Hi Jestine ,

Can i check with you ,  if i measure the VCC pin of the power ic UC3842 and i get the reading not constantly example 11.xx dcv jump to 14.xx dcv then go down to 12.xx dcv and then up again , normally which parts did i need to check . Thanks

HI Chong,

It can be the IC itself have problem, the supply vcc line e-cap ESR value have shoot up or even if there is components shorted in the secondary side preventing the ic to start up. I suggest that you check all secondary output diodes first and filer caps, then optoisolator ic then back to the power ic.  Sometimes corresponding components to the power ic also could have problem



thats again im new in all this fixing computer and dont know much here
are model of my Acer AL1715 can you please send me some pictures so
that i would know were and see pico fuse in the inverter board thanks
again for your time


HI Ronald,

Sorry i do not have the monitor with me. Here are the two types of the pico fuse:





I’m trying to remove a rectangular heat sink from some SMT MOSFETs. The
heat sink has a small wire on two corners that goes through a hole in
the heatsink down to what appears to be a brass ferrule in the PC board.
It’s soldered on the back side.
I’ve tried two irons at once (a 35w and a 40w) AND I’ve tried a 150w
soldering gun, but I cannot get the solder to flow. So I can’t pick up
the solder with either a wick or a sucker.
How in the world do we get these heat sinks off the board to access the
components that lie underneath?
Bill Granger
Bristol, TN

HI Bill,

It could be unleaded solder which is hard to remove. Try apply some flux and use a solder gun that is more than 150 watt.


By the way here is the answer by John Timbs:

 Flow some 60/40 fluxed solder on to the joint this acts as a heat sink for Desoldering led free solder.

 So what I am saying is to solder over the top of existing joint with good quality 60/40 electronics grade fluxed solder then re heat & apply sucker or braid.

The 60/40 solder allows a heat transfer into the higher melting point lead free. You may need to re apply the 60/40 once or twice more.

Plus one other little trick on lead free, flux the joint before trying desoldering  try different types of flex .

& Justin keep up the good work.

Best Regards JOHN

 Contact Name: Mr John Timbs 

Company: Computer Parts Recycling 

Address:  92 Terry St.

Town   :   Albion Park. 

Region:  New South Wales.

Post code 2527

Country        Australia .

where do i find the “drive transistor” and “drive circuitry”. its a Sanyo tv set.


Hi Gavin,

Drive transistor can be found in the horizontal  and color section and also some other section-it depends on the tv and model design. You need a schematic diagram to see it.



HI Grant,

I got one of your book – testingelectroniccomponents

Thanks for the support.

but I still confuse on testing the Bridge rectifier, diode and the transistor.  When I switch to the diode testing mode on the digital multimeter on testing a diode, one way should be open and the other way should come up some value,

Yes you are right but using digital meter to test semiconductor is not accurate because a leaky semiconductor could still be tested good with digital meter but fail when test with analog meter.

and how can I tell if the diode works properly or not by this value.

for a good diode when black probe touch on the cathode and red probe on the anode it should have about .43 or .44 volt. If you reverser the probes there will be no reading. If you get reading this means the diode already shorted.

Like I just test a bridge rectifier (KBJ4J), use the red lead to the – and the black lead to the +, I got 1.15 from the digital multimeter in diode mode; but if I use the the red lead to either ~ and the black lead to the +, then I got 0.65.  Is this situation normal on the bridge rectifier?  Thanks!

Yes it is normal but i suggest that you test it the the diodes individually so that you will not be confused. Some bridge diodes have more than .5 volt so they are good diodes.

Bridge have two similar symbols ie; ~  so we assume ~1 and ~2 . Now begin to test:

+ and ~1 (make sure have one reading if you get two readings means shorted)
+ and ~2 (make sure have one reading if you get two readings means shorted)
– and ~1 (make sure have one reading if you get two readings means shorted)
– and ~2 (make sure have one reading if you get two readings means shorted)

There you go a systematic way to test a bridge. If you have other question please do email me again.



Hi Jestine ,

Just want to know can i replace FFM107 Surface Mount Diode with 1N4007 Diode , thanks

HI Chong,

1n4007 is a general purpose diode thus it ca’t be used. You should use UF4007 which is an ultra fast recovery diode. When you buy this part insist on UF4007 and not 1N4007.



Hi Jestine,

Thanks for your reply, but I still have a question:  do you have any recommendation of the multimeter (digital & analog) for checking the transistors?  Currently I am using the basic digital multimeter with diode check function but I found that checking the degrading transistors is very tricky, like different meters have different reading.  Also, I found 2 identical MOSFETs on the same board, the measured value from the same meter is off around 15% for each other, not sure if I need to replace both or … so frustrating.  Do you have a clear idea on how to find out the degrading diode, bridge rectifier, transistor, MOSFET … Thanks!



I have heard of Sencore meter that was good in testing transistor . Since I do not own any one of these thus i can’t give any recommendation:


You may need to check also the peak atlas tester at http://www.Peakelec.co.uk

For diodes I just use my faithful Sunwa 360TRE analog multimeter. I set to X 10 Kohm and could know if a diode is degrading or leaky.



Dear Justine,
I would like to know how to connect osceilloscope to check the the primary voltage in the switch mode power supply that is 300v to 450v .I mean the settings what type of probe,time scale and volts/div

Hi Babu,
You need an isolation transformer otherwise the ELCB will trip if you want to check the primary side. Just set to highest DC setting in the volt division and make sure the probe is at least x10 and not x1. Place your black probe to the big capacitor negative pin and the scope probe to the positive pin and you should expect a nice one horizontal line indicating a good dc supply voltage.



Hi, Jestine, First thank you, Ihave been receiving the montly newsletter from you, that is great, second, I have a problem with an HP Pavilion f1903 monitor, it turns on, a few seconds and then goes black, I don’t know where to start, could you give me an advide and direction in this problem. thanks.

Hi Victor,
It could be bulged filter cap, dry joins in the high voltage transformer, one of the high voltages transformer is bad or one of the backlight have problem. Either one of these happen the display will shutdown after few seconds.




Hi Jestine ,Would like to have some guide line from you , i got one CRT monitor that can’t adjust the resolution too high . 800×600 is OK but once switch 1024×768 i could hear a sharp sound came from the monitor . Any advice , thanks

Hi Chong,

If the monitor is an old type then it can’t support 1024×768. If it is a new monitor and can’t support 1024 then you may need to checkon the cpu, h/v oscillator and even the s-correction circuit for bad cap and bad fet.




Hi sir in Testing Electronic components ebook i read about smps primary section have a resistor to drain the charge in the mainfilter capacitor therefore if the resistor is open circuit the mainfilter capacitor will store the charge about 300 volt DC

Yes if the start up resistor have open circuit the voltage i the big fat cap will not be drained. Sometimes a bad POWER IC also could prevent the big cap from discharge.

so i come across a tv smps in testing all the components in the primary section found good so i do’nt know if i will replace the power IC the TV will come to life so please sir i need help thank best Regard noel
If the problem is no power and you have check all the components then direct replace the power ic and retest. Before that you also need to make sure the secondary output voltages are good and stable.



Hello Mr. Yong,

I am really enjoying your ebook. I am learning a lot!

I will be using this knowledge to repair Bose Wave Radios for resale. Mostly for a hobby, but will branch out once I have some experience.

My question: Insulation Testers. I see you use the Kyoritsu analog type. When shopping around I find these are very expensive $250+. Would a digital tester also be able to test the capacitor leakage as well? I have found the digital meters, such as the Victor VC60B with 250v, 500v, 1000v settings are much less expensive around $50.00 dollars.

Thank you for your great books, David


Nice to know you like the ebook. Yes you can get a cheaper version. Read my review on DY294 DIGITAL TRANSISTOR TESTER:




Hi i was reading your artical on intermittent failures and i was wondering if i can use a can of compressed air turned upside down as freeze spray? when you flip the can upside down it will freeze things but is it too low a temp or is there an reason not to do this

Hi Chirs,

I have not done that before thus i can’t give to you an answer. The reason I use the freezer was the temperature is below 0! I’m not sure what is the temperature of the compresses air when turn upside down. Perhaps you could try that out and see the outcome.



HI Olaoluwa,

how are you and your family.i hope you are all fine.

My family and I are fine thanks.

thank you for the guide and support,i really appreciate your kind gesture.

sir i need your help with respect to the power rating of a resistor.i worked on a dead 17inches dell tft monitor,after troubleshooting,i discovered that a resistor in the primary section was open.the value of the resistor is 3.9ohms.now i have a problem on how to determine the power rating of the resistor that i am going to use to replace the bad one as i do not have the schematic diagram of the monitor.

i came across two resistor values on the internet.one  is 3.9ohms 1/4W with max.working voltage of 250v and the other one is 3.9ohms 0.6W with max. working of 400v.

sir i would be glad if you could guide me on how to choose the right value.once again thank for the support.

Simple-always replace the resistor with the same or slightly higher rating in term of watts. If your is 1/4 watt then you can put in 1/2 watt. If yours is a 1 watt then you can put in a 2 watt resistor.




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