Set
came for repair in dead condition. No
standby light, no audio thumb sound at switch ON. Seemed fully dead.
Transistors are used in SMPS supply circuit.
Checked
all the transistors, 3 of them, at the power supply circuit. De-soldered them out from circuit, and
checked. All of them was OK. Checked all the four rectifier diodes. Found OK.
If
so, the power supply must turn ON. The standby
light at the front panel should light up.
Here it is not. Suspected the
horizontal output, and horizontal drive circuit. Checked the horizontal output transistor (BU508DF) in
circuit. Found as OK. Checked the continuity of the primary and
secondary windings of driver transformer.
Found OK. Then what should be
the fault. It is sure that, there must
be some short circuit at the secondary side of the SMPS power transformer.
Checked all the rectifier diodes and
capacitors connected in parallel across them.
All were OK.
Disconnected
the Line output transformer {LOT} bower supply, by disconnecting one end of the
resistor [2.2Ohms] here, and switched ON the set.
Front
panel power LED began to light up.
Checked the voltage at all secondary outputs of SMPS transformer and all
other low voltages fed from it to different other section circuits. According to circuit diagram, all was found
OK. Replaced the Horizontal Output Transistor
with a new one. Set worked all
right. Checked all other
functions.
Checked
the horizontal output transistor, which was taken out from the circuit, using a
digital multimeter. Found that, its
Base to Emitter resistance measured very low, in Ohms [Approximately
2Ohms]. It is clear that, the BàE junction inside it is leaky (A rare fault).
Conclusion; Here the SMPS power supply have over load
protection. Even though it is not an
opto-coupler coupled regulator, it will sense any short circuit at the
secondary side of the SMPS transformer. Never go so much as I’ve done
here. The fault is with the Horizontal
Output Transistor (BU508DF) itself. The
kit found with this set is common to most of the BPL models, including 21”
CTVs.
See the internal configuration of BU508DF
Appearance
The similar fault will be occur to most of the Chinese kit, available in local market too.
