The emission current of the monitor is normal, and the zero adjustment knob can be adjusted to zero, but it often gives false alarms, and sometimes the alarm cannot be cleared. First, check whether there is a magnetic strip or a book with a magnetic strip within the detection range of the monitoring bracket, and then check whether the grounding is good. If necessary, you can repeat the grounding, or install an AC voltage stabilizer to effectively avoid interference from the power supply. If it still doesn't work, it means that the instrument is seriously interfered. At this time, set the working selection to low gear, and then carefully find the interference source to try to eliminate the fault.

Analysis: The monitor is an alarm product and has high requirements for the working environment. Otherwise, it is easy to make false alarms under other interference. The monitor's bracket, the relative position of the internal coil, and even the host position of the monitor should not be moved casually to avoid changes in the zero point of the machine and cause human failures.

3 Conclusion

The JX-5 book monitor is a product from the 1980s. It works in a sinusoidal critical oscillation state of several KH and is particularly susceptible to external influences. In addition, the magnetic stripe is very labor-intensive to install and is easily damaged during use, so book thefts still happen from time to time. We look forward to designers and manufacturers producing a book monitor for libraries that has a higher frequency, is easy to install and is not easily damaged, and works more stably.

References

[1] Zou Yunping. Detection technology and electromagnetic compatibility design. Wuhan: Huazhong University of Science and Technology Press, 1992.3

[2] Chen Runtai, Xu Kun. Detection Technology and Intelligent Instruments. Beijing: National Defense Industry Press, 1996.10

[3] "JX-5 Monitor" Instruction Manual