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Electronic Repair Questions/Answers & Testimonials Part 9

By Jestine Yong on August 13, 2008

Hi there,

Thanks for the newsletter. It is much appreciated. I have a question. I have a mitsubishi diamond point v71LCD 17″ monitor that powered on for a sec showed picture and went blank. I used a flashlight on the screen and i could still see the picture but it was dim. I knew the power board was fine and maybe it was the inverter area that had an issue.

I took out 2 three pin black transistors on the primary power supply side and put in wrong replacements without thinking. Soldered and plugged in the set.

No power at all. Now i replaced and put the original transistors back that i had took out previously, soldered back and still no power. Im wondering what i did wrong here? did i blow something. I can’t get any power that turns on the monitor at all. I can send you a pic if that would be more helpful.

Hi C,
Is okay this is what we call as process of learning. This experience will make you more careful in the future. I myself have gone through it many times-don’t worry. Usually when the display dim it was caused by a defective inverter circuit. Since you put in the wrong transistor this may have caused the corresponding to become faulty too. Sometimes it could cause the power IC to get spoilt too. Try rechecking and making sure all the soldering are good and not touching with other components. Is the two transistors placed at the wrong side? Did the two original transistors have problem in the first place?

You have another choice which is to check all of the electronic components in the power section and hopefully you could find out the problem. Yes you may send in some photos but send it to my yahoo account at jestineycs(at)yahoo.com . If you have other questions please do email me again. Have a good day!

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Hi jestine I have some good news I managed to repair my first faulty flat screen monitor after reading your book a short report on display shutdown caused by dry solder joints, I was about to give up the idea of repairing faulty monitors I had not-much success before with soldering and I was hoping that this was all the problem was and after examining the inverter board with magnifying glass I noticed what looked like dry joints on two pins from one of the high voltage transformers -and managed to do a better solder job and now the monitor is staying on and I should be able to sell it rather than scrap it. The monitor is an LG flatron 17 inch many thanks and all the best Bob S

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Hi Stuart,

One last question if I may. When I have found only the fuse gone high, I have checked the surrounding components, but high % of times all read ok, my question is, is it common for “only the fuse to be replaced” that the fuse has come to the end of its life only / nothing really caused it to blow?

(Of course there have been times where there has been more than just the fuse blown}

Yes, there is a chance that the fuse will die itself (fuses have life span too). If you refer to the pico fuse you can directly on the machine after the replacement but in the case of the

Main fuse, i guess the best is to connect a 100 watt bulb in series with the fuse holder and look at the brightness. If the brightness dim down then you can put the fuse back and “On” it.

I have always been interested in electronics & after reading your eBooks I now have a nice collection of lcd’s 90% now working, I think a high success rate & thats down to your help! I had been putting a fuse wire link across smd fuses to test and they have worked fine, but I would not like to sell them like that..

If the fuse wire link is within the same spec then it is okay but you have to solder it nicely. If you could get the original then use it.

I think I have recognized lcd symptoms to avoid & not waste time with, & I am hoping that I have found an extra income with something I love doing!

That’s the same thinking that i have. Do something that i like and at the same time earn money from it.

I hope you are ok with me asking questions, its just for my own piece of mind & confidence

No problem

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Hi Jestine,

Please take your time to have a look of my power point for the defect parts (NEC2412) that i found from my home TV Sets. Currently I confusing on testing Inductor (Coil) with the Dick Smitch Flyback tester. I really not sure the parts is OK or NG after check with the tester.

At the same time, some of the parts i not able to get the datasheet and have not idea about the parts description. Please guide me if you have got the parts description.

Hi D,
The transformer that you tested was a horizontal driver transformer and the reading seems good as the primary winding have less winding compare to the one from the power supply section. If you have other TV most probably you can compare the reading.

As for the coil, it could be good too if the coil is a horizontal size or linearity coil. Why? because dick smith meter is not design to test any coil that have inductance reading of less than may be .3 or .2 milihenry. The coils found in the flyback, power transformer, monitor b+ coil and horizontal yoke coil have inductance value of more than .2 and above! That’s why the horizontal driver transformer has only two LED lit when tested with dick smith meter. In other words, the higher the inductance value of a coil, the higher reading of LED you will get. If it is shorted then the LED will go off or have very low LED.

As for the transistor, I could not find from the internet because it could be their own customize coding.

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Hi Peter,I have a board with the same problem I have put a spare board from another monitor and the monitor works OK so I know the problem is
In the board …all I have is the inverter chip pin 9 Vcc is high but all the other pins have no voltage on them it is such a large area with
all the SMDs on the tracks I am not sure if I am looking at the feedback or perhaps the oscillator circuit.

If the display shutdown and the supply voltage is still present at the input pin of the inverter IC then most probably it could be fault in the backlight, transformer, tuning capacitor or even transistor (some designs using small 8 pins SMD IC that have FET transistor in it.). It could also the fault of the inverter IC since you already tested that there are no output voltages from the inverter IC.

Do you have any more schematics of the Oscillator and feedback circuits
like the one on page 36 of your E-book

Try http://www.eserviceinfo.com as the website have lots of free lcd monitor schematic diagram.

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THANK YOU SIR….

i have some few question about my lcd monitor viewsonic VA702b,model # VS10781;

This monitor has no power and totally dead, it is possible that the high voltage x’former is the problem, and sir give some guide to repair this problem to my monitor,,

i hope u will give some idea,,,,,,,,,,,,,,,,, thank’s and best regards to your profesion,,,,,,,

Hi Mar,
I don’t think the problem with high voltage transformer at the inverter side. No power problem most probably in the primary side and also could be something shorted at the secondary side like a shorted diode. Check all of the components in the power supply as lcd monitor contains less components than the CRT Monitor. Particularly pay more attention on the semiconductor like the power ic, power fet in primary side and diodes in the secondary side. Sometimes a bad mainboard may cause no power too. If you get voltages at the secondary side then chances is high the mainboard have problem.

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Could you tell me what type of resister is a trimmer,&what is its function,

Please go to this website http://en.wikipedia.org/wiki/Trimmer_(electronics)

also if i do not get a capacitor of same value<pf,nf,uf> can i use a higher value, or lower.?

It depends on the circuit. If the capacitor located at secondary act as a filter capacitor then you can use a higher value let say from 1000uf to 2200uf without problem. If it locate in the timing circuit or circuit that related to frequency then you have to use back the same value.

Best Regards,
Jestine Yong

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Hi Tat,i have an electronic enthusiasts for sometime already.

Nice to know about your background!

As what u mention, it is very difficult to have grasped on troubleshooting electronic board. In yr e-book is all the testing will be done without taking the component out for measurement?

Some components you can test on board and some you have to remove it out to accurately test it. Once you have familiar in testing electronic components, you can actually test 30 components in less than 10 minutes!

Will u be using oscilloscope?

No except in the topic of

Crystal. To check crystal we need oscilloscope. I do mention about scope waveform in lcd monitor repair ebook.

What be the starting pt, learning the component testing and then follow by in depth like LCD repair, plasma repair?Yes that’s the right way. Be familiar with testing electronic components first as this is the foundation for all electronic repair work. Even if you could know the color board have problem but because you are weak in testing electronic components, you will have a hard day to locate the culprit!

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Respected Sir

One of my friend bring a DIGITAL Monitor the problen is no HV after inspecting the monitor i saw alot of dry joints and on the yoke connector the Horiz pin which is sold on the board is totally dry and black spoted. I resold the whole board and solve the HV problem now High Voltage is o.k but there is no picture on the screen.So please help me . Thank You

Hi M,
Since there is high voltage then your concentration should be in the screen voltage (G2), G1 (negative voltage) and also the heater voltage (6.3 vdc). All these points can be found at the CRT board. If the screen voltage too low, G1 negative voltage too high and missing heater voltage-all these could cause no display symptom. Hope this helps!

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Jestine, thanks for the reply. By output voltages did you mean the secondary side of the transformer. If so I checked voltages from cold ground to the secondary side of the transformer and the voltages on each pin would climb to a peak and then slowly climb back down and then slowly back up. It this correct?

Hi R,
Yes, thats what we called as power supply pulsating/cycling. Try isolate each of the line after the diode like removing a jumper or coil along the line. Try one line at a time and if the power supply stopped cycling then i guess thats the line that had caused the power to pulsate.
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Hi R,
May I know how much is the cost of the following tester (including postage) if posted to Sarawak:-

(A) Digital Multimeter-Sorry i doesn’t sell Digital multimeter as you can easily get one from any electronic supplier in sarawak

(B) Blue Ring Tester- Kitset is rm200 and fully assembled unit would be rm300 (inclusive transport charge to sarawak)
(C) Dick Smith ESR Meter- No more in production
(C) Blue ESR Meter- Kitset is rm330 and fully assembled unit would be rm430.00 (inclusive transport charge to sarawak)
Among the Blue ESR Meter & Dick Smith ESR Meter, which one do you recommend? Are these two meters the same?

Both meters were designed by bob parker and they are the same except that the newer version (Blue ESR meter) have a better energy saving for battery.

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Hi S,

This lcd use the same board in the lcd 17 dell, and the inverter section is composed of four high voltage trasformer drived by a pair of c5707 and capacitors marked 100v .150 the capacitor of second inverter blow with the transistor the power line vcc in the transistor is stable to 16.9 v and is present when the inverter ic start up tomorrow at work i check winding in the second pair trasformer. can easy check the winding in the transformer?

You need a coil tester to test the transformer winding.

and u suggest to disable one inverter lcd backlight and use only one with 2 tubes?

But you need to do the modification work

can i drive the tubes with external inverter found in another 17 monitor that use the same configuration? i think to use another inverter to connect to power on signal dim signal and power with the same levels how about this?
If have the same configuration and the external inverter could fit into the lcd, then you can proceed to use it.

By the way have you check the P-channel FET i.e the part number is either fu9024n or FQU11P06 for short circuit?

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3 Comments

nowdaygood

August 13, 2008 at 10:55 pm

dear jestine
What's do you think abount LCD monitor repair line ? because now laptop replace desktop day by day.
Bye.

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nowdaygood

August 13, 2008 at 11:00 pm

Dear jestine
I have a second question.I hope you can help me.I have a Acer LCD, Both side all white color 10cm , screen in center Ok.What pasts i need check now ?
Best wish to you.

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admin

August 14, 2008 at 8:57 am

Hi,
I'm getting more an more LCD Monitor repair compare to CRT Monitor. The profit margin is very good and most importantly it is easier to repair than CRT Monitor. As for your second question, chances is very high the LCD PANEL is the cause of the problem. It look like either both the side LCD DRIVER IC have problem or both the IC did not receieve the signal from LCD controller IC.

Jestine

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