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Old 03-24-2018, 11:37 PM   #101
al1415
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

Digi-Key Part Number 296-41938-5-ND $2.02 plus shipping usps.
Part location and info
main board U712
Manufacturer Part Number
TPS54328DDA
Description IC REG BUCK ADJ 3A SYNC 8SOPWR
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Old 03-25-2018, 12:39 PM   #102
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

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Originally Posted by al1415 View Post
In the beginning on i had backlites when TV turned on, then when vizio logo go away just black screen. After plugging and unplugging and checking voltages i get only black screen and no backlites also no voltages going to T-Conn or LED driver board. On the main board their is A Synchronous Step-Down Converter location U712 Pin 2 was 1.3 volts now only .7 volts, also X201 has no voltage or the cap at connector for lvds cable to T-Conn no voltage where as before i lost backlites altogether i did have. I agree though issue is main board just what chip is the question. Could be U7612 breaking down under load maybe??
If U712 is a tps54328, pin 2 is the feedback. It should be at ~0.76V during normal operation according to the datasheet. I don't think that one is the issue. if the output voltage on that regulator is fine, I wouldn't worry about it.
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Old 03-26-2018, 04:07 PM   #103
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

Just another data point- Same model and problems. Replaced eeprom with shopjimmy replacement. Same.
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Old 04-09-2018, 10:53 AM   #104
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

Replace main with a used ebay board. Now it comes on with Vizio logo and than goes dead. So the no backlight problem is definitely a main board issue.
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Old 04-09-2018, 04:07 PM   #105
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

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Originally Posted by enzodude View Post
Replace main with a used ebay board. Now it comes on with Vizio logo and than goes dead. So the no backlight problem is definitely a main board issue.
So the ebay board changed the issue, but did not solve it?
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Old 04-10-2018, 01:53 PM   #106
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

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So the ebay board changed the issue, but did not solve it?
Yes, Original main board had no backlight at all. Replacement would come up with Viz logo than backlight would go out. Also Swapped eeproms. No change.
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Old 04-10-2018, 02:18 PM   #107
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

ok... so with the "new board" that gives you the logo, try disconnecting one of the tcon to panel cables and see what happens. Repeat doing the same with the other.... anything different from before?
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Old 04-13-2018, 01:58 PM   #108
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

Just to throw in an update So far using leakseeker 89 I have found on main board Q210 bad which is on bottom side of board, going through downer boards that i have looking for a replacement transistor.
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Old 04-14-2018, 12:39 AM   #109
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

Quote:
Originally Posted by al1415 View Post
Just to throw in an update So far using leakseeker 89 I have found on main board Q210 bad which is on bottom side of board, going through downer boards that i have looking for a replacement transistor.
Looks to be a MMBT3904. Fairly common if you have spare parts or spare boards. I think I checked that particular transistor on my board with a multimeter and it was OK. I'm not sure what the leakseeker does or how well it works, so I cannot comment. On my board, it at least is not shorted because my thermal camera did not show any abnormal heat coming from it.
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Old 05-01-2018, 08:40 PM   #110
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Smile Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

I have the same P502UI-B1E TV with what I think is the same problem, but I'm new at troubleshooting new TV's.. The last time I worked on them they were mostly tubes.

My TV produces no display and gives no sign of LED illumination. It has no bloated caps, or other apparent physical trouble. When I power it up with the power switch the white "standby" light fades out. Then if it's turned on again the "standby" light flashes once, pauses then flashes 16 times, then goes out.

I replaced the power supply board because I had no 5v "standby" voltage loaded or unloaded, so figured must something on the board and since the board was inexpensive ordered one, but when the new one was installed I still had no 5v standby.

I also didn't want to spend $13 for the service manual. Maybe we split it?

Could it be that this model needs no 5v standby voltage? Like you I'm seeing the 24 volts, the 12v, and the 165 volts on the power board when it's plugged in. I'm sure these questions are neophyte level, but I have to start somewhere. Thanks for any help.
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Old 05-02-2018, 03:46 AM   #111
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

Reading through this very long thread it appear from post #1 there is no 5v at the 5V point. Says no wire.

The checks that have been made are:-
Do you have approximately 400V across the two pins of the large capacitor on the power supply when you come out of standby

CN806
Pins 1 - 4 = 24V
Pins 5 - 8 = 0V

CN805 with backlights on
Pin 1 0V, Pin 2 = 1.5V, Pin 3 = 3.2V, Pin 4,5,6 = 0V
Pin 7 = 4.9V, Pin 8 = 4.2V, Pin 9 = 12.2V, Pin 10 No connection

On the led driver board
R8129 = 4.1V
C8067 = 1.8V

Do you have these voltages?
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Last edited by dick_barton; 05-02-2018 at 03:48 AM..
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Old 07-14-2018, 03:46 PM   #112
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

Adding in my two cents: I've got a board that seems very similar to al1415 - I also traced that "unknown" enable signal back to the mainboard, transistor Q210 - and measured it as bad using my multimeter. I've ordered replacements from digikey, and I'll let you know if it works or not.
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