Insignia NS-42L780A12, no backlight.

[johnboy1313]

I picked this one up tonight. It powers on and off and shows a picture visible with a flashlight to the screen. When first powered on, meaning, the exact moment the LED turns from red to blue, the backlights flash for a split second. I've had two TVs with a bad bulb and they both had a true two seconds to black failure. This one is more like two milliseconds to black. I see nothing out of sorts on the PSU or the inverters. The connector on the PSU shows all voltages that I would expect, 3.3, 5, 12, and 24V.

I can't find a service manual for this one so I don't know what, if any, voltages should be present at the 5 wire connector running from the upper left corner of the PSU to the lower inverter board.

[budm]

Re: Insignia NS-42L780A12, no backlight.

Can I see the whole back side of the TV? I cannot where the 24v connected to on the inverter board.

[johnboy1313]

Re: Insignia NS-42L780A12, no backlight.

Sorry bud, I honestly thought I had attached them. The connector on the PSU is not labeled at all. On the inverter it is.

[budm]

Re: Insignia NS-42L780A12, no backlight.

Ooh, one of those. On the power supply board, there are two 2-pin connectors with black wires close to the High voltage transformer driven by the two MOSFETS (run off 380vdc power supply) mounted on the heat sink to the right of the transformer. Those two connectors carry semi high voltage may around 100~200VAC High frequency, this high voltage is then fed into those small high voltage transformer on the inverter board to step it up to 1000~2000vac.
At this point we know that the bl_on is working otherwise the backlights will not flash on and go into shutdown. So it can be that power supply is not supplying the PFC Voltage booster, or bad small inverter transformer, or bad lamp.
So check first to see if we have correct voltage on those two filter caps, they are connected in parallel, has about 370~400VDC when TV is turn on.

[johnboy1313]

Re: Insignia NS-42L780A12, no backlight.

Yeah, 165v at standby, 390v when on. The PSU is working to a degree, at least. There is picture and sound, just no backlights. I'm not sure if that five wire connector is supposed to have power on it or not. There isn't any.

[budm]

Re: Insignia NS-42L780A12, no backlight.

Post good clear picture of the power supply board both sides, also smaller sections of both side, then I may be able to give you the test points to test. I think the problem is in the high voltage transformer drive circuit.

[johnboy1313]

Re: Insignia NS-42L780A12, no backlight.

Will do. I will try to get to them this afternoon but we've got a big night tonight so it may not be until tomorrow morning. Thanks.

[johnboy1313]

Re: Insignia NS-42L780A12, no backlight.

Pictures

[johnboy1313]

Re: Insignia NS-42L780A12, no backlight.

More pictures

[budm]

Re: Insignia NS-42L780A12, no backlight.

OK, check out that fusible resistor (0.2 Ohms or so, I cannot read the numbers as printed on the body) which the 380V is fed through it to feed the two MOSFETs that drive the high voltage transformer, if it is open, then you probably have shorted MOSFETs.

[johnboy1313]

Re: Insignia NS-42L780A12, no backlight.

It doesn't measure open.

[tom66]

Re: Insignia NS-42L780A12, no backlight.

But does it measure shorted, nearly 0 ohms?

[johnboy1313]

Re: Insignia NS-42L780A12, no backlight.

It has 0.1 ohms printed on the side. So, shouldn't it read nearly zero ohms? It read 0.2 on my meter. I chalked it up to the meter not have the ability to read accurately at that low level.

[tom66]

Re: Insignia NS-42L780A12, no backlight.

That's fine.

[johnboy1313]

Re: Insignia NS-42L780A12, no backlight.

Any other ideas?

[tom66]

Re: Insignia NS-42L780A12, no backlight.

Part # of the ~14-16 pin chip just above the MOSFETs in this image?
https://www.badcaps.net/forum/attach...7&d=1380931566

[budm]

Re: Insignia NS-42L780A12, no backlight.

That is the IC which drive the primary side of the MOSFET Gate drive transformer.

[johnboy1313]

Re: Insignia NS-42L780A12, no backlight.

Oz9976gn
aa5g19.1n
1123c

[tom66]

Re: Insignia NS-42L780A12, no backlight.

Can't find a datasheet for an OZ9976GN unfortunately. You might be able to figure out the functions of some of the pins but not many.

[lookimback]

Re: Insignia NS-42L780A12, no backlight.

I just spent over a week, with a lot of help from Bud, working on a Westinghouse which was doing exactly the same thing. About a half second or less and backlights went out. Turned out the guy I bought it from had tried to fix it himself and bent over a pin inside one of the connectors on the inverter board. Original problem was probably the cap on the inverter board.