Medical Electronic Instrument Maintenance Manual

Author: Chief Editor: Dong Xiuzhen
Publisher:
Publish Date: 1998-11-01
Features: Table of Contents This manual is collectively written by more than ten biomedical engineering experts working online. The book is divided into nine chapters, providing an in-depth introduction to the general principles of medical instrument maintenance. It then thoroughly discusses the maintenance of various commonly used medical electronic instruments, including physiological function testing instruments, biochemical testing instruments, physical therapy instruments, treatment instruments, ultrasound diagnostic instruments, X-ray examination equipment, CT machines, gamma cameras, and ECT, with a focus on fault analysis and troubleshooting. Only a brief introduction to the principles of these instruments is provided, making the book concise and focused. It is suitable for biomedical engineering technicians working in medical instrument management and maintenance in various hospitals, as well as for reference by biomedical engineering students. Excerpt:
II. Basic Methods of Fault Diagnosis The key to diagnosing faults in medical electronic instruments lies in selecting appropriate inspection methods to identify, judge, and determine the location and causes of the faults. The basic methods of fault diagnosis can generally be summarized into 14 types: no-power observation, power-on observation, symptomatic treatment, voltage measurement, waveform observation, signal injection, signal tracing, capacitor bypassing, split testing, component substitution, condition modification, whole-machine comparison, resistance measurement, and component testing. By flexibly and cross-applying these methods based on the instrument's fault symptoms and working principles, effective and rapid fault diagnosis can be achieved.
1. No-Power Observation Method Under no-power conditions, observe whether switches, knobs, dials, sockets, terminal posts, detectors, and indicating meters on the instrument panel are loose, misaligned, jammed, or disconnected. Open the instrument's outer casing to inspect internal components, devices, plug-ins, power transformers, and circuit connections for signs of burning, leakage, mold, breakdown, detachment, or open circuits.
2. Power-On Observation Method The power-on observation method is particularly suitable for checking faults such as sparking, smoking, unusual odors, or blown fuses. These faults typically occur in the instrument's rectifier circuit. When performing power-on observation, attention should first be paid to the operating status of the rectifier tubes.
3. Symptomatic Treatment Method Most instrument manuals provide comprehensive maintenance and adjustment data, such as voltage tables for various circuits, waveform diagrams, and reference tables for common faults, causes, and repair methods. These are valuable resources for instrument repair personnel. Therefore, during fault diagnosis, the repairer can refer to existing data to address symptoms, accelerating the repair process.
4. Voltage Measurement Method Checking whether the internal power supply voltages of the instrument are normal is the foundation of fault analysis. Therefore, fault diagnosis should begin with measuring the voltages of various DC power supplies in the instrument. Even when the fault location is already determined, it is often necessary to further measure the operating point voltages of electronic tubes and transistors in the relevant circuits. This is highly helpful for identifying and analyzing the causes of faults and damaged components.
5. Waveform Observation Method During fault diagnosis, using an oscilloscope to observe the input and output signal waveforms of oscillation, amplification, inversion, shaping, frequency division, frequency multiplication, and modulation circuits in the instrument can quickly identify the fault location and assist in analyzing the cause of the fault, further refining the detection methods and steps.
6. Signal Injection Method Using an external signal source, inject a test signal into the input terminal of the instrument's terminal indicator and sequentially move forward to the input terminals of each preceding circuit. Observe the response of the terminal indicator to determine the fault location and analyze the cause of the fault.
7. Signal Tracing Method Apply an external signal source with appropriate frequency and amplitude as a test signal voltage to the input terminal or the input terminal of the preamplifier stage of the instrument. Then, using an external oscilloscope, observe the input and output waveforms and amplitudes of each subsequent amplification stage to identify abnormal signs.
8. Capacitor Bypassing Method When troubleshooting electronic instruments with parasitic oscillation or parasitic amplitude modulation, the capacitor bypassing method is often used to check and identify the faulty circuit. Specifically, use a capacitor with appropriate capacitance and voltage to temporarily bypass the input of a suspected circuit to the ground to observe its effect on the fault. If the fault disappears, the problem lies in the preceding circuit stages; otherwise, the issue is within the current circuit stage.
9. Split Testing Method Some medical electronic instruments have complex circuit compositions involving numerous components with mutual dependencies and influences. Therefore, during fault diagnosis, it is necessary to split the circuits by detaching one end of the circuit connections or removing relevant electronic tubes and unit board plug-ins to observe the effect on the fault. Alternatively, test the functionality of the split circuit individually to identify the problem and further examine the cause of the fault.
10. Component Substitution Method When diagnosing faults in medical electronic instruments, it is best not to disassemble components and devices in the circuit, especially for precision instruments. Instead, use components, devices, printed circuit boards, or unit plug-in parts with the same model, specifications, and structure to temporarily substitute the suspected parts to observe their effect on the fault. If the fault disappears, the substituted part is the issue, and it should then be desoldered and replaced or further investigated for the cause of the fault.
11. Condition Modification Method The condition modification method refers to intentionally altering the status of semi-adjustable components in the circuit, including intentionally touching the pins, sockets, solder pads, or switch contact points of relevant devices, or even significantly changing the values of relevant components or the operating points of the circuit to observe their effect on the fault. This often reveals issues such as poor contact, cold solder joints, value changes, or degraded performance, allowing for adjustments or replacements to resolve the fault.
12. Whole-Machine Comparison Method When diagnosing faults in medical electronic instruments, having reference values for the operating point voltages and waveforms of the circuit under normal conditions is essential for comparing differences and identifying problems using the voltage measurement and waveform observation methods. Therefore, when technical data is lacking and multiple detection methods still fail to analyze the cause of the fault or determine the problematic location, the whole-machine comparison method is typically used. This involves using a fully functional instrument of the same type to measure the operating point voltages and observe the waveforms of the suspected faulty circuit, comparing the differences between the two instruments to identify the issue and assist in analyzing the cause of the fault. This method is particularly effective for diagnosing complex electronic instruments.
13. Resistance Measurement Method When diagnosing faults in medical electronic instruments, issues such as poor contact at component pins or sliding contacts, cold solder joints, variable resistance values, or capacitor leakage often cause faults. These problems require checking the instrument under no-power conditions using the resistance measurement method to locate the fault.
14. Component Testing Method Quantitative testing of suspected circuits during fault diagnosis in medical electronic instruments helps determine and analyze the cause of the fault. It should be noted that the testing conditions of various component testing instruments differ from the operating conditions of the instrument. Frequently, a suspected component may test as good but fail when connected to the circuit. Therefore, in addition to obvious parameter changes and performance degradation, the component substitution method may still be necessary to confirm the quality of the suspected component.
Among the above 14 basic methods for checking the causes of electronic instrument faults, no-power observation and power-on observation methods help quickly identify damaged components and locations; symptomatic treatment methods are highly instructive for beginners and aid in entry-level troubleshooting; voltage measurement is the foundation of fault diagnosis, as effective testing and analysis can only be performed when power supply voltages and operating point voltages are normal; waveform observation, signal injection, signal tracing, capacitor bypassing, split testing, and other methods help quickly identify faulty circuit sections; condition modification, component substitution, resistance measurement, and component testing methods help identify variable, aging, cold solder joints, damaged, or poorly contacting components and parts; and whole-machine comparison is very helpful for solving difficult fault problems.
Section III General Procedures and Methods for Repairing Medical Electronic Instruments
I. General Procedures for Repairing Medical Electronic Instruments Repairing medical electronic instruments is a technical task requiring both theoretical and practical knowledge. Repairers should neither rely solely on experience, nor engage in theoretical discussions without practice, nor blindly tamper with the instrument in hopes of accidental success. Otherwise, not only will the repair efforts be futile, but the fault may become more complex. Therefore, to perform effective repairs, one must possess a certain foundation in circuit theory and electronic circuit knowledge, understand the correct use and operation of common testing instruments, be familiar with the basic methods of checking the causes of medical electronic instrument faults, and follow a scientific work procedure on this basis.
Typically, the repair procedures for medical electronic instruments can be summarized into nine steps: understanding the fault situation, observing the fault symptoms, preliminary surface inspection, studying the working principles, formulating a testing plan, analyzing test results, identifying and repairing faults, post-repair performance verification, and filling out repair records.