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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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www.1aauto.com 1A Auto shows you how to repair, install, fix, change or replace a blower motor resistor on a 2001 Ford Taurus. The procedure is similar for 1996-2007 generation Ford Taurus and 1996-2005 generation Mercury Sable.

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www.bug-transmitter.com Many thanks to talkingelectronics.In this video we are showing you how to make a very small home made FM transmitter Bug using a single transistor and with a transmission range of 200 meters. This FM transmitter Bug is very easy to build. The size of the circuit board is ONLY 22mm x 10mm. I design the circuit board layout using Pain shop pro X2 ,it takes me 4 hours to get the design right ,i want a very small PCB. To make the PCB i will be using the toner transfer method and Ferric Chloride. Parts used Home made PCB 2n3904 transistor 4.7p ceramic capacitor 1n ceramic capacitor 22n axial ceramic capacitor 1 – 5p to 30p air trimmer 360R resistor 4.7K resistor 10mm Mic 35cm antenna Battery Plug Some solder 19mm heatshrink Total cost just 90p.

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This week, I collaborated with Xander to show you how to make your own printed circuit boards (PCBs) using a laserjet printer.

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Picture become weak or dim usually happen after the monitor have been used for many years. Again it depends on the quality of the picture tube or cathode ray tube (CRT). Some grade A CRT can last for many years, but for the lower grade one it can only last for couples of year. Don’t be surprise that I do come across picture tube that had gone dim in less than a year! If you switch on your monitor for 24 hours without turning it off, then expect the tube to become dim fast. Besides dim (weak emission) , CRT may also develop many problems such as open filament, short between cathode and heater, G1 and G2 short, poor gamma performance, stripped cathode and focus and G2 short. In this article, I’m going to touch on the weak emission problem only and show you a possible way to brighten up the picture tube.

The cathode ray determines the brightness and sharpness of a picture tube. Its electrons are emitted from the cathode surface into the cathode ray tube’s vacuum. The electrons then strike the screen in the form of a ray and cause it to shine. Weak or used up cathodes produce few electrons and emit weak electron rays. This causes the picture to become dark and fuzzy. Sometimes the cathode emission ceased due to a layer of contamination coating covers the cathode surface. This contamination prevents the electrons from leaving the cathode’s surface thus you get a dim picture.

In order to identify whether is the tube that cause the dim or from bad components, one can always use a CRT tester/meter to check it. Some call it CRT rejuvenator, CRT restorer and others named it as CRT regenerator. These testers are designed to test and check CRT and fix it if the picture tubes have problems. I personally owned two CRT testers- the BMR 2005 Muter regenerator and a less feature one I bought from Thailand. With the help of CRT tester, I can know if a picture tube is working or not. If you do not have one, don’t worry because by checking the voltage at the CRT socket pin outs, you will have some ideas if the CRT is good or bad.

From my experienced, I noticed that the older tube can be brightening up with the help of CRT tester but not for tubes that was made in the year 1994 and above. I suspect the getter (made of barium) inside the tube have been used up. The functions of getter inside the tube are to absorb gas that was release when the cathodes are heat up. If the getter fails to absorb the gas, then there is no point to restore a picture tube. I have brightened up many picture tubes with the help of CRT tester and some looks like a new tube. But somehow after one or two days the picture tube get dim again because the getter cannot absorb the gas and the gas will travels back to the cathode surface and turn it into carbonate. Electron emission will cease again. I even have tried some home made CRT restorer downloaded from the internet and the result is still the same-the picture tube will dim back after couples of days.

After done some research and experiment, I found that the best way to brighten up the tube is to increase the heater voltage. Normal heater or filament voltage is about 6.3 volt and if you increase the voltage to about 8 to 9 volts, this means the cathode will produce more electrons and thus the screen will become brighter. Bare in mind that do not apply any voltage that is higher than 9 volts to the heater otherwise the internal filament will burn out and broken. Once it is open then there will be no way to rescue it.

If the picture tube is heading to rubbish dump then there is no harm to try to restore it. About how long it will last, I cannot guarantee it. Some will last even more than two years and some only couples of weeks. Recently there was a badge of Compaq and Hp 15” monitor that have the symptom of blur display with bright picture. No matter how hard you try to adjust the focus adjustment at the back of flyback transformer, the picture still look slightly blurs. Using the methods above I had saved lots of picture tube (I don’t need to replace a second hand one to customer).

It is a simple modification and this only applies to the 14 and 15” monitor picture tube only. First you need to find which secondary output that have output from 12 to 15 volts. Once you have locate it, then solder a wire at the cathode pin of the diode and join it to the input pin of 7808 (output 8Volts) voltage regulator. Please don’t tap from the B+ voltage which is usually 45 volts and above and this may kill the 7808 IC because this IC can only withstand voltage up to 38 volts maximum. The current drawn from the 14 and 15” picture tube is less than the 17” tube and if you use this way to brighten up the 17” tube, the power supply may go into shutdown mode and sometimes will even lead to power blink.

I do know some technicians like to use the method of turning few rounds of wire at the flyback ferrite core to restore the brightness. This method can work in Television but not in Monitor. Computer monitors runs in many resolution and the higher the resolution the higher the B+ voltage and thus making the output voltage at the wire increased. This will lead to a sudden brightness and may shutdown the monitor. Even if it didn’t shutdown, the customer may get annoyed with the Monitor brightness (raster) whenever the customer change the monitor resolution.

Now solder the pin 2 of 7808 IC to cold ground and the output of 7808 IC pin 3 to the CRT heater point as shown in the picture. Remember to cut off the 6.3 volt supply line from the power supply because you don’t need it anymore. What you need is the 8 Volt supply to the filament in the picture tube. You can also connect a low ohm protection resistor in series to the heater point. Just in case if any short circuit happens the resistor will open circuit. Make sure you attach the 7808 IC to the heatsink, preferable the heatsink at the flyback transformer.

This method only work for tube that is slightly blur and not too dim picture. If a picture is very dim even if you raise the heater voltage to 12 volt, you still would not see a bright picture. As mentioned above there is no harm trying to modify it and who knows the tube can be brought to life and serve the customer for many years. Hope you enjoy the article.

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