DAMAGED D2499_Horizontal Output Transistor _ SAMSUNG CZ21V5MJ - Deflection Yoke Fault.
The
set received for repair, in dead condition.
As per information from party, they haven’t used it for a long time; at
about 2 years or so; as there was nobody at home. Might be; because, the set too looks so. Whatever it may be; let me work with it.
I
did plug the set to AC power and switched it ON.
Dead indeed. But, the green power
LED at the front panel of the TV lights up for a moment, and goes OFF. That was all.
No symptom of software loading or any static EHT effect at the front of
the screen.
As
the power/standby LED lights up; at least for a moment, means the SMPS section
circuit is working normally.
These
sets have short circuit protection. If a
direct short circuit occurs at any stage at its main board, the SMPS circuit
will shut down automatically. That is
what happening here. There should be a
short circuited component/s anywhere at the circuit board. Where might it be?
The
set gives me no time to measure any voltage points. It shuts down, as soon as I switched it ON.
Disconnected
the set from AC mains. A meter test done
to all suspected components. Detected a
damage with the Horizontal Output transistor.
Collector to Emitter leak [Direct shorted junction].
If
this transistor (D2499) should get damaged, there are 4 possible causes.
* 3k9pF
[2000VDC] capacitor. [Free-wheeling capacitor]
* Horizontal
oscillator frequency.
* Shorted
winding inside the LOT
* Shorted
winding in the deflection yoke.
* The
3k9pF capacitor [connected to the collector terminal of the horizontal output
transistor to Ground]. Either it might open
or the solder terminal loose contact.
Checked it. No loose
contact. The capacitor showed a value of
4k2pF, with capacitance meter test. It's OK.
* Frequency
of horizontal oscillation. If this
frequency go beyond a specific value, say if it changes very low or high; the
horizontal output transistor will get damaged.
The normal frequency is 15.625kHz.
about +/- 2% deviation can be tolerated.
Not too much.
When
the set receive a transmission, this frequency will get synchronized with the incoming RF signal, as synchronizing pulses is being send by the TV transmitter along with
video information; to synchronize the
picture at the TV end. There are both horizontal &
vertical sync: pulses are present along with the transmitting video signal. At the TV end; the synchronization separator
circuit; built within an IC, will separate these signals and will routed to
horizontal and vertical oscillator section circuits of the TV set; to lock the
picture rock-rigid.
Here
my problem is how I can measure this frequency.
If
I could measure it; the set should switch ON; and stay ON.
Here
the set switches ON and shuts OFF due to the shorted horizontal output
junction. If I rectify this short-circuit, the set
will stay switched ON and I can measure all other voltages.
I
de-soldered out the horizontal output transistor from circuit board. Switched ON the set, without horizontal output transistor.
Now
I can and did measure the DC [+B] supply voltage with volt-meter. This 110VDC supply voltage was OK.
Plugged
the set to AC, and switched it ON.
The
power/standby LED now stay ON, just for 5 seconds and shuts OFF; indicating a
software load up.
Measured
the horizontal frequency at the base foil point of the horizontal output
transistor. Found OK. 15.596kHz.
Switched
OFF the set. Soldered a new horizontal
output transistor of same number in place. [D2499]
Plugged
to AC power, and switched ON.
An
EHT static felt at the front of the screen; but, within 10 seconds the
transistor became very hot.
Either
LOT or the deflection Yoke might be the reason.
Disconnected
the deflection yoke plug from the main board.
Pulled out the CRT base card from the CRT neck and kept it securely without making any
short circuit to it with any other components on board.
Switched ON the set, after plugging it to AC mains.
Checked
the heat of the horizontal output transistor after 5 seconds. It was normal.
Means,
the deflection yoke have the fault. If
the fault was within the LOT, the transistor might have heat up this time
too. It is difficult to detect the
winding short, either to LOT or deflection yoke. The only way is to check it by replacing with
an other good one of same number.
I
too did it.
Switched
ON the set. OK.
On
checking, the deflection yoke the winding of it have corroded. Might have short inside in between adjacent turns. That was the fault.