The complaint in this LIKOM 15' L4031NDG Monitor was no display and has power. Well, no display can be no high voltage, failure in G1, G2 and heater circuit, defective flyback transformer, failure in picture tube and microprocessor or alot of reasons . To be able to find out the answer we have to switch on the Monitor. Upon switching on the monitor, I could not hear any sound from the monitor but the power led is on (green light). You can feel any static at the glass of the picture tube. This could possibly be a sign of no high voltage symptom.
Anyhow, I always be use my faithful Kaise SK9000 hollywood probe to in the event that the high voltage present at the anode. Sometimes seeking can't feel the static at the glass of CRT, doesn't mean how the high voltage isn't present. It's only low in high voltage; say become 10 to 12 kilovolts. Low high voltage and no high voltage are totally two different symptoms. Inside above case, my high voltage probe registered zero v! So it's confirmed no hollywood problem.
There are a lot of reasons for no high voltage problem. A missing B+ voltage, a defective flyback transformer, no horizontal drive pulse from h/v oscillator ic, low or missing supply voltage to horizontal driver transformer and etc could cause no high voltage. The first point that I have checked was the flyback internal capacitor-the capacitance is within the normal range which is 2.7 nanofarad. If the internal capacitor is shorted, it would cause no hollywood. The second point i have checked become okay was the B+ voltage. It showed about 50 + volts and also mean there is supply entering the B+ pin of flyback transformer.
When checking the third point (the voltage at the primary winding of horizontal flyback transformer), Discovered that it has zero volt that is very unusual. The base drive power supply input has about 14 volts but after the resistor it showed zero volts (by right it should have about 12 volt-voltage drop after the resistor). When I placed my finger on the base drive power resistor just after switching off the Monitor, I could felt it was popular. Now this is anything you call a clue. A good working base drive power resistor won't get hot till it burns your finger! There should be something that is now wrong or shorted that makes the power resistor turned extremely hot. From the result we get, day-to-day activities say that the horizontal drive circuit is having problem so my concentration is on this circuit.
A shorted horizontal driver transformer primary winding, horizontal driver transistor, components surrounding this circuit could cause the power resistor to turn comfortable. The horizontal driver transformer primary winding can be check by using a dick smith flyback tester while the horizontal driver transistor and surrounding components can be test with a normal multimeter. The result, surprisingly not even one component in this circuit found to be able to faulty.
Since I was unable to locate the defective components, the greatest to find out the culprit is arrive out with our schematic drawing base on this monitor circuit (using monitor circuit theory to resolve problem).
When the monitor is turned on their own h/v oscillator tda4858 will receive a supply voltage about 9-12 volts from power supply. This will send a square wave pulse to horizontal driver transistor through pin 7 (horizontal driver output) to increase the risk for transistor to move. Once the horizontal driver is switching the horizontal driver transformer is correct and a base signal was produced to turn concerning the Horizontal output transistor (HOT). This HOT would then drive the flyback transformer and 24 kilovolt would appear in the anode.
I asked myself what if the h/v oscillator ic tda4858 instead of producing a square pulse, it produce a constant signal? If it sends out a constant 5 volts signal to the horizontal driver transistor, this transistor would not switch and causing it to turn 'on' all the time. If this happen, the beds base drive power resistor and the horizontal driver transformer primary winding would get. The base drive power resistor and the horizontal driver transformer primary winding now would acts just as being filament or a lamp!. No wander the base drive power resistor was getting hot as soon as the monitor was fired up.
After carefully studied the horizontal drive circuit, I've made the conclusion that it was the h/v oscillator tda4848 that sends a wrong signal resulting in the base drive power resistor to get hot and possibly at the same time causing no high voltage in plenty of. I did not use an oscilloscope to compare and contrast for the square wave in order to prove my theory right because I'm very certain going without shoes was the h/v oscillator IC that output a wrong signal that caused no high voltage.
You will not likely believe this; yes developed this culprit (TDA4858) that caused the monitor no display problem. After replacing the IC, health benefits voltage immediately energized as well as the zero volts measured in the pin of horizontal driver transformer now showed 12 volts! What a relieved in addition to a great satisfaction in my heart. I really believe if we continue to switch on the monitor for a longer period, either the base drive power resistor or even the horizontal driver transformer primary winding would burnt out causing it to open circuit.
Repairing Monitors need patience, understanding how Monitor work and its signal plus the proper way to test electronic materials. If the practical way can't solve a problem, you might still use challenges way allowing you to out. Thus it is in order to know on how to read a monitor schematic diagram. Don't give up and then learn and i believe 1 day you shall achieved what's desired in your heart! Have a good times!