Jestine thanks for your reply. How can I check if the power supply is stable bcz I don’t have any oscilloscope meter,but I have a sanwa meter CX 506A where I can trace any defect in cap [ESR].I’ve already change all the cap in the ps.I’m sure that the ps is not ok but don’t know where is the culprit?
Thanks and regards…………..

Hi Bobby,

If you get the exact voltage from your meter and the needle did not move (constant) then you know you have a good and stable voltage.



HI Sonny,

sir i would like ask how check or test a t.v yoke

You need a blue Ring Tester.
and how can i determine if a t.v flyback is weak or faulty because im getting a blurd picture.

Blur picture could be cause by flyback or a weak picture. Make sure the flyback screen voltage could be tune to more than 250 vdc. The focus wire should have between 3 to 8 kvdc and you need a high voltage probe to test it.



Hi:I’m having a problem with new rubycon, nickelson capacitors.ex-25v-1000uf only read 955uf. 470uf only read 445uf. Are these capacitors garbage, or will the uf increase to where it should be after put in circuit under load?Hope you can help me .

HI Marvin,

Most e-caps if kept for long time even though is new the capacitance will decrease because inside is electrolyte and it will dry up. If your cap meter is good then I suggest that you replace the e-caps because after few months time the value will drop even more causing solid or intermittent problem. Once the value have decreased it will not increase back with or without load.


Hi jestine,thank you.i am just experiencing this problem the first time.a monitor was brought to me with the problem of no power.i opened it and saw four bad capacitors at the secondary power.after replacing them,the monitor powered orange colour which is good but doesnt display the no signal cable on the screen.instead the led turned to red.i connected it to cpu and it didnt display.what is wrong here?

Hi Victor,

It can be mainboard or the power supply output voltages not good. If the voltages from the power supply and the mainboard smd voltage regulator ICs are good then chances is high the MCU may have problem. You also need to make sure the vga signal cable wires are not broken internally. If the MCU did not receive proper signals from the vga connector then it will turn to red led.



HI Victor,

1.why is it the power connector is located at the main board instead of the power supply board?

Either way also can be used but generally designer prefers it to have the connector in mainboard.

2.how can i know if no sign of power or power failure is from the main board using?

if the mainboard is getting the right supply voltage. The power led signal is coming from the MCU. and if the MCU have problem the led will not light up so you will think it is no power.

3.which particular component in the main board can i test for power failure voltage?

The smd type voltage regulator ic.

4.how is the inverter voltage tested and which particular component is to be tested for voltage supply in the inverter?

Find the vcc pin of the inverter ic (find the inverter ic datasheet) and test the voltage of the vcc pin.

5.if i see voltage at the drain of power fet,can it also make me believe the primary power is good or it must be only the power ic?

It is only 80% good because other bad components in the power supply could also cause no power symptom.

6.when can i test the voltage of the big cap of lcd monitors?

if you want to know if there is any voltage present on that cap pin (when the complaint is no power). This mean if you get the voltage at the cap pin your common sense will tell you that the fuse and the bridge rectifier are good.

7.which leg did you say is the drain of power ic where voltage can be tested and how can i trace it?

For power fet is the drain is center pin. For power supply that have the built in fet in power ic the drain pin you ned to check from the power ic datasheet.

8.how many voltage is seen at the drain of pwr fet,pwr ic,and big

if your country is using 230 volt then you should expect about 320vdc at drain and big cap pin. the power ic is about 12 to 18 volt depending on the part number being used.


Dear Mr. Jestine,

i have a laptop service problem, can u guide me please???

there is a resistor which had a value 1.9KOhm according to the schematic.
the problem is, at the one tip of resistor has 3.3V but the other one
0V, i assume this shorted or broken resistor.
but when i measure it with ohm-meter, even with 2 multimeters, there is
1.9kohm show in multimeters display.

is it short/broken resistor or what ???
but if it short, why it has value precisely same with the schematic ??


HI Xav,

The problem is neither short or open. It could be the load problem have components shorted which you need to check. This mean by right it should have out voltage but because of something have shorted in the load it will oull lots of current and this will cause the output to drop to very low. Another ways is to direct replace the resistor and retest.




I’m so grateful for having an online consultant and a very effective online instructor in electronic repair. I hope that you are always in good health. You know, the electronic repair tips that you sent to me really help me a lot in molding my knowledge in this field. I am now confident to say that my knowledge in this field is already professional and it is all because of your full of compassion assistance.

Thank you and may God bless you and your family forever. . . . . .





Hi Jestine ,

I’m looking for the round coil didn’t know you got any ideal where i can get it or can i replace it for the junk board coil . Is all the junk board coil are the same . Thanks


If you can’t find one in electronics shop. Yes you can replace with junk board coil but you must make sure the pins of the junk board coils fits into the holes.



Hi Jaimin,

hope your fine,

I’m fine thank you.

i purchase your ebook :- howtotestelectroniccomponent, smpsrepair, burnt resister value etc

Thanks for the support!

actually i am not a electronic repairer, i am in the field of repairing of printer and its cartridge.



so i have to send the printer board to other dealer but at present i am able to solve some problem,

thanks to you.

Good to hear that-congratulation!

now i want to repair laptop adoptor and printer power supply board, so for that what FET, Bridge Filter capacitor etc is required to stock. in some laptop adoptor i am enable to see the part number for that. so what to do. pls help me.

For laptop you can try to use K2645 (FET) as substitution. For printer power supply you have to use back original part number because substitution part number may not work long. For bridge rectifier you can use 4 pcs of 1N4007. Filter caps comes in many different value like 470uf 25v, 470uf 35 v, 1000uf 16v, 1000uf 25v, 2200uf 35 v and etc.

in some hp formatter board, Transistor Marking with WU1 is gone, i can’t find it from my local market what to do.
Here is the datasheet:


According to the datasheet it is a dta144 PNP DIGITAL SILICON TRANSISTOR. So when you want to buy this part instead of looking at WU1 you should also look for DTA144.
You can try this website to see which one is match with WU1 casing:




HI Boling,

Removing the B+ coil and placing a bulb still has the chirping sound and the bulb didn’t light at all, so the problem should be at the power section and I’m still looking for the culprit.

Not really in the power section. Sometimes it can be shorted component in one of the output voltages line beside the B+ line. You may have to use resistance method to test it out. Please refer to my smps repair ebook.

By the way, I wonder why the ring tester tested the primary winding of SMPS transformer well, but couldn’t assure the power section was all OK?

In order for a power supply to work, all other components or section have to be functioned first. This mean a good primary winding does not mean anything if the power fet is not switching. At least when you know that the primary winding is ok you know that chances is high the power supply can be repaired because it is not easy to find the exact transformer.


Dear Mr Yong

I finally found the isolation transformer sale in Australia. I wish to
known is 300W 240V-240 volt suitable for me use in the electronic
testing like what you are doing in the office?

Best regards

Hi Ng,

Yes it is suitable if the equipment you are repairing consume less than 300watt. Note: For a laserjet the watt consumption could be more than 1000 watt.



Hi Jestine,

This monitor has a chirping sound when switched on and the led indicator is very weak, hardly can be seen. I have checked primary winding of flyback ,SMPS transformer has full led with ring tester. B+ FET and horizonal driver FET are also good. All e-caps are also good. I haven’t taken out the B+ coil to check the B+ voltage.

Have you ever come across this kind of problem and what is causing the chirping sound?

Have a great day,


Hi Boiling,

Chirping sound can be caused by bad components itself in the power section or something shorted in the load. Try remove the B+ coil and place a bulb and see the result.



thanks to you  for the assistance you give you are the best. one question for you. i have a 20 inch tv with the power supply parts burnt. i have the replacement all ready but one part is missing. the part A817A. NTE gives me nte290a as the replacement guide. when i search for  possible replacement i get 2sa733 or 2sa1015 but they are half the size of the one i am replacing i.e 2sa817a.is that right?

Can i use the 2sa733 or 2sa1015 in the stead of 2sa817a? thanks corriete


From my databook 2SA817A replacement is BC640,2N5400,2SA965 and 2SA1533



HI Jaimin,

i want to as some thing :-

yesterday i am trying to repairing dell 19.5 V DC Adoptor, and trying to test resistor,

i mark that, if we put digital meter in buzzer mode and check resistor it will show resistance, but in some low ohms resistor that is 0.22 Ohms it will show the resistance and also make a beep sound that means it short ? or not ? i try to test working resistor then it also give me a beep sound. i solved the problem its a shorting in cable so thats why its not power on.

For your information, to test resistor you need to use an ohm meter and your normal meter can’t test a low ohm resistor. You need to use a low ohm meter like the
BLUE ESR meter that can test low ohm and ESR value.


another problem,

I have one DC Board of Hp LaserJet 1010 Printer

its working fine, but for good printing we require a High voltage on cartridge point. otherwise it gives you a light printing. i can’t solve this problem, i also send this board to other dealer who are enage in this field for many year but he also return it without repair. can you help what the problem.

I have not repair such board and the problem. If you mention that it need high voltage to function then first thing you must make sure the high voltage section is working fine. If it does not function then you may need to check on the high voltage section. It can be the problem of the high voltage transformer and surrounding components or even signal that did not get from the MCU. I suggest that you check all the components in the high voltage section first.



HI Bill,

I’ve been trouble-shooting a power supply that at first looked like a
SMPS, but on the back I found a L6599 Resonant Half-Bridge controller.
While the assembly still has 2 MOSFETs feeding the transformer, I still
wonder if this is a SMPS. I ask because I couldn’t find any info on
this type in your book.

This is a SMPS that use different topology.
The data sheet also says the optoisolator feedback elements can
influence the frequency (and thus the output voltages).
I guess what I thought was the power IC is in fact something else. It’s
under a heat sink so I can’t access it’s part number, but it’s a flat IC
pins 1-4 grounded, pin 6 is gone, pin 5 is at full board voltage (315v
from rectifier) and pins 7-8 still undefined.

There are also a couple of BJTs on the board I’ve not yet figured out.
All of this is on the PRIMARY side. I’ve been chasing that side all day
because I couldn’t get any signals coming out of the presumed power IC,
but now I recognize that the 6599 has a shut-down circuit that has
caused the transformer and FETs to go idle.

Do you have any info on this type of circuit? Is it possible to create
a false input to get the board to turn on? Right now, all I have at the
outputs is the 5vsb standby voltage. The 5,12,24 regulated outputs are
all null.

If you look at pin 5 in the datasheet :


It is a standby pin and the signal is coming from the optoisolator ic. The reason you did not get the full voltages , 5, 12 and 24v are:

1) Either the MCU IC in the secondary side did not send a on signal to the optoisolator ic
2) the optoisolator ic circuit have problem
3) the primary side circuit which include the IC have problem.

Please keep me update on this case. I need to know what type of equipment is this-thanks.



Hi Jestiney Yong!


I received a dead Samsung TV. I worked on it and replace a power supply switching IC. After replacing PSU, TV is now playing. TV plays 100% with full size screen. After a while horizontal width start to shrink. However, when the set is playing normal B+ is 125V and when it is not playing correctly B+ is 110V. B+ voltage drops down from 125V when it reached 110V is when it started to reduce the size of the width and then after shut down. Right now, i am suspecting LOPT. If you have an idea what could be causing this fault please assist ?

Best Regards


Hi Azwindi,

First make sure if the B+ line component like filter cap is good. Remove the B+ pin at the flyback transformer and connect a 100 watt bulb between the B+ line and the ground and power on. If you get a very nice voltage then the lopt or even the yoke coil or surrounding components have problem. If the voltages drop then suspect the power supply have problem. it could be in the primary or secondary side. Check also if the optoisolator IC and related components have problem or not.



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