HI Roland,

Good day Jestine,

Good day!

I would just like to inquire with regards to a burned varistor. I had a electronic control  board that has burned varistor and the part number was also

burned. Is there any way or technique that I can do to replace the burned varistor. i have measured the varistor and has a value of 27ohms which is i believe that is shorted and keeps on blowing the fuse.

Yes you are right by right it should not have any reading.

the supply voltage is 220volts and the fuse that was used was 2amps cellulite slow blow fuse.

You mentioned in your book that you can still operate an smps without the varistor but there will be no more protection against lightning strike or power surges.

Yes you are right.

I dont want to do that is there anyway you can help me with the replacement of the varistor?
No problem, just go to any electronics shop and ask for a 270 volt varistor.

Hope this helps.



What if the display once switch it just dim or back light blink and off , thanks


Hi Chong,

Check the supply to the inverter ic. No matter what the inverter ic have to get the right supply voltage first. If no supply voltage when you turn the set on it will immediately go dim.



hi Jestine

I tried to figure out the primary side of the power supply but I was unsuccessful so now I have attached some photos which you can refer and guide me.

Hi Ashwin,

It is a bit hard to see from the photo. Since the transformer is a linear transformer then it does not have primary side board. It should be ac direcly going into the transformer primary winding and produce output at the secondary windings. You need to check if there is any secondary output ac from the transformer.



Hi Jestine,

Can you point to me an equivalent to the transistor BU508DFI?




Hi Bruno,

This is a common part that is easily available from electronics shop. If you can’t get it then try BU2508DF.



HI Russ,

One thing I have always thought that experienced repair people knew about was circuit analysis.  I always imagined that when looking at schematics I would be able to figure the voltages at any node and if a particular node was zero volts, lower than expected, higher then expected or the same as the supply voltage I would know what component was most likely at fault.  I have been study circuit analysis and it is not an easy thing to grasp and I still have a ways to go, but am I fooling myself? and isn’t there an easier way to learn this subject?

It depends on what kind of circuit you want to understand. You and I knows that for every equipment there are many circuits to make the equipment works. For a tv, it has, power supply section, color, high voltage, h/v, mcu, eeprom and etc. You need to analyze one circuit at a time.

Most of the books are teaching this to engineering students, but I only want to learn this for repair work.  I don’t need exact answers just close enough to for me to see that there is a problem with a component.  Are there any books or websites that teach this subject to repair people?  Something that can show us shortcuts for figuring these things out?  Can this even be done?  Can this subject be streamlined so that general rules of figuring voltages can be learned quickly?

Before you start to repair an equipment you need to understand the function of each of the circuit and what kind of symptom will appear if the circuit is faulty. For example, one horizontal line in tv/monitor is caused by bad vertical circuit. So far I have not seen any books that talks about this kind of circuit analysis and troubleshooting.

For example.  If we see a bridge rectifier with a AC voltage going in, isn’t there a general easy way to find the output? So that if it is half-wave, full-wave or bride we could easily find the output voltage.

If you understand the rectifier theory then you know that when AC in DC must out. So you just put your probes across the DC output pins and expect DC volt.

I have not really seen a book that just says take the input multiply by xx and this should be your output.  Same kind of things when you run across transistors and caps and inductors.  I can almost see some of the answers but I have to think someone else has come up with a simple way of figuring these voltages without having to do nodal analysis and add phazors and imaginary numbers and calculus.  Or is it really just that hard and you have to do a few years of engineering to be able to do this?

It depends on what you want to achieve. For me I would try to make it easy as explained above which is to understand the circuit function well then only troubleshoot. You just have to have more practice on this part. For example, if the complain of a power supply is no power, i would automatically check on the fuse or just place my red probe to the supply pin of power ic to check if there is any supply voltage. With just a simple probing your common sense will tell you which section or sometimes which components is fault. But then again you need lots of practice. I got hold this kind of troubleshooting was because I’ve been exposed to thousands of equipment that have sent for repair.



Hello Mr. Jestine

Hope you are well. I have some problem with ATX smps. The problem is all voltage are normal when taken out from mother board, but when i connect to motherboard, it shut down by rotating the fan few seconds. All capacitors are ok with esr meter. Now i tested on load with 12v 35w bulb in 5v(red) and 12v(yellow) the bulb glows with stable voltage. when i connect it to 3.3v(orange), the smps shuts down. i suspect 3.3v problem but could not found any faulty component (capacitor and MOSFET are ok). Since 3.3v can deliver30amp so bulb must glow dim but not so happened. Please suggest what to check in this smps.

Best Regards

HI Gopal,


Did you connect to original load (motherboard) as some ATX power supplies are very sensitive. In your case try direct replace the components in the 3.3 volt rail. Make sure also the main primary side filter cap are good in terms of ESR and capacitance value.



Hi Jestine,

Thanks a lot for your newsletter. meanwhile, am through with reading the recent ebook i bought from you – lcd monitor, burnt resistor approximate value and the bonuses. in fact after reading your books i can boast of being a professional in lcd repair and others. and it will interest you to know that i have successfully repaired an lcd monitor and another one is now on my bench. thanks a lot for your good work in electronics.




I like to know if removing any IC output pin or one side of a component attached to it to check for voltage or signal test is an acceptable method to check if the IC (Integrated Circuit) is good or defective. This method is acceptable for analog and digital IC or only for analog IC?

HI Manuel,

It depends on what type of IC and the usage. The best is to test them on board together with load. Many ICs nowadays will shutdown if there is no load detected at the output.



Dear Sir,

Just few minutes ago I have read your mail, which will be very helpful for me. I always read your blogs & articles. Yesterday I have made a capacitor discharging tool, taken help from your blog. I have already repaired two lg CRT monitors. Both of them were dead. One of them was only 150MF/400V electrolytic capacitor faulty. Other one were some components faulty ( Power transistor, Power IC, Signal diode 4148 & some resistors). Without your help I could not able to repair those monitors.

I am very grateful to you for your works & responsibilities.

I wish your success in life.

Best regards,

Md. Shahjahan


Hi Ambali,

another thing i got Tv sony 21″ using str 6707 it has a fault of switch on for some seconds then go to standby i referred to your ebook i check the same circuit diag in your eBook the voltage and everything. i tried to follow everything as you wrote changing run resistor opto, SE115 but still the fault is there i change even vertical ic LA7830 no success is there any componet to check or FBT is bad?
last but not least God may bless you ahmen.

First you need to check after the TV is in standby mode does the smps still produce good output voltages? If yes then most likely the CPU or the optoisolator ic area have problem because the CPU signal can’t trigger the opto to turn on the full power supply. Sometimes a corrupted eeprom and bad CPU could cause this problem too.




Thank you for this month newsletter about the helpful electronics website.

You are welcome!

Now I am working on this big CRT monitor, which shows display when no signal plugged in ( it displays: no input signal ), but there is no display at all when injected with generator signal, the led is still steady green and there is a faint sound trying to start up and then shutting down over and over again.

What area should I concentrate on with this kind of problem? Thanks.

If there is a display with no signal this mean most of the circuitry are working. But when you plug in the cable the monitor will try to start up this mean either the CPU/EEPROM or along the 5 input line (r,g,b,h,v) to the cpu have bad components. You may need to check on these 5 lines.



HI John,
I have bought you book sometime now and been reading it.I wish I met you some time earlier in my life.This information is extremely helpful.


Can you please help advise me a you mention tester like the Logic Pulser ,Logic comparator ect.

These tester is more for troubleshooting boards that use logic gates ic such as the TTL 74 series ICs.

Can you please assist me as to where I can get detail information on exactly how to use these tester showing it connected to actual circuits.Which pin to connect and what results to get out on output pins.

Try this link:


You may need to buy those digital circuit books from amazon.com  to understand the truth table so that it will be easier for you to troubleshoot.



Hi Jestine,

Thanks for the newsletter this month.It sure the websites that you gave will surely provide lots of information regarding electronics repair.

I am planning to put up an electronics repair shop in the near future.Can you please provide me some points or articles to consider in preparation of my plan.As of now i’m just accepting repairs at home.

Have a nice day,

HI Amor,

Congratulation but sorry i do not have such article. I suggest that you find more customer first. Once your customer base is already big and you have excess fund after deduct all the expenses then go ahead to rent a shop and hire staff. That’s what i have done back in 1996. Concentrate on marketing because even if you are the best repairer in the world and no ones know that your company exist there will be no activity in your company.




Dear Mr. Jestine,

Can you please help me again in troubleshooting of my LCD monitor, now I have LG Flatron W1934S, the problem is when you turn on the LCD, there’s a power and the display is not clear, it like a redish color and the screen will automatically turn off for about 2-3 seconds but the power indicator is still blue, when you turn off and turn on the same problem happened. I already open the LCD and all the capacitors are goods, I resolder all the pins of the transformer but still the same problem, can you please help me again?

thank you very much!

HI Dexter,

Reddish suggest a weak backlight and a weak backlight could cause it to shut down. Try remove the backlight and see if there is any dark end at the backlight. If have then you need to replace the backlight.





I have a Divx player branded Géant Electronics model GDV-X454 does not work no light comes on, I tested the power supply (SMPS) or I found the following outputs values ​​(see the diagram):

In warm state:

Output voltage  12 v    the  value   found. is   +10,9v
Output voltage -12V   the  value   found   is    -9.0v
Output voltage +5 v    the  value   found   is    +5.04v


The circuit TL431A, the voltages are found: reference voltage Vref = 3.45v  and the cathode voltage Vk= 2.5v, so the condition of boot is not verified

In cold state: the output rectifier diodes and optocouplers EL816 are all in good condition.

But in vain I could not solve the problem

Forward to hearing Pending an encouraging response, please accept, Excellency, my best regards


Lower than normal output voltages can be caused by few factors:

1)  make sure the filer cap ESR value is good in all output lines
2) check and direct replace the optoic and TL431 and retest.
3) the low ohm current sense resistor (usually 0.22 or 0.33 ohm) value could have increase and these value can only be tested by low ohm meter such as the blue esr meter. if you do not have this meter you can directly replace and retest.
4) the transformer secondary winding could have partial short circuit (quite rare).



jestine i need your help with a power supply that i’m trying to fix.
this ps have big sound when it start i was sawing on the pwm current controller and the frecuency was 22 khz i did supouse that these frecuency was too low and i try to get up and the result was i broke this ic but i don’t have where to buy it because here in cuba ic is ob2262 do not have electronic stores and i tried to remplace with another and I found the uc3842 and i put it to work but the voltaje at the out of the transformer was 26V and break the capacitors 470 u 16v that was in the out of the rectifier element what it happend
the feeback do not have to regulate that voltaje


HI Osviel,

Try buy from this website:


uc3842 can’t be used as replacement. You may need to check on other components before you replace the ic and retest.



Jestine Yong works as an Electronics Repair Lecturer in Kuala Lumpur Malaysia and his training website is http://www.NoahtechElectronicsTraining.com. You can check out his repair blog at http://www.JestineYong.com