If the set is dead, it will be rather easy to troubleshoot. If the set is working intermittently, the troubleshooting will be somewhat difficult. Anyway, coming to the exact fault point is very important. Never replace any component/s at the circuit board, without any idea about the fault. Check all the voltages to different test points, and make sure about the faulty stage and proceed.
Dead set. [No power LED]
In this case, troubleshooting should start from the AC input point itself. Check whether there is any break inside the AC chord, which prevents AC to come to the set. Check the continuity of the AC chord with a multimeter. We should check this starting from the AC plug; to the power supply circuit. If there is no AC voltage present at the main power supply regulator section circuit, check the On/OFF switch of the set for damage. If found faulty, replace it.
If AC is present at the supply point at the main power regulator section, then check for DC voltage across the +ve and –ve terminal of the main filter capacitor. If there is no DC voltage, check the main rectifier diodes, and the serial resistor [usually a wire wound fusible resistor (Surge protector)] connected to the bridge rectifier for open. If DC is there across the main filter capacitor, check its potential using a multimeter. If AC input voltage is 220VAC, the Rectified DC voltage should measure up to 300VDC. If not, replace the main bridge rectifier and/or the main filter capacitor used; will usually come up to the value of 100Mfd ~220Mfd 450V. When replacing electrolytic capacitors, make sure about its terminal polarity. If the polarity has been reversed by mistake, the capacitor will burst as soon as you switch on the set, after connecting it to AC mains, and will be dangerous too.
If the DC voltage is found OK, check the start voltage. For any TV using SMPS, this voltage is the most important to note, if the set does not switch ON. The main symptom will be that, you must have to switch the main power On/Off button several times, before the set to switch on. The start voltage is derived from the DC +ve voltage itself, by connecting one or two resistors or 270Koms or a single 470Koms. This voltage is used to trigger the SMPS oscillation. In most cases, this will be the reason for failure of the SMPS to start up. When checking these resistors, do not check it on circuit. Just desolder up one end of each resistor and measure for its resistance. If found abnormal, or open, replace it.
If this voltage is also found OK, probably the switching device [might be a transistor, FET or STR] might be faulty. If it is a transistor, or an FET, it will be easy to measure its characteristics using a multimeter. But, it will be difficult for and STR. The only thing we can do is to replace the STR with another one with exact number. Here one thing should be noted is, if the STR or Transistor, whatever it may be, is found damaged, you should check all the other components related to this SMPS circuit, especially the Zener diodes, low value resistors etc: There is a chance for these components too can be damaged along with the switching device.
It will be best to connect a 100W filament type bulb at the DC output of the SMPS, after disconnecting the B+ load to the main board, and make sure that the output voltage that comes out from the SMPS is steady. Do this test for at least 30 minutes, before connecting it to the main circuit [Line Output Transformer]. Be sure to use the rated fuse only. Usually, the fuse ratings will be marked on the PDB itself, very near to the fuse holder. Never use fuse with higher ratings, which can cause more damage to components. If you note that the fuse is blown up, make sure that there is something wrong with the circuit, a short circuit or an over load can be. Trace the fault first, recify it, and make sure that all the components in circuit are OK. Then only connect the set to AC mains.
Usually a TV [CRT] type will have a working voltage between 95 ~ 135VDC. Make sure about this working voltage by referring the schematic diagram of the set. For some brand sets, there will be a B+ voltage adjustment pot-meter to adjust this output voltage, and for some others it won’t be there, and is a constant supply voltage. Be very careful when working with this type of power supplies. Check all the related components at the SMPS stage, at least twice, before energize it.
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