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.