Testing The LCD High Voltage Transformer

If you already have a Blue Ring Tester I’m sure you have use it to test on the primary winding of switch mode transformer. The switch mode power transformer is under the category of step down transformer because the output voltages is lower than the input voltage. Simply connect the ring tester to the primary winding and check the result. The ring tester is designed to check on a transformer that have more winding. In other words it can’t be used to check on the secondary winding because secondary side have less windings.

Now, if you want to test on the LCD high voltage transformer then you have to test the winding at the secondary side and not the primary side. The reason for it is because high voltage transformer is under the category of step up transformer thus the secondary side have more windings than the primary side. Just use the ring tester and place it at the secondary side and read the result. For accuracy testing, remove the transformer and test it (off board testing)

Unexpected Guest In The SMPS

If you are a seasoned electronic repairer that deals with SMPS repair i guess you would have encountered an unexpected guest before which is the lizard! The lizard got burned inside the power supply be it in the ATX power supplies, Printer power supply or even in Monitor power supply section. It doesn’t happen all the time but once in a long while you may get one of this problem. The effect usually caused the main fuse to blow and in some cases it produced a very bad smell too. Removing only the dead lizard is not enough and you got to get rid of the smell.

I’m not really sure why the lizard want to be in the switch mode power supply but there are few possibilities such as power supply although already turned off, the power section may be still warm and this attract the lizard to go in, next could be hunting for food and last the lizard could be in the wrong place at the wrong time

High Pitched Whistling Noise Coming From Power Supply

If the power supply (SMPS) that is running fine and you could hear a high pitched whistling noise then suspect the switch mode power transformer (power transformer) or the secondary side filter coil (inductor) have problem. The first thing that you need to do in order to confirm whether is the transformer or the inductor problem, you use the handle of the screw driver and gently hit both the components while power is still operating (On). Try knock on the transformer to see if it will quite down or not and then proceed to the inductors. If by knocking could cause the noise to go away then you have confirm that the particular component indeed have problem.

Your next step would be to stop the noise from coming again. Dip the transformer or the inductor into shellac for couple of minutes and then let it dry for a day. Fix back the components on the next day and I’m very sure you could hear the noise no more!

I have also came across problem with dry joints in one of the small ceramic capacitor that caused the high pitched sound. Resoldered the primary section cured the fault

Testing SMPS

If you repair SMPS and wish to test it, make sure the load that you are using is big eough for the SMPS. A small load would not turn “On” the SMPS in fact it would cause the SMPS to shut down or even run at a lower voltage (instead of producing 12 volt, the SMPS now only output 7 to 9 volts). If you don’t get the right amount of load to test the SMPS, you might have thought that the SMPS have problem again.

What kind of load do i need to use? Well it depends on what type of SMPS you are repairing. If you are fixing the computer power supply i suggest that you use a motherboard with at least on or two hard disk. You can buy those used motherboards and hard disk from any computer shops. Some power supply only requires a lamp (light bulb) or resistor to work but i do came across some that only will power up with the original load. Hope after reading this post you will make your own decision on what type of loads that you need to use on the Switch Mode Power Supplies

How To Check Flyback Transformer

It’s simple to check flyback transformer with the help of a flyback tester, digital capacitance meter and an analog meter. First check the internal capacitor of the flyback, it should have between 1.5 nf to about 4.5 nf (depends on Monitor) and check also with analog meter set to X 10 K Ohm to see if the internal capacitor is shorted or not. Next we check the primary winding of the flyback by using a flyback tester. If you use a dick smith meter, a good flyback should shows 5-6 bars and above (again it depends on the size of the Monitors). Sometimes even if you have the best meter in the world, the flyback transformer can breakdown when under load thus your meter can’t test as most of the meter used to check on flyback were out of circuit test. If you suspect the flyback breakdown when under full operating voltage, direct replacement is the best solution

What Could Cause A Transformer To Burn

Whether you are dealing with Linear or Switch Mode Power Transformer, I’m sure you have came across a burnt transformer before. The question is, what could have cause the transformer to burn (shorted primary winding, bad smell from transformer internal parts, open primary or secondary winding and etc)? From my experience, below are some of the possible causes:

1) Lightning strike

2) Power Supply Design fault

3) Low quality of the transformer used

4) Shorted components in corresponding circuit (primary side)

5) Shorted components in the secondary side

6) Decayed glue used in surrounding circuit.

If you know of any other way that could cause a transformer to burn please share your comment -thanks

Could A Smps Transformer Breakdown When Under Full Load

Last week there was a Panasonic kxp1131 printer sent for repair and the problem was no power. After checking all of the components, i found power fet K2651, power IC KA7552 and a low ohm resistor (.75 ohm) burnt. As usual, i do not direct switch “ON” the power until i have tested the surrounding and the important components. Everything looks okay to me and the moment i powered it “On” i could hear a ‘bang’ sound and the same components turned bad again. Since the customer want the set back fast i do not have the time to really examine it. I hope it was not the smps transformer that breakdown when under full load. So far i have never come across one, perhaps if you have ever came across one, you are always encourage to give a comment-thanks!

SMPS Problem

I got these photos from one of my ERG members asked about how to fix this kind of problem. Here are the questions and the answers:

Since I’m still an amateur in this repairing field, most of the bad equipment that I try to repair are from the church’s. Now their fax has problem with no power at all, opening the case I saw the components surrounding the filter caps are darker, I can see it is because the filter caps leak. Now, the first thing to do is to clean the darker surface on top & bottom, but using Philips 390DCS/2 Degreaser Cleaner wouldn’t help. Do you know what cleaner fluid would help in this case. Having no schematics will add the difficulty to repair, so cleaning it is of the first importance. Could the ultrasonic cleaner help clean this darker area of the pcb? Do you often use this kind of cleaner? As you could see from the bottom side of pcb, the protecting green layer has taken off when I tried to clean it with tissue paper.

Yes the first thing is to clean up the area first. You can use a thinner solution where you can easily get it from local hardware shop. As for the ultrasonic cleaner, i have never use before thus i can’t give any comment about that.

Another question is the other board which has three telephone jack sockets has a small round component like fuse but with 3 legs identified as V2 on the board, what kind of component is that? Upon checking its resistance with digital meter, they are all open among the 3 legs, and the coding on its surface is 3P600 ss

It could be a varistor and the 3p600 could be the component can withstand up to 600 Volt.


Since the fluid leakage of e-caps is so corrosive, causing the resistor color coding, diode coding to fade that I can’t see the original type/code, do you think it is still serviceable?

If i’m the one handling this type of problem, i would only allocate half an hour for it. If it can’t be repaired i will just send back to customer. The reason is because the leaked electrolyte already eaten up some components and i’m afraid it may intermittently short circuit some of the adjacent track. Even if you can repair this type of problem, it may not last long due to that the board circuit and track already in bad shape.

Could I switch on any smps power supply safely without load, since the ouput socket has many pins and I don’t know which pin exactly is the output.

It depends on smps. some will work for a moment and shutdown, some will work and some will not have power at all at the output! Try measure the pins where the output electrolytic capacitors were located. Assuming you have seen one that have 1000uf 16 volt that means the voltage across the cap should have voltage somewhere between 8 to 12 Volt DC.

How to easily know which side of smps power transformer is the primary, because this one has both six pins on both sides.

Look for the + sign of the big filter cap and trace from there. It should have a path that lead to one of the transformer oin. Next look at the drain pin of power fet. Trace from there also and eventually it will goes to another pin of the transformer. So both pins is the primary winding.

Note: If you have other suggestions please do give comment-thansk

Shorted Secondary Winding In SMPS

For your information, i came across shorted primary winding in SMPS transformer but rarely on the secondary winding. So far i came across only few SMPS transformer that have bad secondary winding. If you come across any SMPS that produce lower output on all the output lines, don’t just suspect components fault in the primary section or even in the Optoisolator circuit- check also the primary winding. A partial short in the primary winding not necessary will cause the Main fuse and power FET to blow. Sometimes it will cause all the output to have lower voltage. You have to test the primary winding with a coil/flyback tester and the reading should light up 4 LEDs and above. If it has only one or two LEDs lights up then you know the transformer have a partial short. If it has no light at all then the primary winding have a severe short circuit.

So, first thing you must do when you come across any SMPS that produce low output, check on the primary winding before checking on other components

high voltage SMPS

high voltage SMPS

DISCHARGING SMPS CAPACITORS

MOST SMPS HAVE A RESISTOR TO DRAIN THE CHARGE IN THE MAIN FILTER CAPACITOR
BUT THE RESISTOR MAY FAIL AND THE CAPACITOR CAN HOLD THIS CHARGE EVEN YOU HAVE TURNED OFF THE DEVICE

THIS CAPACITOR HAS ABOUT 330uf AT 400V

TO DICHARGE THIS CAPACITOR
- TURN OFF YOUR DEVICE
- PLACE A RESISTOR ACROSS THE TWO LEGS

- CHOOSE THE RESISTOR LIKE ( FROM 2.2 TO 4.7 K ohm 10watt )
- each time check the charge of the capacitor using your meter

thank you

fathi