Electronics Repair Questions/Answers and Testimonials- Part 83
Thank you for the newsletter, also thank you Mr. Kent Liew, the article is amazingly useful
not only for the repairers, but also for the users of LCD TV!
Thank you again, and I wish you realise all your plans, now and forever,
Thank you for your quick response. I didn’t know what to expect. I am certainly going to recommend you and your products to everyone. I will purchase more in the future.
Thank you again
please i will like you to tell me how to test voltage regulator if
functioning or not and how to identify positive output voltage
regulator and negative output voltage regulator from there part
Find out the part number from datasheet website. From there you will know if it is a positive or negative output voltage. You can use your dc voltmeter to test it. Black probe to ground and red to output pin.
I’ve got power supply that all passive component were good. checked all the capacitor value and ESR is good. the problem is, during turn ON, the power supply is good and can function. but after 15minute it OFF and cannot turn ON back. a few hour after that i turn ON back and OK. but still repeat the same problem. Why?
Thanks and Regards:
Try check if there is any dry joints. You may also use a freezer to detect such fault because some semiconductors could breakdown when it is hot.
Dear Mr. Jestine ,
Thank you very much.The articles are very good and easy to understand like spoon feeding.After going through it I could service myself the PSU of my PC model AP200-400watt.
I have noted a very important thing which I never compared or found. When I measure voltage of screen grid in ctv with costly digital meter, it is found 300v but when it is measured with simple digital meter it is found about 65v. This is a very good experience and comparative study for me with high quality meter and low quality meter. This difference is compared only at screen grid but not other points. One more thing is that good meter has stable voltage reading then general meter.
Noted that’s why i do not use cheap meters.
I intend to buy your book aprox in 2 week (i don’t have banc acount and i have to go to make one).
I think will be no problem that the acount is in EURO and the book price is in USD.
I guess the bank will convert the value for you.
I have also a question, if you can help me. I have a Benq PF71V+ monitor who fail after 10 minutes. The display goes black and the power led remains blue.
The condensators are not leaked. Do you think i have to replace them?
Also the IC701is very hot.
Make sure the output voltages are good, nor dry joints in high voltage transformer and check if one of the backlight have problem or not. If all check to be good then directly replace the inverter ic and retest.
I have a doubt about one scenario that was happen to power supply two times.
I couldn’t realise what was really happen according to my experience.Just one and half year
The problem was I received a computer which is not power on.
So I remove the case and power on, Then CPU fan started and suddenly stop.No power to CPU green light also.
So I know its a case of power supply.So I removed power supply from CPU.But
I want recheck it using power good.So I checked it’s voltage it’s 3.8v and then I short PG with ground
using a paper clip.I heard fan is staring and but no voltages in 12V(yellow wires).I was happy I found the problem
Then I installed new power supply to CPU and check it’s mother board and very thing working fine and I send it to customer back.
On next I want to check that power supply again and I did same thing using paper clip.Because I want to repair
Unbelievable I heard fan is working then I tested voltages.Now it’s working 12V,5V 3.3V all are live.
Same thing happened two times in different PSUs.I couldn’t understand it.
If you can please tell what was really happened or are there any problems in powers supply.
It seems that it is a dry joints problem. I suggest that you check the pins of transformer and also all the pins in the secondary side.
Hello Jestine…saya sedang troubleshoot sebuah LCD komputer jenama Acer 19"
Problem LCD ni fius dia blow walaupun saya dah gantikan yang baru.Selepas
saya troubleshoot saya dapati rectifier dengan satu ic 3 pin dah rosak tapi
mengapa setelah replace kedua-dua komponen tersebut fius tetap blow bila
saya switch on the power?Kerosakan transformer boleh ke menyebabkan fius blow?
Transformer jarang rosak. Sekiranya komponen 3 kaki rosak (fet), biasanya power ic pun rosak. Saya cadangkan anda check semua komponen sekali lagi.
Yang kedua ada satu smd resistor berhampiran dengan ic 3 pin yang rosak tadi yang
dah burn serta tak dapat baca value smd resistor tersebut.Ada cara lain tak untuk
replace resistor ini.
Dah burn memang susah dan anda kena cari model yg sama untuk compare ataupun cari schematic. Workshop saya tiada acer 19" kalau tidak saya boleh buka dan tengok nilai smd resistor tersebut.
Dear Jestine Yong,
First of all, I want to let you know that I have found your Ebooks very informative and very well written. I noticed that you have a lot of the same conceptual ideas that I have always felt were very important, not only in electronics work but in everyday life. I just finished reading the latest report that you sent to me by email which effectively was titled 7 ways to save time in the electronics repair line and when I read the last section on how rude people are and how they like to provoke people due to their egos and jealousy and so on, I just had to write to you and tell you that you are very much correct. I am saying this from experience of where I work. I taught myself electronics after being forced into a career change a number of years ago due to an on the job injury. I studied very hard then and I still study any books that I can get my hands on from cover to cover in order to get the best understanding of the material I can. Back in the Spring of 2000, I was told that when I had been injured on the job and because I had been told by the doctor that I would need to find a new career as I would never be able to perform the work I had been trained to do again that the state where I was injured was responsible for retraining me to work in another career field. I already had a couple of college degrees and had spent a lot of valuable time to earn those degrees and now I could not do that work again so I took their offer to send me back to school to learn a new career but only after I had taught myself a lot about the field I had a very passionate interest in and to this day I still do, yes, that new career field is electronics.
After completing the first course in DC/AC Fundamentals, which I already had learned a lot about on my own before attending the class, the instructor who taught the course advised me that I should go and apply for a job at the school as he had seen that I could explain the material to my fellow classmates so that they could easily understand the material covered. Within the two years that it took to complete the program, I went from student to full-time instructor while my fellow classmates went on to graduate with their degree in Electronics Technology. I continues to strive for ways to develop new ideas and ways to teach electronics that not only would make it fun for the students to learn electronics but also to get them actively involved in the lectures and I was successful. That is when the problems began, as the other instructors became very jealous of me and my ability to teach the students the theory so that they wanted to be in the classes, even 3 hours before the class actually started in most cases. The students would approach me and tell me in the next term that followed how much they liked and appreciated how I incorporated a hands-on approach to teaching electronics that really helped them to learn the material and when they enrolled in the next class taught by another instructor that instructor basically read to them from the book and when they asked that instructor to teach like I did where they could do hands-on experiments during the lectures to understand the material better they stated that the instructor teaching them told them that my methods of teaching were stupid and that it was the wrong way to teach electronics. Shortly afterwards, those other instructors would come into my classes and request to speak with me while my students were doing their laboratory assignments in class. Upon allowing them to do so, these other instructors then told me that I needed to stop or slow down what I was doing as I was making them all look bad and they were not willing to devote as much time to teaching the classes as I did. I had already tried to share the material that I was using and my methods for covering the material with them but they just insisted that my methods were too hard and too time consuming for them to incorporate.
One of those other instructors has now become the chairman of the department and in many of our meetings that instructor and others who sided with that instructor would tell me that any of my ideas that I had to make the classes more informative and enjoyable for the students were very stupid and not worth their time and effort and then they would take certain segments of my ideas and submit them to the upper administration as though the ideas were theirs and they were given credit for the ideas. I still to this day work as a full-time electronics instructor but I have learned to use my ideas that I develop to teach my students but I feel like I have had to learn to teach my students using my methods in secret. I am still harassed by other instructors and now it has reached the point that the upper administration has joined in to take great enjoyment to provoke me and they seem to enjoy looking for things to use to complain about my job performance and the only thing they really have to use against me is that I am too detailed and too patient which makes me slow to them. However, my students tell me that I can teach them more in 5 hours than they learn from the other instructors in 15 weeks and I tell them more than just 3 words about how things work as that is about all they get from the other instructors. My students have even told me that they felt lucky in their other classes if they actually were given an explanation that was a complete sentence.
I do not share any of my ideas with the other instructors anymore and when I found your Ebooks I felt I had found the one person who had the same exact ideas and could explain things like I felt they should be explained so I am very appreciative that I found your Ebooks and that you took the time to write these very informative books. Do not worry, your information is safe with me and I will never share any of your methods with anyone from the school where I work. I have really always wanted to and have even strengthened that desire to do electronics work on my own and to start my own electronics business and I feel that through reading your Ebooks and applying your ideas and because of my very strong passionate enjoyment of electronics, which I feel is just as strong as your enjoyment of the field, I now have that very opportunity to do just that and perhaps I can finally do something I really enjoy without the frustration I feel from being treated the way that I am treated at the school where I currently work and I really do plan to ultimately quit working at the school as soon as I can get my business going. I have even thought of starting my own school much like you did so that I can teach things in the way that I feel they should be taught as I really do enjoy teaching electronics to others also.
I want to thank you again for writing such wonderful Ebooks that have given me the ability to strengthen my will and that have given me the inspiration that I thought I had lost because of how others attitudes toward me affected me. Your written work has given me a new will and inspiration to continue what I have always enjoyed doing which is learning about and working with electronics and I feel that I cannot thank you enough for that. Keep up the wonderful and very inspiring work that you have begun as you have definitely helped me and I have read the great comments from others so you are doing a great job.
1) How to check the EEPROM whether it is faulty or not?
You can’t check with meters because there is programs inside. The best it by comparison method. That’s mean substitute it with a good eeprom and if the set start to work again then you know the original eeprom have problem.
2) Can I replace the faulty EEPROM directly?
You can’t because new eeprom is blank. You need to replace an eeprom that have working/good data inside.
3) If I replace the EEPROM, is it all the data inside will be erased?
If you take out the eeprom the data in it will not be erased. It can only be erased with a programmer.
i would like to know how do you know the max base voltage or collector voltage of a transistor by reading the details on the internet?
i have tried to check for d2058 on the internet but i am getting vcbo,vceo and vebo so which one between this is indicating the base voltage and which one is for collector? i was expecting to see something like base vcc. please help me to understand this.
hi sir jestine,how are you?i hve a problem about the philips television 29 inch,having a problem vertical line now i replace the vertical ic the picture was normal when opened after a moment the picture become dis apear,i measure the G2 voltage no voltage is measured,i switch off and on the tv it has picture again after while the picture dis apear again,,which section im going to check and what should be the suspected parts faulty?thanks jackson lor
Make sure the B+ voltage did not slowly increase otherwise the tv will shutdown. Check that all the supply voltages from the smps are good and stable.
Hello sir!! how are you now ? i have something to consult about the transistor that blown out in a smps unit.its DG 2N60,
a 2amp,600v N channel-mosfet.
Vdss=600v, Vgss=30v, package=T0-220f.
I have already searched the internet for replacement but i found nothing reliable since its not available here in our local market.I hope you could help me!!! thank you sir
Sorry my databook does not have this spec thus can’t get the replacement. Try buy the fet instead of finding the substitute:
How is it posible to find faulty components on pcb’s without
It depends on what type of board you are troubleshooting. Generally we need meters to find out the fault.
Testing individual components is not the
challenge, finding the faulty component is.
By testing individual components then only we can find the faulty components. Some components can be look good in the appearance but it is bad internally. Software can’t be used to test on individual component.
How do I recognise what
the fault is and how do I find it among hundreds of components?
You need to understand how an equipment work and every equipment will have different section. Take LCD Monitor as an example. If there is no power then we will check on the power supply board. If there is no display but with power we will check on the inverter or the mainboard.
Once you understand how a circuit work, you will immediately know where to start to troubleshoot and this will save you lots of time.
Hi Jestine ,
I am stuck up in between a repair work of an LCD monitor of ACER brand.
The problem is it shuts down after a few seconds. When the VGA cable is
removed it works showing no signal caption .As soon as the cable is connected the raster switches off .I checked the ON/OFF signal from the Main board and found to be below 0.5V when its OFF and 3V when its ON.
I checked the Power supply board and it has two 5V connections to the main board.These are found to be normal and a 12V bulb connected to it glows continuously.But the 12V supply line to the inverter does not glow properly and sinks continuously.I have checked all capacitors diodes down the line and found be OK. I suspect 12V line as the cause .Is my path correct or i am off track.
Try check all the components that is related to the R,G,B, H and V line in the mainboard. If there is components open or short circuit the display will shutdown. If all the smd components check to be okay then the problem could be a bad MCU.
i have viewsonic e70f CRT monitor(17").its now dead.i checked there is a short in HOT.(ST2408hi)but i can’t find the right one.so if u know the equalent for that HOT please tell me.
Sorry i do not have the equivalent number. You can try C5411 or C5407. Let it run for few minutes to few hours and turn off the monitor and touch the HOT. If it is warm the it is ok. If it is hot then it can’t be used. Make sure to check for the condition of the e-cap located at the primary side of horizontal driver transformer.
1. I have a problem in IBM G54 model OAN 6546 with power blink sympotum . I read your article regarding "Power Blink Problem in CRT Monitor" and i suspected a zener diode in primery side of SMPS,value of 2V4 (2.4V) showing full reading in both side with an anlogue meter.
You need to see it carefully because this small voltage zener diode could be mistaken pointing to two same direction using the analog meter set to x 10 k ohm. A good one should not have the same reading.
2. May i use zener diode of different value in place of 2v4?
The best is to use back the original voltage. Zener diode is quite common in the market thus you should easily find one.
3. Kindly help to release me from this tension becasue there are 2 more monitors of same model also laying with the same fault.
Isolate the problem by using the link below:
Have a good day!
These days i practice a lot on how to test electronics components by following the instructions in your ebooks, my testing skill is getting better and better. now, i have to learn how to order parts online. i find out two schottky diodes shorted on the power board, one is SB5100 , PEC 6329 is written on the second row. Another one is SB3100, PEC 7189 is written on the second row. i search on the internet by using SB5100 and SB3100 and i see that some suppliers have them in stock. my question is do i have to care about PEC 6329 and PEC 7189? do they mean anything? i notice that most of the electronic components have more than one rows of writtings. thank you for your good advice. have a good day!
For schottky diode look for the first row SBXXX or SBXXXX The second row can be the part version or could only be understood by the manufacturer.
If we make a very simple 220AC to 12VDC transformer by using a 220-12 transformer and a bridge rectifier like KBPC2502, what will happen with the 12v output if there is an upsurge of voltage more than 220V (example upsurge 230v~280v)?
The 12 volt will increase and the filter cap will blow together with the circuit that is connected to it.
I wanted to ask a question on the LCD no signal problem if the cause was ever determined, such as connecting to the computer while the monitor and computer were both on??
Sometimes I make a hot connection with the power on to a computer and monitor but have never burned out parts to the VGA input or damaged the computer video graphics adapter. Is it a good idea to always turn off your computers when connecting or disconnecting your monitors?
Generally yes, you need to turn it off first before connecting /disconnecting the cable. Again it depends on the design. I have seen certain design when you connect the vga cable , there will be a small spark causing the CPU to hang. Just imagine if one do it many times in a day pulling out and pluggin in, this will surely cause problem either to the CPU or the monitor itself.
Thanks for your news letter.I want to know what is the Zener voltage of below Zener Doid or other similar Zener diod?
Is Zener voltage 22V ?
Well it can be 22 volt but it can also be any other voltage that is only be understood by the manufacturer because this is not a standard zener diode code number.
I am avid fan of your and also want to learn how to repair a LCD monitor, by doing this, I have searched and study online regarding how to repair a monitor, I’ve learned some basic repairing on your handbook but the one that I encountered now on my problem in my LCD is not included on your book.
Here’s my problem, I have Dell E198FPf 19" LCD monitor, I can used it for about 15-20 minutes and after that the screen will goes black, the power will indicate a color green blinking led and there is also ticking sound inside, I will turn it off and leave for about an hour then I will plug again and the same problem will happened. Can you help me how can I troubleshoot this monitor?
It could be the problem of buldge e-caps. Open the cover and make sure the output voltages are good and stable and check also for any dry joints in the transformer pins.
that’s ok sir. pls. help me to solve my problem, a 29 inches Ganzklar television model JL-29420P. the problem is horizontal output easily get hot after 5 mins.then the base,collector & emitter becomes shorted. I’d check already the yoke, flyback, voltages & of course the h driver and it was found out. i don’t know about the freq’s and signal because i don’t have an equipment w/c is the oscilloscope to monitor this. just only give me an idea how to solve this kind of problem. thanks.
Please refer to this link:
hello! Good day! i just wanted to ask something about the samsung LCD monitor, the 153v model. i just wanted to know what is the value of the diode soldered between the heat sink of the power ic TOP247F and the transformer. Because the code number that’s on my PCB is DYV26C PH. I hope to here something from you.. Thanks! and more power!
This model is no longer in my workshop thus i can’t get the value for you. Try use UF4007 as a replacement.
First, I wish to apologize for the missing images in my first message. I thought they were all included. This time I will double check to be sure they are all included.
Second, in your last message, you said: "You are really good in logical and practical thinking. I like the way you troubleshoot a circuit.". I can only reply to this by saying: Thank you. All that I have learned about electronic troubleshooting I have learned it from you and your books "Testing Electronic Components" (and included reports) and "LCD Monitor Repair" (and included reports). You are a good teacher. So, again, thanks to you.
You are welcome!
NOTE: All images references in the following text are included in "IMAGES_6-13.PDF" document. This document contains images 6 to 13 on 5 pages. I double checked, and all images are in the document. So, if you see only the first page (or the first image) of this document, please let me know and I will send you the images as standalone images.
Now, let’s start the troubleshooting analysis.
Because of my mistake (shorting the FB and GND or FB and SD pins of the PWM Buck DC/DC Converter component – see IMAGE-6), I decided to investigate again all the voltages (from the power supply to the inverter board). Here’s the results.
As you can see from IMAGE-7, the voltages coming from the power supply and going into the digital processing board seem correct. The voltage that looks odd is the one from the digital processing board, connector X802, pin 10, labelled PANEL_PWM, with a 0.1 VDC.
Q-1 (IMAGE-7): I know that "diamond" shape in a flowchart is used for "decision" or "question" with a YES and NO or TRUE or FALSE output. But, in electronic diagram, I did not found the meaning of this "diamond" shape. So, I interpreted it as a "connection" between two points in the diagram. Is it correct?
So, in this case, and if my interpretation is correct, there is a connection between pin 10 of the digital processing board’s connector (X802) and one of the lines of the BACKLIGHT CONTROL (also on the digital processing board) labelled as PANEL_PWM. Am I right, or is it means something else?
Yes you are right.
Q-2 (IMAGE-7): If I interpret the schematic symbol of the V708 and V709 components correctly, it’s an NPN transistor acting as a switch. Is it correct? If I’m right, and because there is a 0 VDC and a 0.1 VDC coming into the base, that means the switch is OFF and the circuit is open. So, the 5 VDC line cannot complete its circuit, hence cannot pass the transistor. Is it still right?
I can’t confirm what type of transistor circuit is this and it is definitely not a on or off transistor circuit. it could be some sort of amplification circuit. If you take a scope at both input to the transistor, i believe there will be a signal waveform.
Q-3 (IMAGE-7): About this 5 VDC line, what’s the "T" connection with the label "5V" means exactly? Is that means that there is a 5 VDC source somewhere? If it’s the case, where is this source come from?
This mean a 5 volt supply source and usually from the smps or from a 5 volt regulator ic.
Finally, in the BACKLIGHT CONTROL circuit (IMAGE-7), because all voltages seem odd (except for the 3.4 VDC), because there is some voltage changes between the "before-I-shorted-the-AP1501-ADJ-component" and "after-I-shorted-the-AP1501-ADJ-component", and because the PANEL_PWM line is connected to the analog board’s connector (through pin 10), I decided to investigate the analog board (see IMAGE-8).
This is where I previously made my mistake. While I was investigating the voltages (the first time), I think that I accidentally shorted the GND and FB or FB and SD pins of the PWM Buck DC/DC Converter (AP1501-ADJ – N581) component. Since then, I observed voltage changes in other circuits (see IMAGE-7 and IMAGE-11). As you can see (from IMAGE-8), on the 24 VDC line and circuit, all voltages look normal, except for the 1.3 VDC of the FB pin, but I’m not sure. The specifications for this component are as the following (see "AP1501_PWM_BUCK_DC-DC_CONVERTER.PDF" document):
– Absolute maximum rating for the feedback pin voltage: -0.3V to +25v (see page 3).
– Typical application circuit for the feedback pin voltage: 3.3V (see page 7).
But, if I take into account the fact that there is two resistors (R581 and R582) on this line, maybe the second resistor (R581) is playing a role in the voltage drop from 8.4 VDC to 1.3 VDC. Here’s the formula I used to calculate the voltage drop with two resistors in series:
8.4V / (15K + 2.7K) = 8.4V / 17.7K = 0.47457627mA
0.47457627mA * 15K = 7.12V drop (across R581)
8.4V – 7.12V = 1.28V
So, maybe the 1.3 VDC is normal voltage. What do you think?
It is a good ic because you get the 8 volt and the 5 volt regulator ic have 5 volt output. Sometimes a minor short will not destroy an IC.
Also, while I was investigating the voltages again, in this 24 VDC line circuit, the moment I touched the PWM Buck DC/DC Converter (AP1501-ADJ – N581) component’s output pin (2) with my multimeter’s red probe, I saw a tiny spark coming from this pin, just between the probe’s tip and the pin. Is it normal? Could this PWM Buck DC/DC Converter (AP1501-ADJ – N581) component be defective, even if all voltages look normal in this circuit?
It is normal and we should see the end result. The end result we got the 5 volt output.
Moreover, because all voltages have not changed in this circuit between the "before-I-shorted-the-AP1501-ADJ-component" and "after-I-shorted-the-AP1501-ADJ-component" and because I cannot see how this circuit is related to other circuits, I just do not know where to look and what to search which could cause voltage changes in the other circuits.
Since you are now already got the 5 volt output you should concentrate on other circuits.
If I think back, here’s the facts. Definitely, voltage changes happened between the "before-I-shorted-the-AP1501-ADJ-component" and "after-I-shorted-the-AP1501-ADJ-component". But now, I’m wondering if this short really happens. Maybe I saw the same spark as I saw when I touched the PWM Buck DC/DC Converter (AP1501-ADJ – N581) component’s output pin (2) with my multimeter’s red probe and a tiny spark came from this pin, just between the probe’s tip and the pin. Maybe the voltage changes are just a coincidence with what I think of being a "short circuit" incident. Maybe I’m chasing two problems when in fact there is only one. Maybe it’s the same original "NO DISPLAY" problem that is simply getting worst. What puzzles me, is the fact that in the circuit, where the "short circuit" (maybe) happened, all voltages remain as before and this circuit seems to be isolated (I mean, not directly connected to none of the circuits where the voltage changes occur). If I follow backward the 24 VDC line that is connected to the input (Vin) of the PWM Buck DC/DC Converter (AP1501-ADJ – N581) component, it goes straight to the power supply. There is no direct connection with the 4.6 VDC line nor with the 0.1 VDC line. What do you think?
Since now all the voltages are good you should now focus on why there was no display. Pay particular attention to the ICs in the mainboard.
So, despite this "short circuit" incident, I continued my investigation with the BRIGHTNESS line (connected to the digital processing board through the connector’s pin 10 – IMAGE-8). What caught my attention is the voltages around the V582 SMD NPN transistor. Because of the way the NPN transistor is displayed (on the diagram) and if I correctly understand how an NPN transistor works, I presume that the current flows from the 5V-MCU source line, goes into the NPN transistor, then goes to the BRIGHTNESS line and finally goes to the connector’s pin 10. Is it right?
To be more specific, and if I correctly understand how an NPN transistor works, the 5 VDC coming from the PWM-MCU line and going into the NPN transistor’s base should be enough voltage to turn the NPN transistor’s switch ON, hence enabling the 5V-MCU source to complete its circuit and then go into the BRIGHTNESS line. Is it still right?
Again this is not a on/off transistor circuit. PWM means pulse width modulation that means it is a pulse going to the transistor.
So, if I’m right and if the NPN transistor works correctly, then why the voltage drops from 5 VDC to 0.1 VDC when it goes through the R584 (1K) resistor?
I have yet to find such transistor circuit configuration but there is a possibility the transistor may have problem
I checked this resistor (on-board), with my digital multimeter, and it gave me a 0.998K OHMS result. I know that it’s an on-board test and it could be inaccurate, but with a result so close to its normal value, could that be an unreliable result?
You should test it with a scope to see if there is any signal.
My next thought was that the two resistors (R584 and R585) connected in series play a role in this voltage drop. Here’s again the formula:
5V / (1K + 10K) = 5V / 11K = 0.45454545mA
0.45454545mA * 11K = 5V drop (across R584 and R585)
5V – 5V = 0V
Now, if the formula is correct, and if the resistors are good, then why to drop the voltage from 5 VDC to 0 VDC?
So, this last question raised another thought. If my understanding about the V582 SMD NPN transistor and the current flow is correct and if the R584 (1K) resistor is good, can the V582 SMD NPN transistor be defective?
If it’s the case (a defective NPN transistor), and again, if my understanding is correct (about the NPN transistor and the current flow), it means that the NPN transistor’s switch is OFF (open circuit) and the 5 VDC source cannot pass the NPN transistor to go to the BRIGHTNESS line and then to the connector’s pin 10. Is it right?
This is not a ON/ off circuit.
Based on all those observations about the SMD NPN transistor, and if those observations are correct, is this makes of this SMD NPN transistor a good enough candidate to be replaced?
You need to check first and it is bad you can replace it.
Now, leaving the analog board and going back to the BACKLIGHT CONTROL of the digital processing board (see IMAGE-7), the PANEL_PWM line is connected to the BRIGHTNESS line that goes to the connector’s pin 11 which in turn is connected through wires to the inverter board.
Here, as you can see from IMAGE-9, I managed to lift the inverter board and under it the connector’s pins was labelled as displayed on the image. The VIPWM pin is the one from which the PANEL_PWM and BRIGHTNESS lines (of the BACKLIGHT CONTROL) are connected to. Also, you can see that the ON/OFF pin was having a 4.6 VDC before the "short circuit" incident of the AP1501-ADJ (N581) component (see IMAGE-8) and now, after the "short circuit" incident of the AP1501-ADJ (N581) component (IMAGE-8), it has a 0.6 VDC. The only common point between those two circuits, I mean the 4.6 VDC circuit and the 24 VDC circuit (which the AP1501-ADJ – N581 – component is connected to), it is that they are both directly connected to the power supply and the power supply’s output voltages seem correct.
This is the main reason why there was no supply voltage to the inverter IC. You need to trace it backward to see at where the voltage had dropped. If it comes from the MCU pin, possibly the MCU may have problem.
Now, continuing my voltage investigation, I tested all 4 fuses of the inverter board and all 4 seem to be in good condition. The full 24.7 VDC goes through each of them (see IMAGE-10).
After that, I checked the OZ960GN inverter IC and now it yields different voltages from the first time (I mean, before the "short circuit" incident of the AP1501-ADJ – N581 – component – see IMAGE-11). As you can see, where there was voltage (before the "short circuit" incident of the AP1501-ADJ – N581 – component), now there is none and where there was no voltage, now there is very low voltage that is appearing; like if something somewhere is leaking some voltage. And, as I said in my previous message, I cannot find this inverter board’s schematic diagram, which is a DARFON V070-001 4H.V078.001 /E5. The only thing I found is the OZ960 IC datasheet (see "INVERTER_IC_OZ960.PDF" document).
I suggest that you lift up the vcc pin of OZ960 and if the voltage comes back that means the ic is shorted and need to be replaced.
The last thing I investigated, is the voltage coming from the pin 1 of the inverter board’s connector. As you can see from IMAGE-12, the voltage source is 24 VDC, then it drops to 13 VDC when it goes through the R107 resistor, and then it drops again to 0 VDC when it goes through the ZD4 zener diode.
First, I tested the R107 resistor with my ESR meter (on-board) and it returns odd result. So, I replaced this R107 resistor (which is a 240K OHMS SDM resistor).
ESR meter can’t test such a high ohm resistor. It could only test until 99 ohm maximum. You need an ohmmeter to test it.
But, no luck, the problem remains with the same voltage drop (24 VDC to 13 VDC) when it goes through this R107 resistor. Because I do not have the schematic diagram of this board, I do not know if this R107 resistor is connected or not in series with other resistors. So, I do not officially know if the voltage drop across this resistor is correct or not. But, since I replaced it with a brand new one, I suppose that the observed voltage drop across this resistor is legitimate. Unless it is connected in series with other resistors that are defective. For now, I’m in the dark about this R107 resistor.
The resistor is ok and you should focus on the components along the output side of the resistor.
Next, the zener diode (with a voltage drop from 13 VDC to 0 VDC). But first, here’s my understanding of how zener diode works. A zener diode allows current to flow in both directions. In the "forward" direction (or bias), no current will flow until the voltage across the diode is about ±0.7 VDC (as with a normal diode). In the "reverse" direction (or bias) no current will flow until the voltage approaches the "zener" voltage (or the breakdown voltage – which is the voltage rating written on the diode), after which the current can flow through the zener diode. Is it right?
Yes you are right.
If I’m right (up to this point), concerning the "reverse" bias and the breakdown voltage, here’s my understanding: When the voltage coming into a zener diode is equal or greater than the zener diode’s breakdown voltage, a voltage drop that is equal to the zener diode’s breakdown voltage occurs and the remaining voltage goes through the zener diode (see IMAGE-13 for an example). Am I still right?
Yes you are right.
So, if my understanding on how zener diode works is correct, then here’s my different assumptions:
#1: If this zener diode (ZD4 from IMAGE-12) is rated at 3.5 VDC (breakdown voltage) and is in reverse bias, then the 13 VDC coming in should be going out as ±9.6 VDC (13V_in – 3.5V_breakdown = 9.5V_out).
#2: If this zener diode (ZD4 from IMAGE-12) is rated at 3.5 VDC (breakdown voltage) and is in forward bias, then the 13 VDC coming in should be going out as ±12.3 VDC (13V_in – 0.7V_drop = 12.3V_out).
#3: The "35" code on this zener diode means 35 VDC or it’s just a numerical value representing a voltage (maybe higher than 13 VDC) and its intended purpose is to drop the voltage to 0 VDC.
As you can see, my assumptions #1 and #2 do not return the result I’m getting when I check the input and output voltage of this zener diode. Unfortunately, I did not found any other zener diode with the code "35" on this board. So, I cannot compare it.
About my assumption #3, is it possible?
In your experience, is this zener diode’s behaviour looks weird enough to be a candidate for a replacement or is it a normal behaviour?
From the link given 35 is a 13 volt zener diode which means the voltage is correct. The best is to get the schematic to fully understand the whole circuit. But as you have said , the schematic can’t be found.
In conclusion, and as you can see from this long investigation, I still do not know if I’m dealing with one or two problems. I mean, do I really short-circuit the PWM Buck DC/DC Converter (AP1501-ADJ – N581), hence creating a second problem, or is the short-circuit never occurs and I’m just dealing with the initial "NO DISPLAY" problem that is getting worst.
I hope that I gave you enough information so you will be able to guide me through this troubleshooting journey.
I suggest that you working it backward. The first thing you know is that the power supply is giving out good power so we can eliminate the power section. Next the inverter ic did not get supply voltage. So you work it backward from the vcc pin of the inverter ic. Find out why there was no voltage is it because there is bad components or the MCU in the mainboard did not send out the "ON" signal to let the supply voltage pass through.