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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