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The master (also a rival) has such main repair experience: This master has been engaged in repair work for more than ten years and also has considerable repair experience. His ability to analyze circuit principles is relatively weak. His repair center is also a special repair service station for a well-known domestic brand. At his repair center, I saw many TVs that were under warranty, awaiting repair, and already repaired. Business was very good, especially with several rear-projection TVs. He was then inspecting a rear-projection TV with a "triple-no" fault phenomenon (no picture, no sound, no power). When this TV was turned on, it would immediately enter standby mode, and neither the remote control nor the power button on the TV panel could enter normal startup mode.
This master also has over ten years of repair experience and possesses detailed electrical schematic diagrams (spanning over ten pages) for this rear-projection TV. After diagnosis, it was determined that the TV was in a protective standby state. The protection circuit of this rear-projection TV had more than twenty protection items (overvoltage, overcurrent, status, and short-circuit protection for various internal loads, as well as overvoltage and undervoltage protection for output voltages). When a fault occurred, any one of these protections would trigger, resulting in the TV entering forced standby mode. In this protective forced standby state, all load circuits were in a state of having voltage but no current, making it impossible to use multimeters or other repair equipment for measurement and judgment. The protection process only lasted an instant, almost not giving time for measurements.
This master's method was to disconnect the control electrode of the ultimate control circuit of the protection circuit (a bipolar junction transistor configured as an equivalent thyristor mutual digging circuit), thereby disabling all protections of the unit. As shown in Figure 1, the base lead of Q933 was disconnected, rendering the protection circuit ineffective.
Figure 1
At the time, I felt this approach carried great risks, but this master said, "There is no other way; only by doing this can the faulty part be exposed. Everyone does it this way."
The result was: when the power was connected and the switch was pressed, a loud discharge sound accompanied the explosion of one projection tube.
As this master said, "The faulty part was exposed." Although the fault was ultimately repaired, the cause of the fault was a failure in the voltage regulation circuit, specifically due to a change in the resistance value of a resistor. Although it was fixed, the loss was significant (one projection tube at the time cost 1600 yuan, and outside the warranty period, the manufacturer did not supply replacements).
In daily interactions with many repair technicians, when discussing similar faults, they all adopted the same method for inspection, stating, "This method exposes faults quickly, and generally, there won't be too much damage; at most, one transistor might get damaged." For ordinary CRT televisions, using this method poses little risk since the operating voltage and current of the cathode-ray tube are relatively high, providing greater tolerance, and the probability of damage is minimal (unlike the rear-projection tubes which operate at their limits).
For televisions with multiple internal protection circuits, they are generally high-end models with better performance, higher prices, and larger screen sizes. If a fault occurs internally (especially overvoltage or overcurrent), the protection circuit reacts swiftly, entering standby mode and effectively protecting components like the display screen. During inspection, it is essential to locate the faulty part and replace the component causing the fault for the TV to function normally. However, if the fault phenomenon is standby protection and the TV cannot be turned on, where exactly is the faulty part? Since it is in standby mode, except for the CPU section, all other parts are powered off and inactive. Even with a multimeter or instruments, there is nothing useful to measure, leading to a deadlock. Some repair personnel then resort to extreme, high-risk methods—disabling the protection circuit—to deal with the fault. This method often yields results, solving 90% of issues with minimal losses (specifically for CRT TVs).
With the advent of flat-panel TVs dominating the market, stylish, trendy, and costly, they are poised to replace CRTs, bringing a wave of repair opportunities. Are you ready with your repair skills? (Do you mean analyzing circuits correctly and determining faults?)
For flat-panel TVs, the end display devices are LCD screens and plasma screens. Compared to CRTs, LCD and plasma screens are far more delicate, with smaller margins and less tolerance for overload, often operating at their limits. Whether it is an LCD or plasma screen, applying incorrect voltage, excessive current, or wrong polarity can irreversibly damage the display. Especially for plasma screens, improper sequencing of power application or abnormal drive waveforms can cause permanent damage. Therefore, during the repair process of flat-panel TVs, adopting the correct repair methods to protect the screen is crucial.
Flat-panel TVs, due to their delicate and expensive screens, are challenging to repair. Manufacturers have thus invested heavily in protection circuits (especially for plasma TVs). The circuit design is highly complex, with comprehensive protection functions. Any minor anomaly in power supply, load circuit characteristics, drive waveform abnormalities, or even a missing incentive signal for a certain load circuit will trigger the protection circuit, forcing components into a protective standby state. Handling such faulty TVs indeed tests your skills and intelligence. Repairing them appropriately not only earns money but also boosts your confidence and courage to overcome difficulties. You'll look forward to encountering more such faulty TVs, improving your analytical abilities and practical experience. Mishandling them, however, leads to losing confidence and the courage to face challenges, making you fear such faults.
Repairing properly and rationally sounds easy, but for a flat-panel TV in protective standby mode, the core part is in a power-off operational state. At that point, multimeters and oscilloscopes are useless (if there is a shorted power transistor, a carpet-style search using the ohm range of a multimeter might still work). The time from powering on to entering protection is instantaneous, making it extremely difficult to capture measurements. Under no circumstances should the method of disabling protection be used (especially when inspecting plasma logic drive sections), or you will regret it.
If this issue cannot be resolved accurately and safely, you can never claim to be able to repair flat-panel TVs!
I have long thought about this problem. In everyday life and during rare opportunities, I maintain contact with some domestic repair experts, constantly collecting methods from various masters for handling this type of problem. These masters all have systematic and solid theoretical foundations, rich repair experience, and have devoted their entire lives to the repair industry (all have been involved in repair careers for over 40 years, possess original scientific education, and are master-level repair experts). Even in extremely difficult situations, they have not lost their pursuit of the interest in repair. For example, Master Wu Jixian from Xuzhou, Jiangsu; Master Lao Liangyi from Ningbo, Zhejiang; and Master Lu Yuming from Changsha, Hubei. Exchanging with them clarifies some issues, and regarding this question, they provided extremely valuable suggestions.
Of course, manufacturers consider this difficulty in repair during design. They adopt computerized techniques to provide certain means of repair. For instance, when a fault occurs, the number of flashes of the power indicator LED on the control panel indicates the approximate location of the fault, facilitating fault determination. It is believed that domestic manufacturers will soon adopt similar or even more advanced methods in their produced flat-panel TVs to indicate the location of faults.
Below, I will introduce a few safe methods for inspecting protective standby states:
Preparation for repairs:
Engaging in repair work is essentially a career, and a career requires pursuit. On this path, every difficult fault is an opportunity for improvement (this is a gift from God). Each successful repair of a challenging fault represents a technical advancement. A mature and highly skilled repair master is forged through eliminating one difficult fault after another. Every fault elimination comes with considerable effort, and only through hard work can one reach the shores of success. If you encounter difficult faults and set them aside, you are abandoning opportunities for improvement. In that case, please step out!
Especially for TVs in protective standby mode, the repair process involves checking each protection branch, repetitive work requiring considerable time. For some individuals, it may be a test of patience, but you must have confidence, patience, and meticulousness.
To repair TVs in protective standby mode, you must understand and be familiar with the principles of the protection circuit, the control flow of protection, the items protected, and the methods of protection control. You should be proficient in analyzing the working principles of each protection control circuit. This is a prerequisite for doing this work well. If you cannot meet this prerequisite, do not attempt to repair this TV.
Having met the above prerequisites, you must also have appropriate tools at hand: a sensitive multimeter, a high-performance soldering iron and large tools, ideally an oscilloscope. "A craftsman who wishes to excel in his work must first sharpen his tools."
After much thought and consulting friends, I believe the following three types of methods are feasible, and several masters have been continuously using them with good results. Below, I will introduce them one by one:
Since this is an introduction to safe inspection methods, it must involve practical operational processes. We will use the simple overvoltage protection circuit in Figure 2 as an example to introduce the inspection method.
Working principle of the overvoltage protection circuit:
(Figure 2 shows the electrical schematic diagram of the overvoltage protection section of a certain factory's rear-projection TV.)
As shown in Figure 2, it is the overvoltage protection for the switching current output. The protection items include 180V, 140V, 28V, and 15V overvoltage protection. In each protection channel, there is a Zener diode, a current-limiting resistor, and an isolation diode.
The Zener voltage of the Zener diode is significantly greater than the voltage of the circuit to be protected. For example, for the 180V input channel, the output voltage should be 180V, so the Zener voltage of ZD1 is selected to be significantly greater than 180V, for example, 190V (too high would lose the meaning of the protection circuit). When the switch power supply causes the voltage to rise above 190V due to some abnormal reasons, ZD1 breaks down, and the breakdown current flows through R1 and D1 to the control triggering circuit Q933, Q934, triggering Q933 and Q934 to conduct and cut off the bias voltage of Q932. Q932 stops conducting, and the relay S901 opens, cutting off the power supply, putting the components into a protective standby state.
The role of the current-limiting resistor R01 is: if R01 were absent, when the 180V overvoltage causes ZD1 to break down, the 180V voltage would be short-circuited, and the huge current would burn out D1, Q933, Q932, and even ZD1. Adding R01 limits the sharp increase in current when ZD1 breaks down, protecting the safety of the circuit. The value of R01 should be selected based on different Zener values.
The role of the isolation diode D1: When the circuit protection causes ZD1 to break down, the current flows through D1 to the base of Q933, initiating the operation of the protection circuit. Simultaneously, the positive terminal of D1 acts as a "three-way intersection," allowing current to flow into other protection channels, weakening the sensitivity of the current channel and causing voltage instability in other power supply circuits, as shown in Figure 2B. (Figure 2B shows the current state without the isolation diode.)
Figure 2B
Once the principle of the overvoltage protection in the switch power supply is understood, the inspection method becomes clear.
First Method: Instantaneous Observation with Multimeter.
From the above principle, it can be seen that whenever the protection circuit operates, there is current flowing through the corresponding branch, but this current only appears momentarily because the protection control relay immediately cuts off the power supply, leaving no voltage or current afterward. People use this momentary control to connect the analog (digital meters cannot be used) multimeter's DC high-voltage range (2.5V or higher) as shown in Figure 3 (using the 180V protection circuit as an example) across the terminals of the isolation diode D1, with the red probe connected to the negative terminal of D1 and the black probe to the positive terminal. If there is no overvoltage in the 180V input, the Zener diode ZD1 will not break down, and there will be no current through ZD1, R01, and D1, resulting in no voltage across the terminals of the isolation diode, and the needle of the multimeter will not move. If there is an overvoltage phenomenon at 180V, at the moment the Zener diode ZD1 breaks down, a small current flows through R01 and D1, generating a potential difference of approximately 0.6V across the terminals of D1. This potential difference also flows through the probes of the multimeter set to the DC 2.5V range, slightly deflecting the needle. Because this potential difference only appears momentarily, the needle does not have time to fully deflect before the momentary voltage disappears. However, the visible movement of the needle clearly indicates a fault. With any movement, the circuit is in protection. The faulty position is clearly identified, simplifying the next steps. (Master Lao Liangyi from Ningbo has successfully used this method for years to safely repair TVs in protective standby mode.)
Because a TV (flat-panel, rear-projection, or higher-end models) has numerous protection circuits, initially, it may be unclear where the protection action occurs. However, once you are familiar with the protection circuit principles and the number of protection items (10, 20, 30), you only need to identify the correct positions (the isolation diodes' locations). By measuring with the probes a few times and pressing the power switch a few times, the fault range can be determined, and the rest falls into place. Although the operational procedure is complex, the time spent is not much—worthwhile.
For this method, ensure good contact between the probes and the isolation diodes, possibly measuring a location multiple times to confirm accuracy.
If you find this troublesome and opt for disabling the protection circuit to quickly identify the fault, it may be faster but carries significant risks. Any resulting damage will bring you greater trouble and hinder your personal improvement.
The first method uses the DC high-voltage range of the multimeter, presenting no problems for experienced masters who can accurately determine the fault location. The key lies in carefully observing the deflection amplitude of the needle (different meters have varying deflections), comparing multiple times to establish a rule. Additionally, manual skills are important; each time the probes are placed to connect the terminals of the isolation diode, ensure good contact. For densely populated circuit boards, avoid accidentally touching other components or creating short circuits with other connections, potentially causing greater damage. The following second method is safer and more reliable.
Figure 3
Second Method: Light Emitting Diode Observation Method:
Since a current flows through the isolation diode during protection, if the isolation diode is replaced with a light-emitting diode, will it glow or flash momentarily? The answer is yes, it can, completely safely and reliably. Particularly for beginners, although it may be a bit troublesome.
Modern light-emitting diodes have extremely high sensitivity, glowing with just a few microamperes of current. When purchasing, use the R×10K range of an analog multimeter (R×10K pen current is 50 microamperes) to measure the forward and reverse resistance; the diode will glow brightly, as shown in Figure 4. When selecting, choose blue-light emitting diodes (high sensitivity, high brightness) and compare those with smaller reverse resistance. Measurements with other ranges (R×1, R×10, R×100, R×1K) will not show results because the forward voltage drop of light-emitting diodes is greater than 1.5V (low ones are less than 2V).
Light-emitting diodes can replace isolation diodes, as shown in Figure 5. They can also be connected in series within the current loop of ZD1, R01, and D1. However, since the forward voltage drop of light-emitting diodes is around 2V, connecting them in series increases the Zener voltage of ZD1 by approximately 2V, which can be ignored for high-voltage protection but should be considered for low-voltage overvoltage protection (less than 10V). Based on experience, overvoltage protection generally occurs in high-voltage circuits. According to this experience, connecting light-emitting diodes in high-voltage protection branches generally reveals the fault. Note: Light-emitting diodes must be welded in the forward conduction state, absolutely avoiding reverse connection, or it may cause greater damage!!
For TVs with protective faults, using the light-emitting diode inspection method, the light-emitting diode flashes clearly upon startup, making fault determination very convenient.
For printed circuit boards with surface-mount components, isolating diodes are inconvenient to install. Alternatively, the copper foil lead behind the isolating diode can be cut, and the isolating diode can be welded in the reverse conduction direction. Once the fault is eliminated, it can be restored, as shown in Figures 5 and 6.
Figure 5
Figure 6
Third Method: Instantaneous Observation with Oscilloscope:
If an oscilloscope is available, use it to observe the "jump change" in voltage on the isolation diode at the moment of startup protection. This is an excellent method. The probe of the oscilloscope is needle-shaped, ensuring reliable contact on the isolation diode without mistakenly touching other components, reacting sensitively, making it a safe and reliable method without needing a soldering iron.
Connection method for the oscilloscope as follows:
Measure the negative terminal of the isolation diode with the oscilloscope probe, and connect the ground line of the probe to the ground wire of the circuit board, as shown in Figure 7.
Figure 7
Initially, select a time base (TIME/DIV) range below 100 milliseconds for the oscilloscope, gradually increasing the time as you gain experience to obtain a relatively stable jump pulse when protection occurs.
Select the trigger polarity: "Positive". Select external synchronization for triggering.
Start learning by setting the Y-axis sensitivity (VOLTS/DIV) of the oscilloscope to 10V/DIV. If the amplitude is too small, gradually adjust to 20V/DIV, 50V/DIV to obtain a basically stable, appropriately sized jump waveform at the moment of protection occurrence. Flat-panel TVs are also considered "high-tech products," so they cannot be treated with outdated methods like "rice and rifles." Modern weapons are necessary; otherwise, people's positions will never improve.
Additionally, for other protections in TVs, such as overcurrent, status, and short-voltage protections, they ultimately convert to voltage pulses applied to the protection control circuit, all containing isolation diodes. Measuring the isolation diodes or replacing them with light-emitting diodes allows quick identification of the faulty location. The principles come from "Hao Ming - High-End TV Repair Training Expert" www.haoming.cc Original URL: http://www.haoming.cc/lcd/359/