Free Report
By Humphrey Kimathi
Disclaimer The reader of this book is expressly warned to consider and adopt all safety precaution that might be indicated by the activities herein and to avoid all potential hazards. The author particularly disclaims any liability, loss or risk taken by individuals who directly or indirectly act on the information Contained herein. The author believes that the information presented here is sound, but readers cannot hold him responsible for either the actions they take or the result of those actions.
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Solving power supply problems (s.m.p.s) using control i.c. Most problems with equipment using switch mode power supply (S.M.P.S) like television and monitors are power related. This is because powers supplies run under considerable stress due to high voltages and currents involved around these circuits. In order to understand how to solve S.M.P.S problem I am going to show you how to analyze common failure mode in CRT television, you can use this principle in repairing other equipment using switch mode power supply like LCD Television and Monitors, DVDs etc. 1. Television is dead Here dead I mean the set does not show any sign of life, power ON LED is not lighted and nothing happens even after power is switched on. First confirm the television cable is not broken inside, you can check this by measuring the voltage across the AC power input connector on the circuit board.
If the voltage is reaching the circuit board, then look for the fuse and see if it is blown, if the fuse is okay check also the surge limiter is open or not. If these two components are ok then most likely there is no short on the primary side of the power supply note: Short in the primary side of the power supply usually cause the surge limiter or the fuse to blow. From now on we shall be using the main capacitor negative as the Ground for all the measurement of Voltages on the primary side of the power supply. I have modified my meter ground probe with a crocodile clip so that I can attach it to ground point of the equipment I am repairing and therefore I use only one hand to probe the circuit. This is a safety precaution because as you know for there to have a potential difference there must be a connection between the live and the ground. So if you are using your two hands and incidentally touch the live and the ground then current will flow through your body and this can be fatal. But if you are using one hand it is not possible. One thing I have leant in electronics is that if you master doing your troubleshooting using voltage tests you will have an added advantage because in electronics it is not always the component that fail but it could be just a
crack on the circuit board that is preventing the voltage to flow or even dry joints and therefore if you concentrate on looking for the bad component you are going to waste a lot of time and end up not even repairing the set. Therefore I encourage you to narrow down your faulty circuit (area) by use of voltage testing only. Fast and valuable method I have come to learn. Now set your meter to DC range of 1000 VDC and use the main Capacitor as your ground: First check the voltage across this main capacitor and if you get around 300 vdc on 220 vac outlet or 150 vdc on 110 vac outlet then it is assumed the supply is okay up to the main capacitor. If there is no Voltage at the main capacitor then check backward looking for open component or circuit board cracked or a dry joint on the main power path.
Be sure this main DC (300 vdc or 150 vdc) is the reaching the power regulator ic Collector/Drain pin Now still using the same ground move your positive probe and measure the vcc in pin of the power regulator ic.
Depending on the regulator used you can check the correct vcc from the data sheet for the ic used from the internet. These two voltages are very important and if any is missing the supply will appear dead, if the vcc in is missing this means there is an open circuit or start up resistor is open. Follow backward and find what is preventing the start up voltage reach the vcc pin. Another hint if you see this vcc is less than Normal and pulsating there is a possibility that there is Short on that pin, to confirm just lift up that pin and if the voltage increases then it means there is a short on the pin and the ic must be replaced. There is also a filter capacitor on that pin usually 100uF/35V please check the esr of this capacitor or directly replace it if you don’t have an esr meter. There is also a resistor linked to the source pin of this ic called the current sense resistor which sometimes open or change value and this can also cause the power supply not to start. Usually it very low ohm less than 2 ohm and around 2 watt so it a big one. If up to here you have checked and they are okay then we should suspect a short on the secondary side of the smps. First measure for short of any of the secondary diode using analogue meter set to X1, when you measure across any diode on the secondary you should not have two reading and if you get two reading just lift up one leg and measure again. If you still get two reading then you can conclude that diode is short. Also when using this method and find that the diode has two reading in circuit and when lifted up you get one reading, it could also mean there is a short on that source (line)
You can follow that source looking for the shorted component; usually the most common components to short are transistors, ic, capacitors, and diodes. Remember when doing voltage testing on the secondary sources (circuit) you should be using the COLD GROUND of the set.
The best ground to use is the turner ground (outer part) of the CRT ground return.
Also note that when measuring the voltage at output of the chopper transformer at the secondary diodes use the diode cathode to measure the dc voltage and when measuring at the anode of the same diode use the ac range in your meter. If you find still there is no voltage at the secondary output or the voltage is low then it the time to substitute for the Main B+ load with a dummy load. You can make a dummy load by using a 60watts bulb/250v ac, a bulb holder, crocodile clip and two pieces of wire (red and Black) and connect them as shown on the picture below.
Note you can also this bulb to discharge the main Capacitor, just clip the bulb across the positive and negative leg of the main capacitor and you will see the bulb flash briefly if the capacitor was holding Charge. Now identify the main supply to the fly back called the B+ and disconnect the supply just before the fly back B+ vcc pin. Don’t cut the circuit board to do this just lift up one side of a resistor( high wattage) in series with this B+ voltage or sometime they use a jumper wire just before the B+ vcc pin of the fly back and attach your dummy load between this point and the cold ground. If the bulb light steadily and you measure between 90 Volts dc to 140 dc depending on the size of the television then you can assume that the smps is okay and the problem is on the secondary side.
Hint: A short on the secondary side can cause the power not to start irrespective how far the short is. If you find that when you use the dummy load the power is starting but when you remove the dummy load the power is blocked then suspect short on the components after the take off point which include the following for this television using IC for RGB amplifiers.
when you find the power supply is not starting and you use the dummy load and find that the power is okay but if you connect the B+ supply again you find the power supply is blocked then suspect component in parallel with the B+ (parallel means one leg is on the B+ supply and the other leg is on the ground) Any component which is shorting the B+ to the ground will make the power supply to appear dead From the circuit above the following components can cause the B+ to go to the ground directly and hence can block the Power supply from starting. 1. Snubber capacitor C403 2. Horizontal output transistor (HOT) 3. C444 4. Fly back Transformer
5. IC901 (TDA6107Q) on the CRT neck board 6. C901 and C902 RGB source filter Capacitor
If any of the components marked on the above picture is shorted to the ground it will cause the power supply to appear dead.
By Humphrey Kimathi(Author)
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