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Old 03-13-2018, 12:30 PM   #21
budwich
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

was just killing time prior to heading to the tennis court... :-) take this with some "grain of salt" as I haven't looked at everything but just the last little "exercise" around the 4 IC's in question. Based on the spec sheets posted, and the observations measured (few to none), it would appear that worry about it not being there is one thing but also there should be some sort of check for other voltages that should be at the chip (Vin) as perhaps it not being there (comment about what pins could be measured, had no voltage) maybe is a reason why the EN is not being sent (by another circuit). Perhaps checking for the Vin in some form may show something.
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Old 03-13-2018, 03:19 PM   #22
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

thanks for your help and i will try harder to trace these lines, but i am still not seeing this resistor you say you see with 220 printed on it
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Old 03-13-2018, 03:24 PM   #23
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

i cant see that resistor either lol even with my glasses on maybe its the 122 marked one?
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Old 03-13-2018, 03:48 PM   #24
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

you can see the markings clearly in the photos i took if you zoom in. there is no resistor marked 220
here are some "see thru" pictures (light behind the board)
i also did it for the underside of the board (which there are no components, but some traces)

im getting 0 on that whole line leading to the EN pin, and where that line starts i cant tell because it just disappears right there near the connector. how can i tell where it "jumps" to?
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Old 03-13-2018, 04:41 PM   #25
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

re-checked and labeled all voltages.

some observations:

the PGOOD pin is 0
it comes from R8205 which says 1003 on it which is 100k
on the left side of it we have 4.87 but on the right we have 0.
should it be 0? or could this resistor be bad? or is 4.87v going thru a 100k resistor always gonna end up with 0? my math not so good
the resistor checks out at 100k tho so i dont think its that...
just poking around half blind here


and i am seeing a very small voltage 0.03 on the EN pin and the resistors leading to it
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Last edited by triplefour; 03-13-2018 at 05:00 PM..
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Old 03-13-2018, 04:46 PM   #26
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

Quote:
Originally Posted by triplefour View Post
re-checked and labeled all voltages.
Does the resistor marked 1003 test ok? 4.87v one side 0v the other
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Old 03-13-2018, 05:04 PM   #27
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

i noticed that as well, and edited my post probably while you were typing yours


"the resistor checks out at 100k tho so i dont think its that...
just poking around half blind here"


also the 1002 resistor checks out at 10k as it should

according to the data sheet, i should expcet to see ~5v input to the vcc pin, and then some kind of voltage out from the PGOOD pin. im getting 4.87 to vcc so that seems good (but i cant really tell where this 5v is coming from since the only input to this board is 24v from the psu... it seems like it comes from the northern part of the board somehow, not from the connector CN806) but i am not getting any v out from the PGOOD pin...

of course im not so great at making sense of what it says in datasheets...struggling to understand!

but something funny is going on... Im getting 24v on the three IN pins, but the enable pin needs to be high, not low. output from LX cannot happen without this. also if LX is not outputting, then PGOOD will be missing as well.

so what enables the enable pin :???? and can i, or should i trick this pin to go high?

i trace the enable pin and it goes to what appears to me as a dead end. maybe someone knows better ...
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Last edited by triplefour; 03-13-2018 at 05:35 PM..
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Old 03-13-2018, 06:14 PM   #28
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

ok well i started noticing something else, which might be a little more important.
after about a minute and 15 seconds of being on, the motherboard resets. it does this in a loop. i noticed the white standby LED goes bright, then fades down again, eventually to off. along with that, the red light inside the optical audio connector goes out, for a few seconds then comes back on. also, the 3v coming from the mb to the led driver board disapears but comes right back.
the voltages coming from the PSU to the MB or to the Driver board do not drop during this loop.
also when i tried the flashlight test again really thoroughly i cannot see any picture
and i cannot get any sound though i have my laptop connected and playing audio via HDMI. the sound on the laptop speakers turns off like its trying to play through the hdmi and like it sees a connection to a tv, but nothing comes out of the tv. suprisingly, when the motherboard resets, my laptop doesnt switch back to single screen and audio coming from the laptop speakers. it just acts like its still connected. who knows what this means. it does the same when i turn the tv off (by pressing power once, then again but a long press)... laptop doesnt skip a beat. when i actually unplug the tv from power, thats when the laptop notices and switches the screen up and puts audio back to the laptop speakers. it might just be that the laptop only has to "see voltage" at the hdmi connector to switch modes, and as messed up as the mainboard may be, its still got some voltage at its hdmi ports. also i noticed, by testing each hdmi port on the tv, that the 60hz one will not trigger my laptop to do anything upon connecting the hdmi cable. probably because laptop cant output 4k at 60hz... so it seems like some things in the mainboard are still behaving as they should... but no audio, no picture, resetting every minute like clockwork.... definitely has me leaning now more towards a mainboard issue, and hoping that it really could be as simple as replacing that EEPROM. unless someone has some other great idea for something i should check, im probably gonna order that soon and try it out.

15$ isnt a lot to lose by trying it, and the mainboard itself is 100$ (a lot to spend on a chance but totally worth it if it fixes a 4k tv)

the only thing that has me worried is that in the descriptions of the problems the eeprom chip replacement is likely to fix i didnt see anyone list "mainboard resets every minute and 10 seconds"
its mainly to fix no backlight problem.

so i really would like to test some more things on the mainboard before just replacing this chip. is it also possible that other failures on the mainboard would immediately fry the new eeprom chip?

Last edited by triplefour; 03-13-2018 at 06:22 PM..
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Old 03-13-2018, 06:51 PM   #29
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

Don't bother working on same model TV here and I replaced the eeprom chip from shop jimmy and did not work. Still have TV in a boot loop mode. TV comes on get Vizio logo, power lite dims than backlites go off at about 30 seconds and at about 1 minute TV reboots the cycle all over again. Have to unplug it to get it to stop rebooting or is it turning on than off(have no clue).
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Old 03-13-2018, 06:59 PM   #30
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

is it also possible that other failures on the mainboard would immediately fry the new eeprom chip?
Do not think so or have not happen to me yet. I have replaced lots of eeproms and discovered other issues after words.
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Old 03-13-2018, 07:11 PM   #31
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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
Don't bother working on same model TV here and I replaced the eeprom chip from shop jimmy and did not work. Still have TV in a boot loop mode. TV comes on get Vizio logo, power lite dims than backlites go off at about 30 seconds and at about 1 minute TV reboots the cycle all over again. Have to unplug it to get it to stop rebooting or is it turning on than off(have no clue).
did changing the eeprom chip change any of the symptoms at all?
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Old 03-13-2018, 07:21 PM   #32
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

Nope noda.
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Old 03-13-2018, 07:27 PM   #33
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

started looking at some mainboard voltages now... specifically near the LVDS connector going to the TCON since it seems like we're not getting much there, or on the TCON itself for that matter.

these voltages also drop when the mb resets

actually the 12v ones dont drop, only the lower ones and the ones on the tcon itself drop when the mb resets

and actually when i checked the voltages a second time, the ones that i had written down as 2.49 are now only like 1.09

and shouldnt that 4F 100 16H cap have some voltage on it?
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File Type: jpg tcon-voltages.jpg (1.74 MB, 9 views)

Last edited by triplefour; 03-13-2018 at 07:43 PM..
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Old 03-13-2018, 07:35 PM   #34
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

Every symptom showing should be the eeprom chip. Shop jimmy solution is replace main board. Chip only for no picture/black screen. He told me that they haven't studied issue yet to come up with a repair kit. I told him that they need to investigate further and come up with a solution or repair kit. Since this is major issue and i have seen post about it every where on the web just no response back on post if main board replacement or chip worked out. Main board is over $100 so chip was a $15 gamble was hoping to solve issue. I have my own electronics repair shop and a lot of people around here do not have money just to throw away. Looks like I am going to have to break down and buy that service manual and figure this thing out.
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Old 03-13-2018, 07:43 PM   #35
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

Yes voltages look all good until main resets and than it acts like power cycling or in a loop. Voltages drop out. My opinion it is something on the main kicking in the protection circuit and turning TV off and than back on after about 1 minute. The board is seeing a fault just have not pin pointed it. Did also notice the black heat shield would get very warm to almost to hot to touch. So wondering about a chip under it. Specifically the end with no tape near the coax cable plug in.
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Old 03-13-2018, 07:57 PM   #36
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

As far as Tcon board I took it all the way out of TV and turned TV back on. And still same issue!!! Not T con board.
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Old 03-13-2018, 08:09 PM   #37
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

The only thing that I did not due was unplug 1 at a time the 4 cables for the back lit strings checking for a bad string. That are on the back-lite inverter board. Reason being did not know what would happen. Would it add to the issue, make things worse. You get the idea. The unknown
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Old 03-13-2018, 08:17 PM   #38
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

i dont think it would make things worse. disconnecting one of the 2 strings (or in this case 4) is a valid test, isnt it?

it would be really nice if you could post some voltage readings. it would help me to know more about my issue, as my board is not even getting as far as yours apparently.
specifically the LED string voltages when they are on,
and the voltages at CN805 on the driver board
and in general it will help anyone trying to fix this tv in the future, to know what those voltages should be while backlights are on

Last edited by triplefour; 03-13-2018 at 08:18 PM..
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Old 03-13-2018, 08:48 PM   #39
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

some more questions.... have you tried disconnecting the tcon to panel cables totally and also "onesy at a time" to see what happens.

I was looking at the posted pdf for the ic, not sure that the "EN" is used as perhaps the ic is being run in "switching mode" based on a pwm input.
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Old 03-13-2018, 08:57 PM   #40
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Default Re: Vizio P502UI-B1E has no backlight, no voltage from driver board to LED's

i tried with the tcon completely disconnected from the mainboard and it didnt affect the mainboards looping or voltages

i didnt try messing with the cables going from the tcon to the panel because i thought we only do that when there are panel issues

might as well try it as ive done everything else. will get back to you but i suspect it wont change anything

Last edited by triplefour; 03-13-2018 at 08:58 PM..
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