HORIZONTAL OUTPUT TRANSISTOR BLOWS UP - REASONS (CRT Based TVs)
- Over voltage from SMPS power regulator.
- Line Output Transformer’s winding short [Internal].
- Frequency of horizontal oscillator is very high than normal.
- An defective free-wheeling capacitor connected between the collector of horizontal output transistor to GND.
- A solder loose to the above said capacitor.
- Shorted winding inside the defection yoke [Horizontal deflection winding]
- A loosely fitted horizontal output transistor to metal heat sink.
Assuming that the SMPS section circuit works normally, we can check this voltage for no load to confirm it is OK. In most TVs, this secondary main output voltage will be within the range of [90 to 135VDC], and must be stable too. It should not vary with time. To check this, locate the smoothing capacitor to this main voltage at the secondary side main supply voltage. The value to this capacitor will be within 100 to 220MFD, 160VDC. This voltage is fed to the LOT winding, and is supplied to the collector of the horizontal output transistor. Either disconnect the supply to LOT , or de-soder out the horizontal output transistor from circuit. Thus, we get an open [No Load] voltage point across the secondary main voltage, across its filter capacitor. Solder two wires from the to terminals of this smoothing capacitor, and connect it with a bulb holder. Insert a [100W 220V Filament Type] bulb. Never use CFL or like other as load.
Plug the TV to AC power, and switch it on. If the voltage is there, the bulb will glow
faintly [at about half its intensity]. All the other low voltages
from the SMPS will be present. The
power LED at the front panel of the TV (If any) will glow, indicating that the
standby voltage [5VDC] is present.
Measure the voltage across the connection that we have connected with
the bulb. It should be correct as specified in the service manual. Keep the set in this condition, for at least
5 minutes, and check the voltage again,
It shouldn't vary by any means.
0.3VDC difference is allowed. It
is OK, but if it varies by 2VDC or more during this time, the power supply is
not stable. Check all the components at
the power control side. Be sure to wear
surgical gloves, when you work with the primary side of the SMPS power supply,
because all the points at that side will be HOT. Refer to service manual for
details about the components used at this stage circuits, and be sure to
replace the same type and number; if a replacement is needed. If there is no voltage variation to the
secondary power supply noted, we could go to next stage of troubleshooting.
Check all the solder terminals at the horizontal output
stage. Practically, it is not possible
to detect these loose solder terminals, as they will be microscopic in nature,
that we can’t detect it without using a magnifying glass. It is best to re-solder all the suspected solder terminals, by
applying a little more solder to each terminals, without making any solder
bridge short in between adjacent solder points.
If the horizontal output transistor found Leakey, that is,
its collector to emitter junction shows a dead short [less than 1Ohms],
the
free-wheeling capacitor should be suspected.
It is a best practice to replace it too along with new horizontal output
transistor. The capacitance value to it
can be vary by 3900 to 9500Pf 2000VDC according to different
manufactures.
Make sure when you replace it that the value presently used must be the
value to the replacement, because, this capacitor has an important role
in
generating EHT, Focus voltage etc:- More over, it determines the width
of the
picture. If you use one that has a
capacitance value more than that of used one , the EHT and Focus voltage
will decrease,
and the picture will look larger than normal. If it goes down, the EHT
and focus voltages will get high, which in turn cause arcing to take
place inside the CRT gun as well as failure to horizontal output
transisor. In some sets, an other capacitor
with a very low capacitance; say [470Pf 200VDC or so] value, too will be
connected in parallel to this free-wheeling capacitor. If there is
such, replace it too. Note the value of this capacitor; especially
when you service Samsung Brand CTVs; because, Samsung do not use any
heat sink to the Horizontal Output Transistor.
When
you replace any component to the Samsung TV circuit, It must be the
original type and number used. If improper replacement have done to
Samsung circuit, frequent failure of horizontal output transistor will
be the result.
Then comes the problem of winding short, inside the
LOT. Normally, we can’t measure this by
ordinary methods, The EHT winding
inside the LOT have several turns of very thin copper wire. Even a
short to its single turn, will cause
to put a heavy load to horizontal output transistor, and the transistor
will
heat up. It will heat up to a
temperature, that we can’t even touch it by bare finger. Finally, it’s
collector to emitter junction will get shorted. Just see, here , by
schematic, the collector to this transistor is
connected to +B voltage, via the windings of the LOT, and its emitter is
connected to Gnd. So, if this
transistor shorts, means, a direct short to +B supply to Gnd. With most
of modern circuits, there the SMPS
supply circuit will have a short circuit protection facility, and it
will
detect any short circuit to the secondary DC outputs, and send a message
to the
main system control, which in turn shut off all the functions of the
SMPS
oscillator and control. The set will
remain dead, as a fuse failure, but not.
So if you get a set in dead condition, first check whether the AC mains
fuse has blown up. If it has blown, make
sure that the fault lies within the SMPS regulator control section [HOT]
side,
and not with the secondary side. If
fuse is found opened, check all the components at the main power
regulator
section. Suppose; during checking, you've found one among the four
rectifier diodes have shorted. Do not replace the damaged one only.
Replace all the four [In some sets, a rectifier stalk will be used]. The best way to troubleshoot for a dead set is, first check the mains fuse. If it is found OK, de-solder out the horizontal output transistor. Plug the TV to AC mains, and switch it ON. If the failure is only with the horizontal output transistor, the power supply will switch On, and standby light will glow. This is true to all local CTV kits [Chinese]. Then find out the reason for the blow up of the horizontal output transistor. In some cases, a loosely fitted transistor to heat sink metal plate might be the trouble. When replacing the horizontal transistor, be sure to fix it firmly.
If LOT is suspected, the only way to check it is to switch ON the TV; after replacing all the damaged components; set for a very short time, say, for 5 seconds or so, and switch it OFF. Feel the temperature of the horizontal output transistor by bare fingers after disconnecting the AC cord from AC Mains Socket. It can be just warm, but not too hot. If LOT is faulty, the horizontal output transistor will heat up much within this time period. If so, you have to replace the LOT too.
If the horizontal oscillator frequency shift beyond limit, the horizontal output transistor will fail. To make it sure, before inserting the transistor check the frequency at the base terminal of this transistor with a multimeter have frequency measure range.
Check the frequency, either at the base terminal of the horizontal output transistor or at the collector terminal of the horizontal drive transistor. It should measure within the range of 15KHz. If not, make sure that the horizontal oscillator stage is faulty. In most modern one chip circuits, this section is within the main IC, and the replacement of that IC is the only remedy, no repair to it.
Shorted winding inside the deflection yoke is a very rare case; one out of thousand; but could happen and can be measured by taking the resistance of the horizontal winding terminals. Resistance value of this winding will be below [3-Ohms] to most television In most case, the failure to horizontal output transistor will be the reasons as described above.
Troubleshooting to any equipment is an art. A deep knowledge in the subject won't help anybody to to do a repair work too easily Practical experience will do much. [An Ounce of Practical Experience is more than a Ton of Theory].
