(Pure dry goods) Easily master the secrets of medical equipment maintenance and repair

Generally, instrument systems consist of three parts: the operator, the environment, and the instrument itself. Faults usually originate from one of these three components. Therefore, after receiving an instrument fault report, the first step is to understand the fault phenomenon. Based on an understanding of the instrument's working principles and comprehensive analysis, determine the source of the fault and take appropriate measures.

1. Fault Analysis

Analyze the fault phenomenon, and based on an understanding of the instrument's principles and panel operation, determine the approximate location of the fault. This is the most common and important method in repair and runs throughout the entire repair process.

Substitution Method
This method refers to replacing questionable components or IC boards with normally functioning ones to help determine the source of the fault. This method is often used to determine board-level faults.

Comparison Method
This method involves comparing the working characteristics, waveforms, voltages, etc., of a faulty instrument with those of a normal instrument of the same Model to analyze and determine the cause of the fault. When using this method, attention must be paid to the external conditions of the instrument during comparative measurements: the settings of control switches, dials, buttons, and power supply must be the same.

Measurement and Analysis Method
Measure the voltage, current, resistance, and waveform parameters of relevant points according to the circuit diagram, then analyze whether the measured parameters match the measured circuit diagram, thereby discovering the cause of the fault. This is an indispensable basic troubleshooting method in repair. There are usually two methods: de-energized measurement and energized measurement.

Signal Tracing Method
This method uses circuit segmentation to trace signals to analyze faults, and is also one of the most basic repair techniques. This method can quickly and generally determine the fault location, and then the measurement and analysis method can be used to determine the specific faulty component.

Local Testing Techniques:

Isolation Method
Isolate a questionable part of the circuit, provide a separate power supply, and test and analyze to determine whether the circuit is normal. Or, during power supply circuit repair, if there is a short circuit in one of multiple loads, the isolation method can quickly find the faulty load.

Signal Injection Method
For logic circuits, appropriate logic signals can be injected from the input end, and the normality of each output can be measured step by step, thus judging whether the entire circuit has a fault. For analog circuits, appropriate analog signals can also be added for testing according to the principle.

Variable Temperature Method
For faults caused by poor thermal stability leading to parameter changes or intermittent faults, testing can often be performed by locally increasing or decreasing the temperature to analyze and find the fault location. A hair dryer or cryogenic agent can be used to create the external conditions for local heating or cooling. When using it, attention should be paid to: firstly, it should be local; secondly, it should be appropriate to avoid damaging normal components.

Tapping Method
This method can help determine faults caused by poor contact due to cold solder joints or connectors.

2. Intermittent Fault Repair

Most intermittent faults are caused by cold solder joints, poor connector contact, broken wires, short circuits in individual traces on the PC board, or deterioration of component thermal stability. The most practical method for repairing intermittent faults is to combine fault analysis with board substitution, tapping, or variable temperature methods to locate the fault. For cold solder joints or broken connections between IC pins and the board, the common practice is to resolder, clean the suspected area, and repair it.

The last resort for repairing intermittent faults is to turn intermittent faults into permanent faults, i.e., performing a strength test under certain conditions, using methods such as vibration and heating to change the intermittent fault into a permanent fault. However, this method is not easy to master, and improper use may exacerbate the fault, so it should be used cautiously.

Example of Nova3natriX500C Pulse Oximeter Troubleshooting:
After powering on, all three LCD screens display 888, and all LEDs are on. The audible alarm sounds for about 10 seconds, pauses, and then repeats.

During inspection, it was found that this fault was intermittent. Initially, the CPU board, such as the Reset circuit fault, was suspected, but this possibility was ruled out after board replacement testing. This instrument has three plug-in circuit boards including the power supply board, display control, and analog-digital processing. After carefully reading the maintenance manual, it was found that if the power switch and soft key 2 are pressed simultaneously, the instrument will Enter a special self-test state as described in the fault phenomenon, except that it should automatically Enter the working state after ten seconds. Inspection of soft key 2 revealed that the touch head was slightly deformed, leading to poor contact, and the 74C922 encoder encoding caused the fault. Repairing this switch fault resolved the issue.

3. Interference and Noise Countermeasures

Interference and noise are common problems with electrocardiograms, electroencephalograms, and many medical instruments, and are one of the most troublesome faults for maintenance personnel. Once interference and noise are confirmed, the source must be distinguished, i.e., whether it comes from outside or inside the instrument. Interference and noise are essentially of three types: ① inseparable noise from the signal; ② inherent noise, such as poor thermal stability; ③ interference noise caused by the self-excitation of transistor circuits and integrated circuits. The main method for confirming whether it comes from outside or inside is as follows: observe whether the problem always exists; if so, it usually comes from inside; if not, it is mostly external interference.

Common causes of internal noise and interference in instruments include the following two categories:

(1) Component Failure
One of the failure modes of metal film resistors and paper, ceramic, and mica capacitors is to cause interference and noise.
Dust or other foreign objects exist under the wiper of the potentiometer.
Semiconductor circuits including integrated circuits self-oscillate.
Connectors, switches, and relays age, corrode, accumulate dust, have loose connections, or accumulate static electricity.
Relevant circuits, such as poor power supply filtering.

(2) Mechanical Interference Sources
Broken wires or cables.
Loose mechanical connections in the instrument, such as loose bolts or screws connecting circuit boards.
Poor layout causes interference. Signal lines should generally be kept as far away as possible from power lines, and small signal lines should be kept as far away as possible from level signal lines.
High-voltage circuits accumulate dust, leading to discharge.
Shielding deterioration, loose ground connections, etc.

General prevention and handling methods:

(1)Commonmethods for dealing with internal interference and noise problems，First, perform a thorough preventive maintenance check, cleaning dust, especially from high-voltage circuit boards. Use a cleaning agent to spray the connector interfaces, reconnect the wiring, and tighten the connection screws. If the problem persists, use a fault analysis method in conjunction with measurements. Observe the interference or noise waveform in the suspected circuit section using an oscilloscope to determine if it is power frequency interference, similar to the main circuit frequency, or similar to the scanning circuit frequency, etc. Gradually approach the faulty circuit and locate the faulty component.

(2) The following methods are commonly used to deal with external interference and noise problems：
Try to make the instrument insensitive to external interference, such as using shielding technology, filtering technology, or improved grounding.
Use shielding technology, filtering technology, or improved grounding on the interference source to reduce interference radiation.
Keep the instrument away from the interference source and change the working environment.

4. Do not exacerbate the fault

During maintenance, maintenance personnel must be careful and cautious. The main points to note are as follows: Use appropriate tools during instrument disassembly and do not operate roughly. All connecting cables, connecting wires, and socketed integrated circuits must be accurately reinstalled after disassembly and cannot be reversed. In addition to marking them one by one to help with accurate installation, it should be remembered that even if some circuit boards, cables, and IC integrated circuits have notches to identify reverse insertion, some do not have this protection measure. The addition of input signals or artificial short circuits on the circuit needs comprehensive consideration and cannot exceed the allowable range to avoid damaging components.

Electrostatic damage. Many large companies in the United States have conducted many experiments, proving that static electricity often carried by the human body can cause catastrophic damage to many electronic components. Therefore, extra care should be taken during maintenance. Sometimes the consequences of electrostatic damage do not immediately appear and may manifest as faults later. People generally believe that only CMOS is particularly sensitive to static electricity, but this is not the case. JFETs, low-current thyristors, ultra-high-frequency transistors, large-scale integrated circuits, etc., are equally sensitive, and most other electronic components are also susceptible to electrostatic damage to varying degrees. Many people believe that only unmounted components are susceptible to electrostatic damage, but this is not entirely true. Only when protective circuits are designed for sensitive terminals in circuit design is this view correct, but in fact, this is not feasible. In addition, many people believe that low-temperature environments can cause electrostatic damage, and this view is also not comprehensive. High humidity makes it difficult for static electricity to accumulate, so there is less electrostatic damage compared to low-temperature environments, but not none. Due to the rapid development of integrated technology, low-voltage components have become dominant, and preventing electrostatic damage should be given high priority by maintenance personnel.

Common preventive measures include:

Treat all electronic components as susceptible to electrostatic damage. Before maintenance, release static electricity, such as touching a grounded conductive body or using a dedicated human body grounding wrist strap. When holding electrical wires, hold the non-conductive part at the edge. When soldering, the soldering iron power should be appropriate, and its shell should be grounded if possible. Operate on an "anti-static workbench" if possible, and properly place and transport static-sensitive components.

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