ATX Power Supply - The Simplest Way To Repair It

I had stopped repairing ATX power supply many years back due to the new one cost very cheap. It's not worth to repair it because the spare parts sometimes were much more expensive than getting a new power supply. Searching for ATX power supply spare parts was not easy as many of them you can't even find them on the internet. Not only that, many complicated and different designed by power supply manufacturers had eaten up our precious troubleshooting time too because of we need time to understand how all these different designed power supply work.

Some of the power supply designs were using the PWM IC (UC3842) and power FET, some use the double transistors while some use only a single power IC in the primary side. Because of the manufacturers wants the design to be made into compact size, many secondary or even primary power supply circuit were build into a modular board (smaller board). This made troubleshooting even more difficult because many times the meter's probe can't reach to the testing point.

The real reason why I had stopped repairing ATX power supply was the profit margin. If you charge to high the customers rather buy a new unit with one year warranty given. If you charge too low, you may end up in the losing side because of the components replaced, electricity and etc. If you charge reasonable, the profit margin gained can't even cover your time spent on troubleshooting it. I'm here not to discourage you to stop repairing ATX power supply, however if you have the time, have contacts getting cheap power supply components, easy to access many power supply schematic diagrams and etc then you may go ahead to repair it.

Okay back to the article, one of my customers had asked me to repair his ATX power supply. I told him to get a new one (since it was very cheap) but he said he couldn't find one that suits his customer's CPU. He wanted a power supply that is either same size or smaller then the original one with same or higher specification but all he could find was a standard size power supply!

As a favors to my customer, I would do my best to help him to repair the ATX power supply. When the power supply was switch on, measurements were taken. The results were over voltage. The 12 volts line shot up to 13 + volt and the 5 volts line became 5.6 volts. After the casing was removed, I found the inside was very dirty and I used a vacuum cleaner and a brush to clean off the dirt. Then I saw four filter electrolytic capacitors had bulged at the top casing.

As you know, we as electronic repairers can't just see things at only one side; we have to see the other sides too. What I mean was, try to see if there are any suspicious components that contributed to the failure of the power supply such as broken components, dry joints, loose connection, decay glue and etc before start checking the suspected area.

What I saw was at the primary side there were some components covered with decayed glue as seen in the picture. I have to carefully remove it by scrapping off the layers of the decayed glue while preserving the outer layers of the components. Once it was done, I clean it with the Thinner solution. Decayed glue could cause serious or intermittent problem in electronic equipment because it can be conductive.

If you repair any ATX power supply, make sure you check the fan too because some power supply failure was due to heat caused by a faulty fan. The purpose of the fan is to suck out all the heat generated by the components inside the power supply. In order for the fan to run smooth, you can service it by using a Philips oil base spray as shown in the photo.

Once the four electrolytic capacitors were replaced and the decayed glue removed, I then have to plug it into a junk motherboard together with a hard disk to test the performance of the ATX power supply and measure all of its output voltages. It seems like the output voltages were back to normal. Once everything is okay I then test it in a working CPU to check for the display.

The reason I test it with a junk motherboard first as a way not to cause my good motherboard to go bad just in case if the output voltages is still very high. Better safe than regret later. By the way you can't test a power supply without load otherwise it may turned on for a while and then shut down. If you do not have a junk motherboard you can always at least connect a hard drive and a wire jumper to its connector to turn on the ATX power supply.

Do you know that most ATX power supply is using dual schottky barrier rectifiers to convert the ac wave into DC voltages? Even in LCD Monitor power supply, a dual schottky were used in the rectification too. For your information, checking schottky diode is different from checking a normal diode. With analog meter set to X10 K ohms, it should show some leakage reading when doing the reverse bias test. If this component becomes faulty it is very easy to get a replacement, of course you have to get one with the right specification. That's all for today, hope you enjoy the article and come back more often to check out for any latest repair techniques that I'm going to share to you.

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Troubleshooting And Repair Power Supply The Easy Way

Troubleshooting linear power supply was quite easy as compare to switch mode power supplies (SMPS). AC voltage enters to the primary side of linear transformer and then converted the AC into a lower or higher AC voltage depending on the secondary winding. The output AC voltage is then rectified and filtered by a diode and capacitors to produce a clean DC voltage. If there is a problem in the linear transformer circuit, I can say that it is very easy to locate the fault. This is somehow different in the case of a switch mode power supply. The designs were complicated and some technicians found it quite hard to fully understand how the switch mode power supplies work.

The working principle of switch mode power supply is different from the linear type. First the AC voltage will flow to a full wave rectifier (bridge rectifier) which produces an uneven DC output and then filtered by a large capacitor (usually 220 micro farad and up to 450 volts). The clean DC voltage will then flows to start up resistors and to the input of switch mode power transformer. Once the voltage passed through the high ohms resistor (start up resistors) the voltage would drop to a value where it then flows to the VCC supply pin of Pulse width modulation IC.

Once the PWM IC received the voltage it will output a signal to drive the transistor (or FET) and produces a changing in magnetic field in the transformer primary winding. The changing magnetic field induces voltage in the secondary windings. Each of these AC voltage produced by the secondary windings is then rectified, filtered, and regulated to produce a clean DC voltage. One of the main DC output voltage is the B+ that supply to flyback transformer (for TV and Monitor Circuit)

The output from the B+ voltage supply is then connected, through a “feedback” loop (which consist of optoisolator ic and an error amplifier TL431 IC), back to the PWM IC. When the voltage from the B+ supply rises or drop a bit, the PWM IC will act to correct the output.

If you still do not understand the above explanation, please do not be discourage because you can always buy technical books and schematic diagrams and read it till you get the whole idea of how a SMPS work. You can ask a repair friend or even surf the internet for a better and easy explanation.

Here I would like you to download a free SMPS article by Sencore and I found it to be a great help for you who are still struggle on how SMPS work and how to troubleshoot when it fails. You must ask your self what is the purpose and its function of the components in the SMPS circuit and how to check them if they fail. Find out on your own the function of these components in SMPS circuit:

Bridge rectifier,

Filter capacitor,

Start up resistors

Chopper/Power FET

Pulse Width Modulation (PWM IC)

Current sense resistor

Switch mode power transformer

Optoisolator/optocoupler

Error Amplifier IC (TL431)

Secondary diodes

Secondary filter capacitors

Push yourself further by searching the internet for the datasheet of a PWM IC part number. For example, UC3842 PWM IC is mostly used in SMPS. Do you know what the function of pin 5 of this IC is? Do you know which pin the VCC supply enters? Do you know what the actual voltage that flow to the IC is? Do you know which pin that drives the power FET? Can I get a replacement for this IC? And so on………

Let’s take a soldier as an example. Soldiers not only good in handling rifle but also knows all the details about it. They know how to dismantle and assemble back their rifle fast (imagine in the middle of war the rifle jammed-they can repair it fast). They know how much each bullet cost, how far the shooting distance, how big is the diameter of the bullet, how many cm the length of the bullet and so on. Hope you don’t get bored with the soldier’s story, did you get the ideas?

Any SMPS that comes across my repair bench, I would not immediately repair it, in fact I will take couples of minutes to analyze the circuit design and see it from all angles before I begin to repair. Troubleshooting SMPS is not limited to only one procedure in fact many electronic repairers have their own unique ways and methods to solve SMPS problems. Some prefer to use light bulb to isolate SMPS faults while others like to use resistors. Troubleshooting SMPS is fun and flexible but in some cases could make you get very frustrated too.

Remember, don’t limit yourself to only one or two sources to get you understand and be able to repair SMPS. If you have the budget, get the books that have related to SMPS repair-study and start doing practical about it. Share your problems with other fellow electronic repairers and the most important thing is don’t give up. There’s lot of mountain in the journey of our live and you yourself have to climb and conquer it. All the best!

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Understanding Power Supply Repair Safety

It is very important to know the power supply repair safety before you begin to dismantle the equipment. If you accidentally caused a small amount of current to flow through your body, you may get a shock and even death! Always be alert when deals with power supply. Understand how electricity and power supply work can help you to avoid from electrocuted. Always wears a rubber shoe and have an isolation transformer in your technical department or working area. You may find out more information about isolation transformer by searching on the internet.

If you want to check the hot section in a power supply, always use the one hand 'rule' so that it would not complete a path for current to flows through your body just in case if you were to accidentally touched a live connection or cable. If you are tired and weak, do not repair any electronic equipment or else you might end up causing more problems for the equipment by replacing a wrong component. Take a break and come back to tackle the problem when you are fresh again.

Always use original part numbers for replacement when dealing with components in the critical section such as the fuse area. If the fuse rating is 3 ampere, then you must replace it with the same value which is 3 ampere. Sometimes a slightly higher ampere such as 3.15 ampere may be acceptable. Do not use a 10 or 15 ampere for replacements otherwise fire may occurred due to over current because the fuse may not melt.

The main filter capacitor with a high capacitance and voltage must be installed in the correct location. The positive marking at the capacitor must be inserted to the hole where it printed (+) and a negative marking to (-). A wrongly installed filter capacitor can cause the filter capacitor to explode and may injure your face if you are near to it. In some severed cases, the blown capacitor may burn the power supply as well.

Lastly, don't because of this information explained above scared you away from repairing a power supply. Just make sure you understood the power supply repair safety first and things will become easier. Understanding the safety procedure above can protect your life.

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