Toshiba 42SL417U LCD TV 3 Bilnks -Try to repair

[Sevenqi]

Toshiba 42SL417u, LED backlight LCD TV
Symptoms:
When press power button on the remote, hear relay clicks, no backlight, the front LED blinks 3 times pause then repeat the same blinks. It can also power off with the remote control, after power off, no LED blinks.

Open the cover, check the standby voltage, 5V, OK, there is 12V as well. Powered on, the output voltages look good, the backlight inverter gets 24V too (At this moment, the front LED repeatedly blinks 3 times of course).

I knew there is a thread this forum had the same symptom got fixed by replacing the main board. But it's hard to find now, and expensive, so I tried to deep further to see if I could find something.

Main Board part#: V28A001251A1, Inverter board part #: T420HW08 V0 LED Driver BD. (I borrowed the clear pictures from ShopJimmy.com).

1. Check main board voltage:
5v and 12v ok. There are three Buck Regulators (IC 273,IC290 (Both on the left near bottom),IC271(On the right near bottom)(part number AOZ1094DI) in this main board , IC273: input 12V, output 5V, IC290: input 5V,output 3.3v, IC271, input 5v, output 1.28v), from what I guess, the voltages are ok (IC273 gets input from IC290 output, IC290 gets input from IC273 output, IC273 gets input from 12V power supply.

2. Check LED Driver Board (aka Backlight Inverter):
There is no (0) voltage on BL_On, but get 24V. This means the inverter does not get backlight on command from the main board.
Tried: disconnected the BL_On from main board, connect the PS's 5v to BL_On with a 1.2K register, hope this to force the backlight on, powered on, the backlight flashing and black, same symptom as before.
Check the backlight inverter output voltages: There are 12 pins on this connector, I only got 24V on two pins.
Does this mean the backlight inverter is bad? What should be the right voltages for a LED backlight ?

To answer this question, I check another Toshiba LED backlight tv (Toshiba 40sl500u), the output connector has only 8 pins, I measured the voltages, surprised, there are two pins have 183v output !

Stop here. My question is, does the LED need 183v to lit up ?

[ben7]

Re: Toshiba 42SL417U LCD TV 3 Bilnks -Try to repair

Check the wiring to the led strip. Also, check for shorted LEDs.

Yes the backlight needs about 183v to work. All the LEDs are in series (possibly series and parallel!), and white LEDs normally operate at about 3v.
183/3 = 61 LEDs
There might be a few more or a few less leds, it is hard to tell exactly.

[Sevenqi]

Re: Toshiba 42SL417U LCD TV 3 Bilnks -Try to repair

I checked this model (42sl417u), there are two bars one on left end one on right end, there are 72 LEDs on each bar. It marks 94v-0, does it mean the input voltage is 94V?
There are four pins in the connecotor, not sure how it works.

[budm]

Re: Toshiba 42SL417U LCD TV 3 Bilnks -Try to repair

That is flammability rating of the circuit board. 72 LEDs per bar is more likely to be split in half, 36 LEDs per string. Some LEDs backlit do use 180V for LEDs, it is all in the design choice.

[Sevenqi]

Re: Toshiba 42SL417U LCD TV 3 Bilnks -Try to repair

Thanks for your replies, ben7 and budm, so I assume the dirver board output voltage is around 108v. I'll diagnose further base on that.

[budm]

Re: Toshiba 42SL417U LCD TV 3 Bilnks -Try to repair

I just look at the second pictures on your POST 1, it looks like it has 8 outputs, you can see 8 copper test pads close to output connector. Can you get the P/N of the U201/301? The 4 filter caps with e Positive legs connected to the output pin 1 and 12 are rated at 160VDC. So we know it cannot have out put higher than 160VDC.
If you can find the spec sheet of those two ICs which are for LED driving controller, then you may be able to get some ideas how it works.
The LED return lines are 8 connections to those pads, they are more likely for the current sensing on the return low side of the LED string to maintain constant current through LEDs.

[Sevenqi]

Re: Toshiba 42SL417U LCD TV 3 Bilnks -Try to repair

Hi budm, the pn is max16814b, looks like this one:
http://www.maximintegrated.com/datas...ex.mvp/id/6290

[budm]

Re: Toshiba 42SL417U LCD TV 3 Bilnks -Try to repair

Yes, that makes sense now since you can see labels on the board for 8 outputs (Output1 ~8). So the IC has built-in boost regulator circuit, the out put is from that D1 in the spec sheet, which will be D201 and D301 in each section. And you can see the two large inductor plus 2 MOSEFT, instead of 2, in that section also.
So start checking the voltages feeding the IC. Dowload the spec sheet to get the pin out.
Q502 and Q504 MOSFETs are used for B+ switch to supply the B+ to the LED drive circuits. You can see the pads with Vin1_, Vin2_1, there should be steady voltage there ( you can measure the DC right at the big tab of 502/504) when you turn on the TV and nothing when you turn off the TV.

[Sevenqi]

Re: Toshiba 42SL417U LCD TV 3 Bilnks -Try to repair

When I turned on the TV, carefully watched the LED flashing (less then a second), looks like the upper left LEDs were not lit up.
I measured the driver board output voltage, the left side (part of it is not working) is up to 105v, but the normal right side only up to 90V.
So if part of the LEDs are not working, does this the cause of the Backlight System to be in protected mode ?

[budm]

Re: Toshiba 42SL417U LCD TV 3 Bilnks -Try to repair

The reading of 105V may indicate open circuit reading (or over voltage condition), means no load connected. You may have to check each LED to see if one has open circuit, since they are connected in series, it will only take one bad LED to cause the string not to light up. Yes there must be some kind of protection circuits some where.

[Mahaffey1990]

Re: Toshiba 42SL417U LCD TV 3 Bilnks -Try to repair

Hello My name is brett, i have a toshiba 42sl417 .... my question is ... my tv turns on and works but the only things that do not work are the hdmi ports every port on my tv works but those also the hdmi ports are on the same board that all the other ports are on so how is it that the hdmi ports are the only things that do not work i can also tell you that the devices i connect to the hdmi ports on my tv see the tv but its like the tv dose not notice that there is any thing connected to it ... do you think that buying a replacement board will fix this issue please help the borad's part number is "V28A001251A1" and this is what the board looks like that all the ports and hdmi are on http://tvrepairtips.org/files/styles...?itok=Ds18jJw1