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Vizio E550VL Force backlight to turn on    
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Joined: 31 Jan 2012
Posts: 15
Location: Torrington, WY

PostPosted: Mon Jan 12, 2015 5:40 pm    Post subject: Vizio E550VL Force backlight to turn on Reply with quote

I've got a TV that is giving no end of headaches. I got it with a bad power supply (blowing fuses), so I replaced the power supply. It turns on now, but the backlight doesn't work. The power going to the backlight inverters was the correct voltage, and a backlight was available (it's 4 separate boards on this TV), so I replaced all 4 boards. Still no backlight. The only part left to replace is the mainboard (which is not available), and a repairable problem on the board (bad EEPROM, 2 chips to solder) can cause the backlight to fail, so I ordered the chips and rebuilt the mainboard.
Still no backlight.
At this point, I'm sure that I've gotten a DOA part, but I don't know how to tell which part it is. The backlights are getting the voltage they should get, which tells me the powersupply is good. I have a second believed-good backlight inverter set that I tried, with the same results, but it's possible it's bad as well, meaning that we can't rule out the backlight inverters. The mainboard is the most likely culprit, but I've replaced the potentially bad chips with no results; the whole part isn't available from a reputable supplier, so the only remaining option I can think of is to force the backlight on. The problem is that I don't know what to send the backlight to force it on. The cables to the backlight inverters have ground & 24v that are working correctly, and they have 3 pins at the end, labelled as follows:
A(B/LON/OFF), B, C(DIMMING). B does not have a wire. When the TV is pwered off, A has 0.12V, which rises to 0.2v when it's turned on, then gradually falls back down to 0.17v. when the TV is off, C has no power, then rises to 0.75V when the TV is turned on.
A, B, and C are also present at the power cable between the mainboard and power supply, and the voltages are exactly the same as on the cables running to the inverters; I think the power is all going out of the power supply, but the ABC signal is originating in the mainboard and passing through the power supply, eventually reaching the inverters.
My question is this: What voltages should be on these pins? If it's supposed to be 5, 12, or 24v, I can easily send some there to test it and see if the backlight fires up, but if it's supposed to be 2V and I put 12V to it, I could burn something up. Does anyone have any info on this?
The TV is a Vizio E550VL. The backlight is a 75021071 or VIT71881 Rev. 6, also used in Sanyo/LG/Toshiba.
Thanks in advance.


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Joined: 21 Jan 2009
Posts: 2395
Location: Far, Far Away

PostPosted: Mon Jan 12, 2015 7:15 pm    Post subject: Reply with quote

Try no more than 5 vdc on pin 11.

Joined: 31 Jan 2012
Posts: 15
Location: Torrington, WY

PostPosted: Thu Jan 15, 2015 11:28 am    Post subject: The verdict Reply with quote

As it turns out, the backlight inverters will power on any time there is no power at all on pin #11. Pin #13 does not effect the backlight's operation either way. If you simply cut the wire for pin #11, the backlight comes on and everything works. Unfortunately, after about 5 minutes, the mainboard crashes, and fades to a black screen. Resetting it doesn't fix the problem unless you let the TV sit unplugged for at least 15 minutes to cool off; then it'll start back up and run for another 5 minutes before overheating again.
I've at least verified that the inverters are working correctly, which was my main concern. The problem is definitely in the mainboard. Now I just need to either find a replacement or fix this mainboard.
Unfortunately, nobody has these in stock.
Since it's acting as if it's overheating, but never physically gets real hot (at least to the touch), I'm wondering if perhaps there's a solder problem. I'm going to try reflowing the board, and I'll post back the results.

Joined: 31 Jan 2012
Posts: 15
Location: Torrington, WY

PostPosted: Tue Jan 20, 2015 3:22 pm    Post subject: The Solution Reply with quote

The mainboard was definitely bad. I replaced it with a new one I picked up on eBay, and all was well. Hopefully this can help someone else.

Joined: 28 Dec 2006
Posts: 2879
Location: Overland Park, KS

PostPosted: Wed Jan 21, 2015 8:01 am    Post subject: Reply with quote

well done. Thanks for posting your conclusion.

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