POWER SUPPLY {SMPS} TROUBLESHOOTING
A typical SMPS power supply regulator circuit (Used in Akira 21" CRT based TVs) is shown here for reference. This is a transistorized one, and there are IC based power regulators too are used with some brand TVs. What are the causes can be when a set is dead, is in concern. It is advised to use an analogue multi-meter, like Sanwa YX360TRF or such, as it will be easy to measure and detect the faulty components. Digital ones too can be used, but the analogue one is better for troubleshooting SMPS power supplies.
Suppose the set is dead. You get it for repair in dead condition, and the owner says that it has been stopped while working. before do any checks, set the multi-meter to 400V or more range to measure the voltages, and check whether there is any residual voltage remains across [C507 – 120u/400v]. Because, if the switching transistor, V513 -2SC4429L is open, i.e., collector to emitter junction is opened, by any way, the accumulated electric charge to the main filter capacitor won’t discharge, and will be there. If we touch to this voltage point, by the multi-meter; which has been set to measure Ohms range, the meter will be damaged. If you find any residual storage voltage with this capacitor, thoroughly discharge it before measuring any components; by set the meter to Ohms range. BEWARE of this; always.
First, Check the AC cord for continuity. Then check the DPST switch [Double Pole Single Throw]; used. If anyone among its contacts is faulty, the set won’t get power. You can check this too using the meter set to Ohms range.
If the AC cord and the power switch is found Ok, check the glass cartridge fuse used [F501]. If it is found open, make sure that there is some short circuit at the primary side of the SMPS. Never insert a fuse, and switch ON the set, until the fault has been detected and rectified. It might make things worse. Check the [C501*2Nos the degaussing posistor. Here a three terminal posisotr is used. It is a common fault seen, that this posistor has got shorted due to a voltage spike. Just de-solder out this posistor and check. It is advised to check all the remaining circuit, without connecting this posistor in place; and connect it at the end of service work. Check the four rectifier diodes VD503~VD506 for shorted junction. De-solder one end of each diode, lift it up, and check using the meter set to Ohms range. You should have some knowledge in electronics to do these works. If any of these rectifier diode found shorted, replace all the four, even if the other three are good. It will be the best practice.
If all of the above check is OK, check the resistance across C507 (the main filter capacitor]. There should be a resistance. It should’t be ‘0’ Ohms or nearly ‘0’ Ohms by no way. If a short is detected while measuring this, make sure that the switching output transistor 2SC4429L id defective. If the collector to emitter junction to transistor is got shorted [a common fault]; the resistance measurement across the C507 will show almost Zero, by measuring by reversing the meter prods. De-solder out this switching transistor, and again check the resistance across C507. It should show a high value.
If you found the switching transistor is short, check all the other components related to its circuit. There is possibility to 2SC3827, 2SA105 and the Zener diode VD519 to get damaged. It will be best to replace these components too at the time of switching transistor; recommended. Replace the opto-coupler VD515 (PC817B too.
Check the resistance value of R521, R520, R522 too for its value determination. Especially R521 & R520 should be checked. Both are 100K 1/2W resistors, which in turn supplies the voltage to the SMPS oscillator and drive stages. Voltage regulation is achieved by using the opto-coupler.
Here we have to know some more about the voltage regulation process. Suppose that the SMPS is working, and there are voltages at its secondary winding. The output +B voltage is used to sense it. See the resistor network and the transistor together with controls the opto-coupler. Refer the circuit diagram. A small voltage is taken for reference, and it is amplified to an extent to drive the opto-coupler, such that if any change in the pre-set voltage trends to go out of control, this network will adjust it automatically. That is; here the voltage is set to 109V. It can be adjusted slightly by turning the preset resistor RP551. The Zener diode used here, connected to the emitter of the transistor (6.2V). Check this Zener diode too for any reverse leak.
If all are found OK, insert the switching transistor; screw it tight to its heat sink.
Re- check once more all the components. Disconnect the +B voltage, that goes to the LOT {Line Output Transformer}. Connect a 100W filament type bulb in place [Across C563 – 100MFD 160V]. Insert the fuse of same rating used. Power ON the set after connect it to AC wall socket. Check how the connect bulb glows. It will glow bright at first, and gradually its intensity will go down, and will be steady within seconds. Check the 109V point, and make sure that the voltage is correct. If it show any deviation (+/_ 0.01% is normal); correct it by adjusting the B+ adjust variable resistor discussed earlier. Keep it switch ON at least for 15 minutes, and make sure that there is no significant change in +B voltage. If all are found OK, switch OFF the set. Unplug the AC cord from AC power socket. Connect the +B to circuit, after removing the bulb. Check all the other low voltages generated at the secondary of SMPS. If the +B is OK, all the other voltages too.
If all other circuit parts at the main board is OK, the set should switch ON and the standby light will be there, Power ON the set with its remote control.
Solder in the degaussing posistor in place, which we have de-soldered out earlier. If the AC fuse now blow up, check replace the degaussing posistor, as it is faulty. Without this posistor, even you can see the picture with no problem; but colour patches will be there on the screen.
Solder in the degaussing posistor in place, which we have de-soldered out earlier. If the AC fuse now blow up, check replace the degaussing posistor, as it is faulty. Without this posistor, even you can see the picture with no problem; but colour patches will be there on the screen.
Troubleshooting is an art. You should have knowledge about how to check each component, and ability to understand the schematic diagram as well.
The most needed is patience and concentration.REFERENCE SMPS CIRCUIT
CLICK ON THE CIRCUIT DIAGRAM TO ZOOM IN