Knowing how to read electronic schematic diagrams and test electronic components was not enough to make us good electronic repairmen. Many faults cannot be detected even if you know how to accurately test and check all electronic components because those faults can only be detected by simply performing a voltage test. Do you know that a good number of Monitor problems I have to perform a voltage test to locate the faulty components? Yes, if I don’t have the voltage test, I may not be able to find the fault and need to return the monitor to customers. What happens if you return many monitors to the same customers? Of course, your business reputation would suffer and worse if your competitors (other electronic repairmen) could fix it. You would definitely lose customers and it is very difficult to regain their confidence in your ability to repair monitors.
I know many electronic repairmen, especially the novice ones, who don’t have the courage to do the voltage test, especially the computer monitors and television, because they were afraid of the high voltage and get electrocuted. Well, if you want to work along these lines and choose not to learn about voltage testing (for fear), then you’re really missing out on something that can help you fix any piece of equipment quickly. After all, voltage testing isn’t as dangerous as you might think. With some guidance from repair friends or even if you try it on your own (handy) and isolation transformer protection, I don’t see any other reason why you wouldn’t want to do the voltage test. In addition to this, of course, you must first know the security guidelines in this field of electronic service.
The real challenging question now is even if you know how to perform the voltage test; this does not guarantee that you will definitely be able to solve electronic problems! As to why you can’t resolve the issues depends on many factors, such as:
– How well you understand the functions of the electronic circuit (assuming I am talking about the Monitor circuit). Some monitor circuits can be very complicated to understand.
– How good you are at tracing the electronic circuit (assuming you are tracing from the source to the faulty components) because many electronic repairmen will lose track and be unable to go any further to pinpoint the fault.
– How knowledgeable you are in understanding each function of the electronic component. For example, if no signal flows to the base of a transistor, then of course no current will flow between the collector and emitter. Another example, what do you expect the output voltage to be if 100 volts pass between a 10 ohm resistor and a 10 k ohm resistor?
– Sometimes, in order to solve a problem, we accidentally create even more problems because we don’t have the ability to firmly hold the meter probes, causing them to touch other pins while we measure a component (for example, an integrated circuit or a transistor ). This would cause the entire equipment to shut down or even burn components. How many of you have done that before?
– We also need to know what is the setting range of the meter to test the electronic circuit accurately. For example, would you test secondary output voltages when your meter is set to AC range? Or would you expect a reading when the red probe of your meter touches a cold ground?
From above, these are the things you need to know if you want to be able to successfully perform a voltage test on any piece of equipment. It’s not that easy to absorb everything I’ve just mentioned, but if you have the passion to do it (I mean learning everything, reading e-books, visiting forums, doing your own practice test, etc.), then you’re giving yourself a chance. to learn something that will really benefit you.
In this week’s article I’ll show you how performing a voltage test has helped me solve an unusual problem with a computer monitor.
Well, this MIMOS branded monitor (this is an OEM monitor) came with the complaint that it cannot be horizontally resized (there is no change when the horizontal resize control is pressed). The width (size) of the screen was large and both sides of the screen curved inward (pincushion outward). FYI, the horizontal size and the pincushion circuit were located in the same area and both circuits worked together to achieve a desired display. If one component in one circuit fails, it would also affect another circuit! That is why, when the horizontal size circuit has problems, the cushion circuit is also affected (the screen curves inwards).
Normally when I see this kind of problem I usually check and test all the components in the horizontal size and pincushion circuits area with my ESR, analog and digital meter. Most of the time, I was able to resolve the issue quickly. Common faulty parts for the horizontal size width problem in a monitor were generally dry joints, shorted modulation diode and pincushion transistor and open capacitor along the horizontal size coil line. But somehow I couldn’t locate the faulty components in this monitor because all the components I checked were well tested!
Since I have verified that all components in the horizontal size and pincushion circuit area are ok, now the only way to figure it out is by using the voltage test. In general, whenever you want to do a voltage test, you have to start from the source first. However, if you are experienced enough, you can always skip some section and start tracing the circuit that you think will fail. Since there are a lot of electronic repair newbies on this ERG member site, I would first show everyone how to trace back from the source.
Now think carefully, what causes the screen to become small or large each time the horizontal size control is pressed via the on-screen display (OSD)? There must be a signal sent from somewhere to the horizontal size circuit to control the screen. I am right? The pincushion transistor has a base pin and if this voltage varies (due to the horizontal size control being set to min or max), the display will also change. You can choose to trace the starts backwards 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 then you can also troubleshoot and fix other monitor issues like can’t control brightness, contrast, vertical and horizontal height. You can use this method that I am about to show you on TV and other electronic equipment as well.
You first need to understand what the function of the digital to analog conversion (DAC) circuit is. Since most modern monitors use digital signals (communication between CPU and EEPROM and other circuits as well), there must be some circuit to convert the digital signal to analog signal to control the pincushion transistor and other electronic circuits as well. Our normal multimeter can’t measure digital signal, it can only read analog signal. Older monitors have the DAC IC located outside of 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 its complete circuit. In other words, the DAC is necessary to communicate with the analog world!
In this faulty monitor, I don’t see any DAC IC near the CPU, so I assumed that the DAC was 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 when the monitor is powered on, you can perform the voltage test on all the CPU pins. Assuming you’ve taken all the necessary safety measures and precautions, now it’s time to turn on the monitor and begin voltage testing.
After controlling the monitor, select the horizontal size control through the OSD screen. You can see when the “+” and “-” buttons on the front panel are pressed, the number on the OSD screen indicates the minimum and maximum value that represents the size of the screen being adjusted. Since this is a faulty monitor, even though the number on the OSD screen is changing (because you are pressing “+” and “-“), there is no effect on the screen and the screen remains large and curved. But I could somehow measure the pins on the CPU to locate which of the pins is changing voltage when the set button is pressed! Try one by one of the CPU pins until you see one that has an effect when the horizontal size control was pressed. The voltage is approximately 0 to 5 volts (depends on the monitors design) and should momentarily go up or down when you change the horizontal resizing. Congratulations! You have now determined the source of the horizontally sized circuit. One tip, a bad CPU or corrupted EEPROM data might not cause signal on the horizontal size source pin!
Once you know the pin, it’s relatively easy to find the fault. Start tracing from the pin and see where the voltage was missing as shown in this photo. Surprisingly, the voltage is held well even down to pin 5 (inverting input) of the LM324 quad op-amp IC. Since this is an OP-AMP IC, it requires two inputs (inverting and non-inverting input) to produce an output on pin 7, but the voltage on pin 7 would not vary. And if this output will definitely not vary, you can’t control the horizontal size. The voltage on pin 5 of the LM324 IC varies and pin 7 does not vary, so the problem is most likely with the IC itself or components related to pin 6 (non-inverting input). I saw a wire coming out of the top of the dash and upon inspection; it was of B+ voltage (about 66 VDC) and passed through a resistor before reaching pin 6 of the OP-AMP IC. Using my analog meter to measure across the high ohm resistor (820k), I found that it was zero voltage. To really confirm if the resistor fails, I desoldered one of the wires and checked with my ohmmeter. Guess what? The resistor has an open circuit! Incredible and it was true! Replacing with a new resistor restored the screen to normal.
Conclusion: Imagine if I didn’t use the voltage test method, I don’t know how much time I’m going to spend on solving this type of problem. And what’s worse, I may not be able to solve the problem and return the monitor to my customer. With constant learning and practice in electronic repair, you will have no problem fixing any electronic failure in the future. Don’t give up and continue learning from the ERG member page. Thanks for being my member.
