Set uses TDA 8172 à VERTICAL OUTPUT / QFJAF 6818 à HORIZONTAL OUTPUT / AN 7522 & AN 17821A à AUDIO OPUTPUT / NOM 8370-B-6NF à SYSTEM CHROMA JUNGLE / KA5Q1265 à POWER REGULATOR / TDA9850 à AUDIO DECODER / ET-5A5E-AF108M à TUNER
HORIZONTAL OUTPUT TRANSISTOR BLOWS OFF FREQUENTLY
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Replace the capacitor fitted very near to the
horizontal output transistor, [freewheeling capacitor], there may be one or two
of them, which comes to a value of 3900pf to 6700pf 2000V or so. It is
connected to the negative rail and the collector of the horizontal output
transistor. If it gets opened, or its value gets reduced, this fault will
occur. If so the transistor you replace will get shorted as you switch on the
power to TV.
The second one is deflection yoke short. If so the transistor said above will overheat within seconds. Even a single winding inside the horizontal deflection yoke shorts, it puts a tremendous load to fly back transformer, which in turn effects the transistor.
After replacing the above said capacitor, you do one thing to make sure about whether there is a short inside the yoke or not. It cannot be measured by normal methods.Unplug the base card from picture tube and place it somewhere near without getting any of its contacts to other parts inside your TV. This time only the EHT chord will be connected to picture tube. Unplug the horizontal yoke connection from main board.
Switch on the TV, and wait for some time, say at about 10
seconds, and check whether the transistor gets overheated or not If it does
overheats now also, the winding or EHT rectifier diodes inside your fly back
transformer has some trouble, and it should be replaced with same type and
number. If it does not overheats now, the deflection yoke's winding have some
short in it. Ok.
Refer the fly back output circuit here. Pin-3 of the FBT is connected to secondary
main power supply voltage [126VDC].
Pin-4 is connected to Horizontal Output Transistor. From pin-10, a boost up voltage [200VAC is
taken out, which in turn rectified by a diode, which fed to video output stage
circuit at the base card.
If horizontal output transistor is found shorted check and
replace C455, C456. Don’t mine whether
it is good or bad. Replace it with new
good ones, and make sure that they are good.
There are two types of horizontal output transistor failure
occurs. The first one is rapid, ie, as
soon as the set is switched ON, and the other is slow, fails after some time,
say 15 minutes or so. If the failure is
of first type, suspect the capacitors stated above and the input supply
voltage. If the fault occurs as stated second, the fault should be of
secondary regulated power supply. Anyway,
it will be best to check this secondary main supply voltage for stability. In most TVs this voltage range between 90 ~
135VDC.
The best way to check this voltage for stability is using a
filament type bulb [100W] as load.
Connect a bulb across [C528B] at power supply section. De-solder out the horizontal output
transistor. Now there is no load to this supply line, except the bulb we
connected. Switch ON the set. Measure the voltage across [C528B]. It should be within permissible range. A tolerance of 0.01% is allowable. If it is more than this range, adjust it
with [VR501]. Keep switch it ON for at
least 15 minutes, with bulb loaded.
Then measure the voltage again, and make sure that it is stable.
Now, unplug the base card from picture tube base board, and
keep it at a secure place, without making any short circuit to chassis. Unplug the deflection yoke connection plug
from the main board. Check all the
above said capacitors, insert new horizontal output transistor, and solder
it. Be sure to unplug the CRT base
card, before unplugging the yoke connection plug. If you have a frequency counter, the work will be very easy. Measure the frequency of oscillation at the
collector of horizontal driver transistor.
It should be around 15625Hz. If
this frequency is out, that is, it does not come under the limits, it too can
cause the horizontal output transistor blow out problem.
If this frequency is found OK, then you can switch ON the
set without connecting the yoke and base card.
Switch on the set. Switch it off
after 1 minute or so. Check the heat of
the horizontal output transistor. It
can be warm, but not too hot. If it
heats up in this condition too, horizontal oscillator section to set should be troubles
hooted.
If it is only warm now, you can now connect the deflection
yoke plug, and CRT base card.
If you haven’t tampered the set too much previously, you
will win. After horizontal output
transistor replacement, no further adjustments need to be done. Anyway, it will be best to check the screen
and focus voltage. A slight touch up
only needed.
NB: Never connect the TV with base
card is in position while deflection yoke connection is isolated. Keep it
unplugged.