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 other 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 check and detect the
faulty components. Digital multimeters 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 at 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 any one contact 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 (burned), make sure that there is
some short circuit at the primary side of the SMPS circuit. 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]; and the degaussing
posistor. Here a three terminal degaussing posisotr
is used. It is a common fault seen, that
this posistor has got shorted due to some 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 solder 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 slightly,
and check its forward and reverse resistance using the meter set to Ohms range.
You should have some knowledge in electronics to do these works. If any of these rectifier diodes found
shorted, replace all the four, even if the other three measures good. It will be the best.
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 ‘Zero’ Ohms or
nearly ‘Zero’ Ohms by no way. If a short is
detected while measuring this, make sure that the switching output transistor
2SC4429L is defective. If the collector
to emitter junction to this transistor has got shorted [a common fault]; the
resistance measurement across the C507 will show near to Zero. 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 this circuit. There is a possibility that the 2SC3827, 2SA105 and the Zener diode VD519 might have damaged. It will be best to replace these components
too along with switching transistor; recommended. Replace the opto-coupler VD515 (PC817B too.
Check R521 & R520. Both are 100K 1/2W resistors,
which in turn supplies the voltage to the SMPS oscillator and drive
stages. Failure of any one among these resistors will cause a starting problem, when you switch ON the set. (Start-up problem). You might have to switch On and OFF the main power switch many times, to start the TV ON. This is a common fault found with most of the sets. Voltage regulation is achieved
by using the opto-coupler.
Suppose that the SMPS is working, and there are voltages at its
secondary winding. The output +B voltage
is used to sense the voltage regulation. See the resistor
network and the transistor together which 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 +B voltage trends to go out of range, 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 220V filament type bulb (not CFL or other types); [Across C563 – 100MFD
160V]. Insert the fuse with same rating
used. Power ON the set after connect it
to AC wall socket. Check how the connected bulb glows. If all are OK; the bulb will glow bright at
first, and then gradually its intensity will decrease, and will get steady within
seconds. Check the 109V point, and make sure that the voltage is correct and steady. 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. {Line to Load check} If all are found OK, switch OFF the set. Unplug the AC cord from AC power socket. Connect the +B to circuit, after disconnecting the
bulb. Check all the other low voltages generated at the secondary of SMPS. If the +B is OK, all the other voltages too will be.
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. It will work normally. Switch Off the set, disconnect it from AC mains wall socket and solder in the degaussing posistor in place, which we have de-soldered out earlier. If the AC fuse now blow up now, after the degaussing posistor is re-connected to circuit, check replace the degaussing posistor, as it is faulty. Without this posistor, 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.{Please co-operate with my English, as it it entirely different to that mothertounge of mine. Just catch up the ideas I do try to express before you.}
REFERENCE SMPS CIRCUIT
CLICK ON THE CIRCUIT DIAGRAM TO ZOOM IN
