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Electronics Repair Questions/Answers and Testimonials- Part 79

By on May 26, 2011

Hi Cedrick,

I have some questions regarding the power suply voltage anyway I have an idea about this before, but to make sure from the real source.My question is What is the voltage after the rectifier from the power supply of primary section?

It depends on the design. Different power supply have different output voltages

2. In the Secondary Section of power supply or in the cold ground is it all DC voltages?

Yes secondary side output after the diode are all DC voltages when measure between the cathode (red probe) and cold ground (black probe).

3.Do you have any idea about the voltage tracing technique to identify the shorted electronic components so that we can easilly identify the fault?

If there is a shorted component the output line will turn low and what you need to do is to check or isolate all the components that are related to that supply line.

4.What is the best test equipment to check and easilly identify the fault in  electronic components which is affordable in our pocket.aside from Analog and Digital  tester?That can perform many checking method.

There is no best equipment for this because different components need different meter to test it. For example checking  e-caps you need ESR meter.



Hi Jestine.

Thank you very much for your educative articles. I have been in the repair
business for 16 years. I find the information in your articles very
practical and is applicable to the novice and pro alike. I have learnt a lot
from your articles since I came across them on google. I am looking forward
to buy the troubleshooting and repair of SMPS book soon.



I a sure you that there notting unintresting on all what you send I want to be pior electronic man and I am still to learn more about electronic. this is a fild which no one can end it.I do benefit a lot from you.Last week I encount one problem on one Parck bell Laptop,without you I would not able to solve it now is working fine.I really thank you.



Hello Jestine,

Thanks for your reply and advise. I love reading your ebook and learned a lot.

Tried my new knowledge in troublesshooting and you know what, i fixed our 19″ LG monitor no power, found faulty filter capacitor in secondary circuit. Amazing! That was my first success and also i have a SMPS for HP 8000SFF computer, the problem was voltage droping from 19VDC to 0VDC, i just checked and resoldered the primary big filter capacitor and the signal is steady to 19VDC.




HI Walter,

How are you

I’m fine thank you.

I need to know what is the correct way off testing a chopper transformer
Photo attached
I know I need to test it qwith low resistance and with a multimeter , but
I am not to sure off what pins goes where
and I know u said that normally there is not a problem with primary
is the primary side the side with the least pins so according to photo
primary side is facing me
I have a Tv Hi sence that the filter cap poped and there is no power
I have replaced the filter cap and HOT ic and suspect LA7840 power ic now ,
but also not sure how to test it according to
alldata it gives me the Volts for pins , ias that the voltages that I need
to get on each pin

Secondary winding usually have no problem only the primary winding. Resistance test is not accurate because a shorted winding can have a good resistance reading. You need to use a blue ring tester. From the power primary side locate the power fet center pin (if the power supply is using power fet) and follow the circuit till it reach one of the transformer pin. Next look for the big filter cap positive pin and see where the circuit go. It should end at one of the transformer pin. These two pins are primary winding pins and should be tested with blue ring tester.



Hi Mr. jestine yong,

I am beginner in electronic repairing, I have 2 LCD HP 1940, and there is a same problem in both power/inverter boards(Benq 48.L1F02.A02), I replace the pair of transistor C5707s but same 2 transistors blow out after a few hours of use. I am attached the picture of inverter board and highlighted the related transistor with red circle. Please help me to solve the problem




Hi Bashir,

Make sure no dry joints in the high voltage transformer and that the tuning capacitor value is good. Check also for the p-channel fet (fu9024n or fqu11p06) for leaky condition.



HI Rohit,

hi jestine how r u .hope you are fine.
Fine thank you.

1.why tuning capacitors are used in back royer inverter circuit?

The collapsing magnetic field of the inductor generates an electric current in its windings that charges the capacitor, and then the discharging capacitor provides an electric current that builds the magnetic field in the inductor. This process is repeated continually.

2.value of smd resistor 5493?any web help?
Sorry no info on this. Try compare the value of the same part number in other part of the circuit if have.

3.if the power and inverter section built in one board,if power section had a problem , is there any jumper so to disconnect inverter,so we can check the power section.if not what we have to do?

You can disconnect the pico fuse. If no pico fuse you may need to remove component along the 12 volt line or cut the circuit track and test and joint it back later.


For this component the best is to direct replace it because it is an IC. Checking with meter offboard is not accurate.

5.current sense resistor normally what range in psu?why it is used?

0.01 to 1 ohm and use to sense current draw from secondary side and feedback to the PWM IC. If too much current draw the PWM IC will shutdown the generator.



Hi Yong:


I was so happy while I have received Blue ESR Meter yesterday. But I discover the probe wire lenght a little bit short . I feel unconvenience to check electronic component. Could you please I can modify wire with other kind of probe wire by myself.Otherwise, Do I need to recalibrate the meter and how to do the caliberation after rewiring?Thanks


Hi Michael,

The length of the wire is meant to be short because if it is too long it will cause the easr reading to be inaccurate. Yes you can replace it with the probe type but make sure it is no longer than 30cm on each wire.



Question: I am working on a dvd player and it has a problem with the spining motor..I replace the motor and it worked for two days then again, it stopped working. when I checked the new motor that I put in, it was fine but noticed that not voltages reaching the motor to make it spin. I disconnected the motor and checked the voltages on the sockect, I am reading 3 volts there but the moment I put the motor socket in, the 3 volts are gone. there were two bad capacitors on that board..470/10 volts. I replaced them but no difference. any ideas. Thanks

HI Waleed,
There is a possibility the motor may have problem or the supply voltage have components breakdown when under load. Try put a 3 volt lamp to see if the supply voltage will sustain or not.



Hi Kasirye,


I am so happy that during my internet brousing for information on electronics repair, I  came across your well resourced website. Thank you for the good work you are doing in empowering us with knowledge especially us from the developing countries.

You are welcome!


Coming to the point, being a graduate in Bsc. Mechanical Engineering, I do have the fundamental knowledge in electrical and electronics. I have plans to set up an electronics repair shop for DVDs, TV sets (CRT and Flat Screen types), Audio Systems,Microwaves, Computers and Office Equipment(Faxes, Printers,Scanners etc),Cell phones etc. Please I need your advice on what basically I will need in the start-up of business in terms of:

  • The necessary reading/consultation/reference resources (manuals, textbooks, multi media DVDs, catalogues etc),
  • Testing and Repair equipment,
  • Electronics Repair Supplies (components)
  • Management software (for a repair shop) and
  • The workshop layout (graphically) in terms of working areas/benches, spares storage, customer reception area etc.

I suggest that you focus on one or few equipment repair first because if too many you will be confused and you will not be able to specialized it. Once you have selected the field then get/buy information related to that field. Different field needs different type of test equipment. The common tester you need are:

analog/digital multimeter
digital capacitance meter
Blue ESR meter
Blue Ring Tester
oscilloscope (optional)

Start from a small rented place first (unless you have lots of budget to rent bigger space) until you have many customer base. From there you move on to a bigger place and you could also hire an assistant.

Before you start to repair customer equipment you must know how to repair first. Buy junk/used equipment and see how you can solve the problem. You can buy it from ebay or from the equipment shops. Doing the real practical work is totally different what we have learnt from the college.



HI Walter,

Will the flyback transformer cause the sense resistor to blow

If it is shorted-yes the current sense resistor in primary power side can blow.

Can you maybe tell me what pins it is to test on flyback , because there is no marking on the board to tell me what is the B+ or anything else so I think pin  4 is the one that goes to the HOT collector but dont know where B+ is

Follow one of the output voltages (usually from 40 to 110 vdc depends on model) from the power supply and this will lead to one of the pin in the flyback. From the photo pin 3 is the B+ input, pin 1 is the collector pin (going to HOT). So pin 1 and pin 3 should have resistance and need to be tested by blue ring tester.



HI Harlene,

I have a question on a Acer 24 inch monitor that rolls at start up
I think it is due to heat build up but I don’t have a schematic and I
don’t know where to obtain the key components to repair it.
I shows a very clear screen.
Model # AL2423W
Manufactured in 2006
So it is only 5 yrs old
Can you help with this problem?
Thanks in advance.

Sorry i do not have the schematic either. Try check if there is any bulged or bad e-caps at the secondary side of power supply. If all the voltages are good and stable then suspect the MCU in the mainboard have problem.



Dear Sir,please be good enough to explain me how does the remote
control(Infra Red)makes the crt tv to standby position,does it cuts off the
oscillator pulses to the HOT drive transistor base?, a bit of theory
explanation please,thank you for your kindness.plgperera

Hi Plgperer,

Before you press the remote control the power led is in red color. The moment you press the remote control, the CPU will receive a signal from the remote control circuit and send a “On” signal out to either a relaycircuit or an optoisolator circuit to fully turn on the main power supply. At the same time the CPU also send a On signal to turn on the green power LED. When the main power supply is fully functional the B+ voltage will be deliver to the flyback and at the same time the pulse from the h.oscillator ic will be channeled to the HOT. With both voltage and signal present the flyback could energize and output the high voltage.



HI Koniel,

I hope you are doing good.

I’m fine thank you.

I have a question for you
regarding replacing a faulty capacitor. Attached is the picture. I have
this mono capacitor (blue) and would like to use the other as a
replacement. Can i do these. It connects the rectified output to ground
on the PS circuit of a 6KVA UPS. Will there be any alterations on the
circuit operation?

It depends on the circuit.Some circuit have to use back similar component but in certain circuit you can use it as a replacement. I suggest that you replace and test run it and make sure that it don’t get hot after running for about 10 minutes.



HI Amosi,

I hope your fine,thanks for sending the Testing Electronic components

like a proffesional book am going to read it,it is very

useful,educative and informative.After complete read it i will buy

another book from your.

Thanks for the support!


I have problem Mr Yong i try to repair Hitachi Tv model no C 1475MN

with Vertical problem,the Tv screen is narrow does not occupy the

whole area from the top to the bottom of the screen,i try to test all

component arrond Vertical IC no TDA 8356,and i change it but the

problem not solved work fine for a short time the problem appear

again. I need your help sir to solve this problem.

Have you check the supply voltage? Sometimes vertical ic have two input and the voltages have to be about the same. It can also a diode/capacitors that breakdown when under load in the surrounding circuitry.



HI Norman,

 I am trying to fix my dead Infocus LP640 LCD projector (keep blown an diode at secondary 12V output) and might need to purchase your electronic component test and repair courses to bring it back to live.
So far I have a DMM, Signal generator, 30MHZ CRO and 0-32V DC power supply. Are these good enough? What other test equipments should I have in order to take full advantage of  your electronic component repair courses that promoted in your website?

If you want to speed up your repair work you need:

an analog meter
digital capacitance meter
Blue Ring Tester and
Blue ESR meter.

In the above case you may need to trace along the 12 volt line to see if there is any leaky or shorted component that had caused the diode to blow. You need to replace the diode with original part number.



Hi i’m rahimah munir. i’m working as a repairer of electronic equipment like rectifier, UPS, inverter and etc. i had few question on my repair job every day,

1) some rectifier had their own control card, my question is: why some IC from different manufacturer cannot use on the control card. like i had found 1 of the IC’s is faulty,then i replace with new IC and same part number. the diffrerent is manufacturer. after replace, its still cannot work. all the passive component is good. after that, i replace new IC with same manufacturer from the original and its can work. why its happen like that?

Good question- I had this problem before with a power IC UC3842. Since the IC manufactured by other factory there will be a slight changes in the design, components/materials used and etc. The outcome of the product will not be totally same with the original one. In many cases if you use that particular IC on other type of equipment it will be working fine. That means it also depends on the original equipment circuit too. If it is too sensitive and no tolerance other IC cannot be used.

2) E-cap value is 1000uF 100V. measure with digital multimeter showed 856uF. it is still can use or not?

Generally it can’t be used because later the value will drop again and causing intermittent problem. It depends on the circuit design too whether it is sensitive or not.

how we can know that some e-cap can use,because sometimes i checked the value still OK but can’t work. after replace with new one, its can work. why?

When checking an e-cap you should use Blue ESR meter because digital capacitance meter will give you a false reading. Don’t trust the capacitance meter.

3) why some rectifier cannot turn on? i mean during turn ON, relay keep clicking( ON/OFF) continuesly. all the passive component is good. i suspect the e-cap has faulty. but check the value is good.

The diode could be breaking down when under load or the capacitor have problem (need to check with ESR meter).



Hi Rohit, input protection & emi filtering circuit x and y capacitors are there.what x and y stands for?

It is use to distinguish how a capacitors is connected. I’m not sure about the meaning-perhaps taken from the x and Y coordinate. Both x y capacitors are connected differently in the circuit.

2.on some pcb’s 94v-0 is written. What is that for?

It stands for fire rating level.

3.where to place the black probe , to read ac voltages produced by secondary winding of SMPT?(HOT GROUND OR COLD GROUND).WHAT ABOUT WAVES AT THESE POINTS.FULL SINE WAVE OR WHAT?

You are not allow to check the ac voltage because it is a high pulse ac and your meter will register a false reading. If you got the dc voltage at the secondary side this means that the AC voltage is present. It is a sine wave and you need to use a scope that is fast enough to capture the reading.


It is the size of the power transformer that determine the output current. The bigger the transformer the bigger the output current.


It is in thick liquid form and easily available from hardware shop. Many use shellac to paint on the wood.


If you can get the diagram that will be good because you will know which opto is for which function. The opto can act as

to sense overvoltage, to sense over current, act as a switch to fully turn on the main power supply. In some power supply the designer can put in 3 opto to sense overcurrent in all the output voltage line. That’s why if have the diagram for that equipment will be good to know the function.

Sorry i could not locate the datasheet for this part number. If it is located in the primary side then most probably either the pwm ic or pfc ic.


You need to check where is the resistor located and make sure the corresponding components are good before you turn it on again. It can be bad components that burnt the resistor.



HI Dindo,

Hi hope your doing fine. I need again your advice because I have this problem with Acer Laptop display is dim. I open up the LCD display and dettached the backlight connector and attached a backlight that I salvage from a Benq LCD monitor although its a bit longer i was thingking that if the inverter is ok the backlight should light atleast.but the backlight didn’t light so I was thingking that the inverter is faulty but before that i checke if there is supply voltage coming from the mainboard to the inverter board so I check the connector on the inverter board pin 1 and found that there is 18.63V present so I assume that the inverter board is faulty because there is input but no I change the inverter board but to my suprise the display is still dim.Would it be that there is problem with the mainboard which causes the display to be dim?Thank you and wish you more success.

You need to measure the backlight connector to see if there is any ac voltage or not. Measure it with analog meter. If the high voltage transformer have problem there will be dim display too. Make sure the transformer primary and secondary winding are good.



HI Boling,

This monitor brought in with problem of having green color too much ( if it is given a signal with white color, the display result is green ). After replacing 3 ICs which are: LM2439T, LM1269NA & LM1280NA, the result is still the same. I had also checked the VGA cable, found OK. Now the voltages on CRT board:
G1  -33 V
G2  159 V
KG, KR, KB all are only about 12 V
HT  7.3V

I also found that the 3 color ( RGB ) Vout at pin 6,7,8 of LM2480 is still around 33 V, but after passing a diode it becomes only 12 V. Please see the attached photo. Pulling the CRT board out also gave the same voltage of KG, KR, KB ( 12 V ).

What is causing the display to have much green color, since all the 3 cathode colors voltage are about the same?

Why are the 3 colors Vouts of LM2480 after passing a diode become so low ( 12 V ), even  after pulling the CRT board?

Is the heater 7.3V still OK?

By the way, in this monitor many of the CRT board components are moved to the mainboard.

There are only two possibilities- it is either the tube problem or the eeprom data setting that need to be adjusted. You need the model softjig in order to do the setting.  Without the softjig it can’t adjust the color setting. The heater voltage is in acceptable level and why the 12 volt low after passing a diode most probably there are circuit that is still drawing current.



Hi Boling

Here is another weird problem, this monitor was manufactured in May 2008 and the inside looks quite new. The problem is sometimes the menu’s leds ( 2 out of  8 )will light on its own and sometimes the display disappear for a brief moment suddenly. This happens after turning it on for about 10 minutes. I have replaced 3 e-caps which esr value is still acceptable, which are at the eeprom & mcu Vcc supply but no help. I cannot see the coding on mcu surface because it is shielded with iron sheet.

What seems to be the problem, do you think? By the way the display is actually perfect.

If the supply voltage to the mcu and the eeprom are good and stable, suspect either a bad mcu or data corrupted in eeprom ic. Make sure the crystal frequency is correct and no dry joints in the ICs pins.



Hello Mr Jestine Yong,
I hope things are quite ok.Mr Yong, I am repairing  a  SANSUI
television set and its model is SV2118Si.The problem is that it
has insufficient height in other words the picture is compressed
in the middle of the screen.I have tried to replace the vertical amplifier
ic but there is no change.Do you have any idea or suggestion?Please,assist.


HI Collins,

Try check the supply voltage. A lower than normal supply voltage could cause such problem. Check also for surrounding components especially the resistor (make sure got a precise ohm value), diode (make sure did not breakdown when under load) and capacitor. If all check to be good then suspect the vertical yoke coil have problem.



Mr. Yong what do you sugest to test digital, analog, and IC chip in circuit without power. Really i want to beable to test most of the componets without power connected?

Hi Breck,
You can use Huntron tracker.




thanks sir.i have two questions again. i cannot find the 3.58 mhz crystal to buy. is it possible to use a 3.579545 metal case instead?

You can try it out and see the result. Hope the circuit will accept some tolerance in the crystal frequency.

what would happen to the function if i change ?

It is either working, don’t work or intermittent problem.



HI Riaan,

I have just started on your book. Opened up my first monitor. Comes on but no backlight. I unplugged backlights to get the board out.(Smps and inverter on same board).
When I powered up ,the Ic’s with mosfets inside, started burning.
My question is, does the inverter board not like having backlights removed? Or do you think something else could be wrong?
Monitor has two backlights. I have ordered parts from UTsource. Still waiting for them.

Usually it will not burn because the feedback circuit will shutdown the inverter circuit if there is no backlight is detected. Most probably is something else problem that had cause the part to burn. Try comparing the top and bottom backlight circuit resistance. Any differences in the resistance is a clue for you to start troubleshooting on that particular circuit.



HI Boling,

This monitor brought in with blown fuse, relay casing broken and coil open. I directly replace the 12V relay, 3A fuse and posistor. Now the set is working, but the degaussing function doesn’t work.

What is causing the degaussing function fails to work?

Dry joints in the degauss area, bad/broken degauss coil, bad cpu/eeprom (did not send an on signal to a driver transistor to trigger the relay and the driver transistor itself have problem.

Is it OK for a monitor to work without this function at all?

Yes it can work but if there is color patches in the future, customer may not be able to remove it
unless he has the external degausser.




Ok so, I ve just repaired my second lcd monitor, thanx to you!
I replaced resistor down to ground in feedback circuit with preset until shutdown stop! Still bit of flicker, but ok! My question is, I found two resistors down to ground in feedback circuit! Which one do I change? Both resistors alters brightenes a bit!

Different model have different circuit. Try test on both resistors and see which one yield the best result. If you got a new lamp that will be better because this is only a temporary fix.

Man, you have a gift! I’ve got a lot of ebooks, but yours just kick all others asses! Brilliant!



HI Alan,

could you please help on ic’s, I have taken lots from old working tv sets but I put them in plastic sealer bags, I have heard that these cause static, so I am wondering if these will be no good now.

No problem with that as I also put in that way. Unless you are talking on CMOS IC that is extra sensitive.

Jestine, is there any way I can test ic with a multi meter out of circuit please

For testing ic we need an oscilloscope. First find out the part number of the IC then look for the datasheet of the ic. Look for the supply voltage (vcc) pin, input and output pin. Make sure the voltage is good and stable when measure the ic on board. If the voltage is good then you need an oscilloscope to check for signal at the output pin. No waveform could be the ic have problem or the surrounding components have problem.

The Microcontroller ic and EEPROM ICs store programs  and we can’t test with scope. We can only test the supply voltage and only through replacement we know if it good or bad.



HI Rohit,

1.yesterday i fixed a dvd psu .i found all the diodes in the bridge are short.such kind of problems happen in psu?i thought that at least one doide must be ok.

Yes it can happen but very rare. I have seen this kind of problem before. there any substitute for optoisolator ic.the present board  i am troubleshooting had (NEC,2501,MC041).CAN I REPLACE IT WITH 817.

The best is to use back the original one and if you really could not find the replacement i guess there is no harm in trying it. But make sure all other components are working before you put in the replacement.

3.another board is having a high voltage problem.i checked all components,it seems all r ok.viper 22ic is used,tl 431 and nec mc04 optoisolator is 5 volt line i am getting 12 v and 12 volts line getting 24v.can i check after putting load.

May i know what is the complaint? You can test any power supply with the load in. smps high dc applied at the primary of smpt it changes to required voltages at the secondary.the secondary voltage is ac how?

The secondary is always ac then after it has passed thru the diode it became DC.

when we applied dc how it converts into dc.?

I suggest that you read page 31 for more info.



HI Rohit, to find a ceramic cap shorted?

Use a x10 k ohm analog meter to check it out. If the voltage is more than 250 v to 2 kv then you can use an insulation tester. article was removed in there any other website for this information?

Nope because i could access it: smps secondary section,trimmer is there to control the total output power of smps.if trimmer damaged ,after placing a new trimmer how to adjust the output power or how to adjust the trimmer.

It depends on what equipment you are repairing. If you are repairing a tv/monitor you could see the size of the tv/monitor when you are adjusting the trimmer. a vcd psu unit secondary ,12v line filter cap is not bulged.but after 5 min its get hot.

Some cap will get warm but not hot. If it is hot i suggest that you direct replace the cap and retest. Make sure also the line voltage is indeed 12 volt and the replacement cap voltage rating should be higher than 12 volt.

5.when 12.3 v passes through one end of 47k resistor ,what may the voltage at the other end.

You need to see the whole circuit and we can’t calculate the voltage just base on this component value.



HI Theuns,

Hi there JUSTINE thanks for link on diagram for above still the smps does not switch can you please help dont understand the diagram above ..i have vo volts on the vcc pin on the ic is a NCP1203

HOW DOES VOLTS COME TO VCC PIN   i have 300v on pin 8   i change the ic and power fet Q4 all other resisters and diode ok smps tested 8 bars with shorted turns tester on primary winding what trigger the secondry winding in primary thanks for help

Please read page 7 (START UP SEQUENCE) and see figure 2 where pin 8 (HV) is connected internally to the VCC (PIN 6).

I suggest that you check all components in the power side which include the opto isolators. Make sure no components shorted in the secondary side otherwise the power supply will not be turn on. Try direct replace D5 and retest. If there is a cap shorted or open resistor in the primary side there will be no power too.





  1. Daniel F

    May 27, 2011 at 9:56 am

    Hi Jestine,

    In many technical documents I read it always says that the secondary windings on transformers rarely fail.
    In my limited experience of repairs this isn't the case. I often find smtp transformers with bad secondaries but the primaries are fine according to the Blue Ring Tester.

    Have you found this too? Also, other than checking the output of the secondary diodes while in circuit, have you found any other way to accurately test secondary windings?

    Thanks as always.

  2. admin

    May 27, 2011 at 9:13 pm

    HI Daniel,

    Yes from my experience secondary winding rarely fail. So far after repair ten of thousands of equipment i came across only few that have problem and it was confirmed bad after comparing it with a good one in term of resistance test.

    Sencore lc 103 have the feature (ringer test) that can test low inductance value like the secondary winding. If you have check that the primary winding is good, all the secondary diode and filter cap are good and no shorted load and yet one of the output have problem (o volt or low voltage) then suspect the transformer have problem. I will post this to my blog and see if there is anyone have a better ways on how to test the secondary winding.


  3. eleno lopez

    June 4, 2011 at 11:54 pm

    hello Mr justine
    This Morning I sent a Money Order for finding out burnth resistor ebook package may I know sir if you allready received it nad also sir may I know how much is the blue ring tester and if I order it what would be the procedure.

  4. admin

    June 5, 2011 at 3:09 am

    HI Eleno,

    I have replied to your email.


  5. Antonio G. Dalisay

    June 17, 2011 at 10:31 am

    Thanks justine i have a lot of knowledge regarding question and answer
    when i read before i have many book or manual i buy,mostly ic manuals and
    transistor others.but when i found this website.i'm so glad and also the
    transistor easy to search by internet.
    thanks so much.

  6. admin

    June 17, 2011 at 9:24 pm

    HI Tony,

    You are welcome and glad that you like the website.


  7. Getachew

    September 13, 2015 at 10:31 pm

    v good


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