Hello everyone! I'm a little new to TV repair and I was looking for a little help with this Samsung LCD TV. It will power up (red LED goes out) and you hear the startup sound as well as sound from the HDMI input I had connected to it. But the LCD screen stays dark. I opened it and noticed a blown 3.15A fuse on the power supply board (BN44-00198A). It wasn't the main fuse (obviously) but after looking over the board a bit I saw it protected the control circuitry for the high voltage transformer that powers the inverter board.
After a little testing I found two MOSFET transistors (FQPF9N50C) that were shorted source-gate-drain. I replaced these and the fuse and powered the TV up. Fuse stayed intact but still no backlight.
Now to show my newbiness. I'm an automotive tech with 13 years of experience troubleshooting and testing low voltage DC circuits. I have some understanding of how electronics work and I have a DSO and know how to use it. But I'm not exactly sure what I'm looking for. I did test the power source for the fuse that was blown. It comes directly off the bridge rectifier and the two 450V 2200uF filtering caps. I was reading close to 400V but it seemed to have a lot of AC riding on top of it. Again this may be normal I'm not sure.
I diagrammed the main circuit for the two mosfets (minus the small SMD control circuitry) but I'm still a little unsure what they do. From the diagram and what I've read it looks like they convert the DC into a high frequency AC for the transformer. There's also a small SMD 30K resistor and a zener across the gate and source. Any information on how this works or how to test it would be much appreciated. Unfortunately I don't have a high voltage probe to test the transformer output directly.
Thanks in advance!
After a little testing I found two MOSFET transistors (FQPF9N50C) that were shorted source-gate-drain. I replaced these and the fuse and powered the TV up. Fuse stayed intact but still no backlight.
Now to show my newbiness. I'm an automotive tech with 13 years of experience troubleshooting and testing low voltage DC circuits. I have some understanding of how electronics work and I have a DSO and know how to use it. But I'm not exactly sure what I'm looking for. I did test the power source for the fuse that was blown. It comes directly off the bridge rectifier and the two 450V 2200uF filtering caps. I was reading close to 400V but it seemed to have a lot of AC riding on top of it. Again this may be normal I'm not sure.
I diagrammed the main circuit for the two mosfets (minus the small SMD control circuitry) but I'm still a little unsure what they do. From the diagram and what I've read it looks like they convert the DC into a high frequency AC for the transformer. There's also a small SMD 30K resistor and a zener across the gate and source. Any information on how this works or how to test it would be much appreciated. Unfortunately I don't have a high voltage probe to test the transformer output directly.
Thanks in advance!
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