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Samsung LE40C530 no backlight

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JLB:
Hi folks, I'm new to this forum, wonder if can contribute.

By now, have this baby in my hands, and my experience and electronics equipment aren't that far.

Facts:
Inverter board replaced was first suspicion of the owner, who ordered one and replaced it. No results.

When I got access to the unit, checked the power board BN44-00340B.

PFC-OUT reads only 155V. I don't have any information on how much should be there, but my guess it should be way more isn't it?

Mosfets QP801 and QP802 show 155V at Gate (pin 1) and Drain(pin 2) ? I'm puzzled to have the same readings in both drains and gates, I wonder those are damaged and internally shortned.

Best thing would be to replace the full power board, but I wonder is there anything else I could check?
BTW, I don't have an oscilloscope, just a multimeter...

No 800V output at CNI801, actually getting 0V here.

I wonder this rings any bells on you from your past experience?

Many thanks!

Turnip:
Hi JLB,

Wonder if you're reading primary voltages to chassis rather than Neg of main cap. Had a no-go myself, and after changing the P/S discovered it was Main board.

Best of luck, guess you have P/S shematic as I do - Chris.

JLB:
Hi Turnip, thank you for your reply.

Reading primary voltages against the chassis.

Checked all voltages on the Main Board and TCON board, as for these boards there are expected readings on testing points in the schematics. All seemed within acceptable readings.

It is a pitty the schematics don't show expected readings for the IP board and for the Balance Board !

Hopefully the main board is ok here...

downunder:

Hi JLB,

As Chris pointed out, PFC voltage is measure using the negative terminal of CP801 or CP802 [100uF 450V] as the earth point. In fact you can just read the voltage across either capacitor, as they are in parallel. Same outcome. Expect a reading of around 380V. If QP801/2 were shorted, the mains fuse will have blown instantly.

Possibly they're getting no drive from the drive IC, ICP801 (FAN7530) which is quite troublesome in Panasonic power supplies.

Not sure about the 800V you're expecting at CN1801. An LCD power supply has no need for such a high
voltage.              Bruce

JLB:
Hi Bruce,

Just replaced the IP board by a new one, still no luck.

My former IP power board was giving 324V at the capacitors, which looks a bit low comparing to the 378V the new board shows there. The former board is v1.3 the new one is v1.4. 

Replaced back the Inverter board, checked the HV at one of its connection points with the backlight, reading is 0V there.

No backlight, sound ok, image is there when one applies a tangencial flashlight to the screen.

I wonder now this TV has the same problem Chris described, the Main Board and the IP and Inverter boards are all ok?

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