How to judge whether the IC in the electronic circuit is working normally

Accurately judging the functionality of an IC (Integrated Circuit) in a circuit is crucial when repairing TV sets and audio-visual equipment. Incorrect judgments can result in the unnecessary replacement of ICs and persistent faults. Therefore, it is important to have proper methods for assessing ICs.

Firstly, it is necessary to have a solid understanding of the IC’s usage in the circuit, its internal structure, and main electrical characteristics. Analyzing the internal electrical schematic diagram may be required. Additionally, if there are different pins for ground DC voltage, waveform, and ground forward/reverse DC current resistance values, it provides more favorable conditions for preliminary judgment.

To judge the IC, begin by identifying its location based on the observed fault phenomena. Then, locate the faulty component within the IC based on its position. Occasionally, multiple judgment methods are needed to confirm that the IC is indeed damaged.

There are generally two ways to check and judge an IC in the circuit: online judgment and offline judgment. Online judgment refers to testing the IC without soldering it into the printed circuit board. Specialized equipment is often required for this method. Usually, the DC resistance method is used to measure the positive and negative resistance values of various pins relative to the ground pins. These readings can be compared with those of known good ICs. Alternatively, the substitution method can be employed, where the suspicious IC is inserted into the same type of IC in a properly functioning equipment to assess its quality.

Of course, if conditions permit, we can also use an IC tester to quantitatively test the main parameters, which is more secure.

2. Online judgment

There is also online detection and judgment, which is a method for judging the connection of integrated circuits on printed circuit boards. On-line judgment is the most practical method for overhauling integrated circuits in TV sets and audio-visual equipment. Several situations can be explained:

2.1 DC working voltage measurement method

One of the commonly used methods to determine whether an integrated circuit (IC) is functioning properly is by measuring the DC working voltage value of each pin relative to ground. This method involves comparing the measured voltages with the nominal values indicated in the schematic diagram or maintenance technical data.

When measuring the DC working voltage of each pin, it is important not to hastily conclude that the IC is defective if the individual pin voltages differ from the specified values. Before confirming the IC as faulty, the following factors should be considered and ruled out:

1. Reliability of the provided nominal voltage: Sometimes, there may be discrepancies between the indicated voltage values and the actual voltages in specifications, schematic diagrams, and other data. It is essential to gather more relevant information for comparison. Analyzing the internal schematic diagram and peripheral circuits, as well as performing calculations or theoretical estimations, can help verify the accuracy of the voltage.
2. Understanding the nature of the nominal voltage: The provided voltage is typically indicative of the working state of the IC. Since the behavior of each pin in an integrated block varies significantly with different input signals, you can modify the position of the band or video switch and observe whether the voltages stabilize. If the voltages become normal under specific working conditions, it suggests that the nominal voltage corresponds to a particular operating state. Therefore, the measured voltages may differ depending on the test’s working conditions.

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(3) Pay attention to the change of the pin voltage caused by the variable components of the peripheral circuit. When the measured voltage does not match the nominal voltage, it may be that individual pins or peripheral circuits related to the pins are connected with variable resistance potentiometers or switches (such as volume potentiometers, brightness, contrast, video, fast forward, fast rewind, recording switch, audio amplifier switch, etc.). Depending on the position of these potentiometers and switches, the voltage at the pins will vary significantly. Therefore, when the voltage of a certain pin is inconsistent, the position change of the pin or the potentiometer and switch related to the pin should be considered.

To prevent measurement errors when assessing the voltage of each pin, it is essential to consider the following factors:

4. Avoid measurement errors: Errors can arise from differences in internal resistance of the multimeter or variations in DC voltage. Generally, the labeled DC voltage is tested using an instrument with an Ω resistance greater than 20k/V. If a multimeter with a resistance less than 20 kΩ/V is used, it may result in a lower measured voltage than the actual target voltage. Additionally, note that measuring voltages on different voltage ranges can yield different readings, particularly when dealing with a wide range where reading errors can be more significant.
5. Analyze pin voltage discrepancies: When a pin voltage deviates from the normal value, assess its impact on the IC’s overall operation and observe corresponding changes in other pin voltages. This analysis helps determine whether the IC is functioning correctly or not.
6. Evaluate abnormal pin voltages: If the voltages on all pins of an IC are within the expected range, it is generally considered normal. However, if the voltage on certain pins deviates significantly, begin calculations based on the maximum deviation from the normal value. In such cases, it is crucial to examine peripheral components for any potential failures. If no faults are found, it is likely that the IC itself is damaged.
7. Consider dynamic receiving equipment: In the case of devices like TV sets that receive signals, the voltage on each pin can vary when no signal is present. If the pin voltage remains relatively stable regardless of the signal strength or the position of adjustable components, it indicates possible IC damage.

(8) For devices with multiple working modes, such as VCR, the voltage of each pin of the IC is different in different working modes.

 (9) The above points are when the IC in the circuit is not faulty, the measurement result is different from the nominal value for some reason. Therefore, in general, when testing the DC voltage or DC resistance of the integrated block, especially when recording the empirical data of the measurement, a test condition should be specified. The signal source is a standard signal under a certain field strength. Of course, it would be more representative if the switch position of each function can be recorded. If the above factors are excluded, when the individual pin voltage is still inconsistent with the nominal value, further analysis of the cause is required, but there are only two possibilities. One is the failure of the integrated circuit itself. The other is the peripheral circuit of the chip. 2.2 AC working voltage measurement method

To approximate the AC operating voltage of an IC and understand its signal changes, you can use a multimeter equipped with a dB jack. Follow these steps during testing:

1. Set the multimeter to the AC voltage range.
2. Insert the test leads into the dB jack.

For multimeters without a dB jack, use the following alternative method:

1. Connect a 0.1-0.5uF capacitor to the common test lead.
2. Set the multimeter to the AC voltage range.

This method works well for ICs with low operating frequencies, such as the video amplification stages of TV sets or field scanning circuits. However, due to the varying natural frequencies and waveforms of these circuits, the measured data will only provide an approximation rather than exact values. Keep this in mind while interpreting the results.