Amount Of Current That Is Flowing In A Cercuit Voltage Testing – Every Repair Technician and Engineer Should Master This Skill

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Voltage Testing – Every Repair Technician and Engineer Should Master This Skill

Just knowing how to read electronic schematic diagrams and test electronic components was not enough to make us good electronic repairmen. Even knowing how to accurately test and inspect all electronic components, many faults go undetected because those faults can only be detected by voltage testing. Did you know I have to do a voltage test to find the faulty component? Yes, if the voltage test is not done, I may not find the fault and have to return the monitor to the customer. What if you return multiple monitors to the same customer? If your competitors (other electronic repairers) can fix it, your business reputation will suffer and worst. You will definitely lose the customer and it will be very difficult to regain his trust in your ability to repair monitors.

I know many electronic repairers, especially beginners, do not have the courage to test voltages, especially computer monitors and televisions, because they are afraid of high voltage and electrocution. Well, if you want to work in this line and choose not to learn voltage testing (out of fear), you are really missing something that can help you repair any appliance faster. After all, voltage testing isn’t as dangerous as you might think. I don’t see any other reason why you wouldn’t want to do a voltage test, with some guidance from repair buddies or by doing the (practical) test yourself and protecting it with an isolation transformer. Apart from this of course in this field of electronic servicing you need to know the safety guidelines first.

Now the real challenging question is knowing how to do a voltage test; This does not guarantee that you will definitely be able to solve electronic problems! Why you can’t solve the problem depends on many factors such as:

– How well you understand the functions of electronic circuits (assuming I’m talking about monitor circuits). Some monitor circuits can be too complicated to understand.

– How good are you at tracing electronic circuits (assuming you are tracing from the source to the faulty components) because many electronic repairmen will lose track and not proceed to find the fault.

– How knowledgeable you are in understanding every function of an electronic component. For example, if there is no signal flowing through the base of the transistor, then of course there will be no current between the collector and the emitter. Another example, what will be the output voltage if 100 volts is passed through a 10 ohm resistor and a 10 k ohms resistor?

– Sometimes to solve a problem we accidentally create more problems because we don’t have the ability to hold the meter probe tightly so that it touches other pins while measuring the component (ie IC or transistor). This will cause the entire equipment to shut down or burn components. How many of you have done this before?

– You also need to know what the range of the meter setting is to accurately test the electronic circuit. For example, would you test the secondary output voltage with your meter set to the AC range? Or would you expect a reading when the red probe of your meter touches cold ground?

From above, these are the things you need to know if you want to be able to successfully perform voltage testing on any device. All the things I mentioned above are not that easy to absorb but if you are willing to do it (ie learn everything by reading electronics books, visiting forums, doing practice tests yourself etc.) then you are giving yourself. A chance to learn something that will really benefit you.

In this week’s article I’m going to show you how voltage testing helped me solve unusual computer monitor problems.

Well, this monitor with the brand name of MIMOS (this is an OEM monitor) comes with the complaint of not being able to adjust the horizontal size (absolutely no changes when the horizontal size control is pressed). The width (size) of the display was large and both sides of the display were curved inward (pincion out). For your information, the horizontal shape and the pincushion circuit were located in the same area and both circuits worked together to achieve the desired display. If one component in the circuit fails, it affects the other circuit too! Therefore, pincushion circuitry is also affected when there is a problem with the horizontal shape circuit (the display curves inwards).

Normally when I see a problem like this, I check and test all the components in the horizontal shape and pincushion circuits area with my ESR, analog and digital meters. Most of the time, I was able to solve the problem quickly. Common fault areas for horizontal shape width problems in monitors are usually dry joints, shorted modulation diodes and pincushion transistors, and open capacitors in line with horizontal shape coils. But somehow I couldn’t find the faulty component in this monitor because all the components I checked were fine!

Since I’ve checked all the components in the horizontal shape and pincushion circuit area are fine, the only way to solve it now is to use a voltage test. Generally, whenever you want to test voltage, you need to start at the source first. However, if you are experienced enough you can always skip some sections and start tracing the circuit as you see fit. Since there are so many electronic repair newbies on this ERG member site, I’ll show you all how to find out from the first source.

Now think carefully, what makes the display smaller or bigger when the horizontal size control is pressed through the On Screen Display (OSD)? A signal must be sent from somewhere in the horizontal shape circuit to control the display. am i right The pincushion transistor has a base pin and if this voltage changes (because the horizontal size control is set to min or max) the display will also change. You can choose to trace backward start from the base pin of the pincushion transistor or start from the source. I will guide you to trace from source because once you know how to trace from source, you will be able to troubleshoot and repair other monitor problems such as not being able to control brightness, contrast, vertical and horizontal height. You can use this method that I am going to show you in TVs and other electronic devices as well.

First we need to understand what a digital to analog conversion (DAC) circuit does. Since most modern monitors use digital signals (as well as communication between the CPU and EEPROM and other circuits) there must be some circuit to convert the digital signal to an analog signal to drive the pincushion transistors and other electronic circuits as well. Our normal multimeter cannot measure digital signals, it can only read analog signals. In older monitors the DAC IC is located outside the CPU (such as the TDA8444 IC used in the NEC XV 14 monitor) while for modern monitors the DAC IC is built into the CPU. I guess now you understand why a digital circuit needs a DAC as part of a complete circuit. In other words, a DAC is needed to communicate with the analog world!

In this faulty monitor, I don’t see any DAC IC near the CPU so I assume the DAC is already built into the CPU. Now, this is the most challenging part where you have to carefully adjust the position of the monitor and yourself so that you can test the voltage on all the pins on the CPU when the monitor power is “on”. Assuming you’ve taken all the necessary safety and precautions, it’s time to turn on the monitor and start the voltage test.

After you grab the monitor, select the horizontal size control through the OSD screen. As you can see when the “+” and “-” buttons are pressed on the front panel, the numbers in the OSD display indicate the minimum and maximum values ​​representing the size of the display being adjusted. Since this is a faulty monitor, even though the number in the OSD screen changes (because you are pressing “+” and “-“) the display has no effect and the display remains large and curved. But you can count the pins on the CPU to find out which pin is changing voltage when the adjustment button is pressed! Try each CPU pin until you see one that affects pressing the horizontal size control. The voltage is about 0 to 5 volts (depending on the design of the monitor) and should go up or down momentarily as the horizontal size adjustment changes. Congratulations! You have now determined the source of the horizontal shape circuit. A word of advice, a faulty CPU or corrupted EEPROM data may result in no signal on the horizontal shape source pin!

Fault finding is relatively easy once the pin is identified. Start tracing through the pins and see where the voltage is missing as shown in this photo. Surprisingly the voltage to pin 5 (inverting input) of the quad operational amplifier LM324 IC is fine. Since this is an OP-AMP IC, two inputs (inverting and non-inverting input) are required to produce an output at pin 7 but the voltage at pin 7 does not change. And if this output doesn’t change for sure, it can’t control the horizontal size. If pin 5 of the LM324 IC changes voltage and pin 7 does not, the problem is likely to be either in the IC itself or in the components associated with pin 6 (the non-inverting input). A wire was seen coming from the top of the board and inspected; It is from the B+ voltage (about 66 VDC) and passes through the resistor before reaching pin 6 of the OP-AMP IC. Using my analog meter to measure across the high ohms resistor (820k), I found it to be zero voltage. To make sure the resistor was at fault, I desoldered one of the leads and checked with my ohmmeter. Do you recognize? There is an open circuit in the resistor! Unbelievable and it was true! Replacing a new resistor returns the display to normal.

Conclusion- Imagine if I didn’t use the voltage test method, I don’t know how much time I would spend troubleshooting this type of problem. And worst of all, I can’t fix the problem and return the monitor to my customer. By continuously learning and practicing electronic repair, you will have no problem solving electronic faults in the future. Just don’t give up and keep learning from the ERG member page. Thanks for being my subscriber.

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