Toshiba 50L1450U Won't Power on, Power Board seems ok. No light.

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Toshiba Television

Repair guides and support for your Toshiba TV.

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Christopher Hill @chillin014

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投稿日: 2020年7月8日

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Toshiba 50L1450U Won't Power on, Power Board seems ok. No light.

I just picked up this Toshiba 50L1450U to repair. It does not power on, there isn’t even a stand by light. When I check the Power Board I see no issues, the voltages match the markings and the voltages follow through to the main board connector. Is this indicative of a bad main board? I’m not even sure what to check on the main board to diagnose.

LED strips tested okay as well.

Other potentially not relevant tid bits :

-The power cord was a polarized cord/connector and somebody shaved down the square side to make it fit like a non-polarized connector.

-the boards are mounted to the plastic backing panel rather than the metallic TV casing. Huge PITA during removal because the boards have to be removed and then somehow re-affixed to the back of the tv for proper grounding which isn’t entirely possible due to the mounting orientation.

Thanks for the help.

These are the circuit boards, apologies for the quality. Basically the power board powers the LEDS directly and the mainboard. From the main board there is a harness to the standby light and the t-con. It’s actually quite minimal.

I measured everything I knew how to measure - 5v, 12v, ground. I don’t know how to measure “BLON” or “DIM”

@jostewcrew I removed the shield but this doesn't look like the traditional t-con setup I see in other TV's. No ribbon cables. It looks like the cable is going directly into the LCD boards, I see no fuse. Attaching a photo momentarily.

Out of curiosity I tested the Standby/IR LED with my LED tester. At first I got it to light up no issues but then as I continued testing I could NOT get it to light up again. Only a very dim flicker.

Ok I’ve tried removed some more panels and it looks like the tcon is integrated with the buffer boards. I didn’t see a fuse but will continue to investigate.

<---This website does not play nice with picture uploads. Preview shows image, no image posts. Try to post as question update - website is not responsive. Try to post picture from phone, photo shows as text. Super glitchy.

Power board model #

@jayeff

@jostewcrew Okay I received the power board from ebay and it was curiously missing the main fuse. So I swapped in the fuse from my other board and immediately upon plugging in the power cord the fuse blew. I am thinking this replacement power board may have a short which might explain the missing fuse in the first place. What do you guys think?

I’m going to buy some more T4A fuses and try again.

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@chillin014 , in my experience, when the led's check out along with the power supply, it is usually the signal from the main board, feeding back to the power supply thats the cause. if youd like, post some images of the boards interconnections and some close ups of the psu and the mb. also, please list the voltages going from the psu to the mb at the main connector. list the pin, pin label and the reading. post back here with findings for further assistance if needed.

jostewcrew さんによる2020年7月8日

Struggling to upload pics

Christopher Hill さんによる2020年7月8日

@chillin014 heres a guide that may help:

iFixitでの質問に画像を追加する方法

jostewcrew さんによる2020年7月8日

@chillin014 ok... with the power supply. .. please list the pin, pin label and voltage reading with the set off, and once again with the set turned on. this will be for the connector leading to the mainboard from the psu. take the readings on the psu side.

(i.e. pin 1- 12v2 - 11.9v)

however you want to format it is your choice, just indicate off or on on the reading.

jostewcrew さんによる2020年7月8日

@jostewcrew understood.

Here are the readings with the TV plugged in. Pressing the power button renders no difference in the readings, it's identical/unresponsive.

Pin 1 (12v2) : 12.55

Pin 2 (12v2) : 12.55

Pin 3 (12v2) : 12.55

Pin 4 (GND) : continuity with ground (not exactly 0, but very close)

Pin 5 (GND) : continuity with ground (not exactly 0, but very close)

Pin 6 (12v2) : 12.55

Pin 7 (GND) : continuity with ground (not exactly 0, but very close)

Pin 8 (N.C.) : no pin/ no reading

Pin 9 (GND) : continuity with ground (not exactly 0, but very close)

Pin 10 (5v1) : 5.13

Pin 11 ( ACD) : 3.54

Pin 12 (PWON) : 3.31

Pin 13 (BLON) : 0

Pin 14 (5v1) : 5.13

Pin 15 (DIM) : 0

Christopher Hill さんによる2020年7月8日

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jostewcrew @jostewcrew

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投稿日: 2020年7月9日

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from your testing:

your power supply is generating the required voltages and are holding them steady to feed to the main board. the main board from there supplies its other required voltages and feedback signals for the logic function of the unit. i.e., the blon voltage comes from the main board to tell the power supply to kick on the backlight circuit. for the back lights to come on, it requires 3.3v which is supplied by the main board if functioning. ( you can test backlights in your case by disconnecting the cable to the main board and jumping pin 12 and 13 on the power supply. (pin 15 is for dim, which is fed by pwm from mainboard. this may also require 3.3v , if backlight doesnt turn on with 12 and 13, add in pin 15..this will be at full brightness, so test only and disconnect.) this will close the signal that was supposed to be from the main board and trick the power supply into turning on the backlights)

in addition to this, the standby indicator light also is fed by the main board. it is stepped down from the 12v fed by the power supply (which is steady) and created by the main board to feed the required voltage to the circuit. the fact that you have no light is another clue. i was trying to test voltage at the fuse to the tcon because its voltage is also supplied by the main board. if it wasnt present, the mainboard is not creating the rail, again pointing to its failure.

i believe changing out the main board is going to solve your problem. here is a link to a vendor that is reputable and has one in stock:

https://www.tvpartstoday.com/75036660-To...

if any further assistance is needed, please update here and let us know the resuts.

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Hi again. I bought a replacement main board but the tv is acting exactly the same as before. I suppose it's possible the replacement board is bad too.

I tried to bridge the terminals like you suggested (pins 12, 13 and 15) but the backlights did not turn on. I've checked the voltages at the LED connector on the power board and get readings of about ~43 V and ~86 V so I'm not sure why they do not turn on. Powering the LED connector with an LED tester shows about 45 V per terminal pair, AND I can see the LED light shining through as well.

Where do you recommend I go from here? Can we still rule out the power board? The other thing that I think is weird is that there is no light from the standby LED. Yet if I check the harness to that IR receive chip there is 3V from the main board to every single pin except one. I wonder if it's burnt out...or if I burnt it out trying to test it with that LED tester.

Christopher Hill さんによる2020年7月16日

@chillin014 ... that is possible that the ir board is damaged. go ahead and disconnect it and see if the onboard buttons will turn on the tv. if it is combined with the ir board, youll want to force on by jumping the connection at the main board it plugs into.

if this doesnt work, you can try and disable the blon pin from the connector on the power supply side and this should disable the pwm circuit and enable backlights on at full brightness without dimming. this is only for continued testing so if it functions turn set of and post back. (everything else hooked up as normal)

" I've checked the voltages at the LED connector on the power board and get readings of about ~43 V and ~86 V so I'm not sure why they do not turn on. "

what is the difference in voltage measurement here?

"I tried to bridge the terminals like you suggested (pins 12, 13 and 15) but the backlights did not turn on."

did you start with just 12 and 13? was this test with the mainboard still connected?

jostewcrew さんによる2020年7月17日

Ok. Disconnecting the IR board had no effect. It's a separate component, not integrated with the buttons as far as I can tell. Not sure which connection to jump.

I bridged 12 and 13 alone, and then tied in 15 and have tried this with both the main board disconnected and connected. Am I doing this to force the 3.3 Volts back to the mainboard through 13?

How do I disable the Blon pin? By removing it from the connector?

Not sure what you're asking about the voltage differences for the LEDs. There are 4 sets of positive/negative LED terminals on the connector (VLED 1, 2, 3, 4). The VLED set 1 and 4 output is 43V and the VLED sets 2 and 3 output is 86V. It's doubled, as if those connections are lighting up more strands. I don't if this has any significance but I find it odd that the power board is delivering power to the LED connector yet they are not turning on.

I'm losing my patience with this %#*@ thing but thanks for continuing to help me.

Christopher Hill さんによる2020年7月17日

@chillin014 ... the bridging was to trick the power supply to turn on backlights by supplying the voltage that was supposed to come from the main board. dont worry about trying this anymore.

blon will be removed from the connector by taking it out of its slot or by cutting and re splicing later on the power supply side. this is is just another way to trick the power board into turning on the backlights like the other method. i wouldnt worry about this now either.

i was taking in all of the info you presented from the diagnostic about the backlights and power supply testing fine, but with four strings of leds, they should all be carrying the same split. ( it could be this way but ive never seen it as of yet) with no standby light and the voltage at the backlight connector the way it is, its kind of now sounding like a power supply issue. please retest your connections leading to the mainboard from power supply again and see if anythings changed. we need to find out how your leds are configured because it doesnt sound right.

its time to call in the big dogs on this one.... @jayeff , @dan ...can you make sense of this sirs?

jostewcrew さんによる2020年7月17日

@jostewcrew

Just wondering if the Standby voltage is sufficient for the mainboard.

I think that the ACO (pin 11) might be the Standby voltage lead and was wondering if 3.54V was good enough and that it should be 5V instead.

What is the board number of the power board? There may be a schematic out there for it although Toshiba are not known for releasing information.

jayeff さんによる2020年7月17日

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@chillin014 , in my experience, when the led's check out along with the power supply, it is usually the signal from the main board, feeding back to the power supply thats the cause. if youd like, post some images of the boards interconnections and some close ups of the psu and the mb. also, please list the voltages going from the psu to the mb at the main connector. list the pin, pin label and the reading. post back here with findings for further assistance if needed. — jostewcrew
@chillin014 heres a guide that may help:

iFixitでの質問に画像を追加する方法 — jostewcrew
@chillin014 ok... with the power supply. .. please list the pin, pin label and voltage reading with the set off, and once again with the set turned on. this will be for the connector leading to the mainboard from the psu. take the readings on the psu side.

(i.e. pin 1- 12v2 - 11.9v)

however you want to format it is your choice, just indicate off or on on the reading. — jostewcrew
@jostewcrew understood.

Here are the readings with the TV plugged in. Pressing the power button renders no difference in the readings, it's identical/unresponsive.

Pin 1 (12v2) : 12.55

Pin 2 (12v2) : 12.55

Pin 3 (12v2) : 12.55

Pin 4 (GND) : continuity with ground (not exactly 0, but very close)

Pin 5 (GND) : continuity with ground (not exactly 0, but very close)

Pin 6 (12v2) : 12.55

Pin 7 (GND) : continuity with ground (not exactly 0, but very close)

Pin 8 (N.C.) : no pin/ no reading

Pin 9 (GND) : continuity with ground (not exactly 0, but very close)

Pin 10 (5v1) : 5.13

Pin 11 ( ACD) : 3.54

Pin 12 (PWON) : 3.31

Pin 13 (BLON) : 0

Pin 14 (5v1) : 5.13

Pin 15 (DIM) : 0 — Christopher Hill
Hi again. I bought a replacement main board but the tv is acting exactly the same as before. I suppose it's possible the replacement board is bad too.

I tried to bridge the terminals like you suggested (pins 12, 13 and 15) but the backlights did not turn on. I've checked the voltages at the LED connector on the power board and get readings of about ~43 V and ~86 V so I'm not sure why they do not turn on. Powering the LED connector with an LED tester shows about 45 V per terminal pair, AND I can see the LED light shining through as well.

Where do you recommend I go from here? Can we still rule out the power board? The other thing that I think is weird is that there is no light from the standby LED. Yet if I check the harness to that IR receive chip there is 3V from the main board to every single pin except one. I wonder if it's burnt out...or if I burnt it out trying to test it with that LED tester. — Christopher Hill
@chillin014 ... that is possible that the ir board is damaged. go ahead and disconnect it and see if the onboard buttons will turn on the tv. if it is combined with the ir board, youll want to force on by jumping the connection at the main board it plugs into.

if this doesnt work, you can try and disable the blon pin from the connector on the power supply side and this should disable the pwm circuit and enable backlights on at full brightness without dimming. this is only for continued testing so if it functions turn set of and post back. (everything else hooked up as normal)

" I've checked the voltages at the LED connector on the power board and get readings of about ~43 V and ~86 V so I'm not sure why they do not turn on. "

what is the difference in voltage measurement here?

"I tried to bridge the terminals like you suggested (pins 12, 13 and 15) but the backlights did not turn on."

did you start with just 12 and 13? was this test with the mainboard still connected? — jostewcrew
Ok. Disconnecting the IR board had no effect. It's a separate component, not integrated with the buttons as far as I can tell. Not sure which connection to jump.

I bridged 12 and 13 alone, and then tied in 15 and have tried this with both the main board disconnected and connected. Am I doing this to force the 3.3 Volts back to the mainboard through 13?

How do I disable the Blon pin? By removing it from the connector?

Not sure what you're asking about the voltage differences for the LEDs. There are 4 sets of positive/negative LED terminals on the connector (VLED 1, 2, 3, 4). The VLED set 1 and 4 output is 43V and the VLED sets 2 and 3 output is 86V. It's doubled, as if those connections are lighting up more strands. I don't if this has any significance but I find it odd that the power board is delivering power to the LED connector yet they are not turning on.

I'm losing my patience with this %#*@ thing but thanks for continuing to help me. — Christopher Hill
@chillin014 ... the bridging was to trick the power supply to turn on backlights by supplying the voltage that was supposed to come from the main board. dont worry about trying this anymore.

blon will be removed from the connector by taking it out of its slot or by cutting and re splicing later on the power supply side. this is is just another way to trick the power board into turning on the backlights like the other method. i wouldnt worry about this now either.

i was taking in all of the info you presented from the diagnostic about the backlights and power supply testing fine, but with four strings of leds, they should all be carrying the same split. ( it could be this way but ive never seen it as of yet) with no standby light and the voltage at the backlight connector the way it is, its kind of now sounding like a power supply issue. please retest your connections leading to the mainboard from power supply again and see if anythings changed. we need to find out how your leds are configured because it doesnt sound right.

its time to call in the big dogs on this one.... @jayeff , @dan ...can you make sense of this sirs? — jostewcrew
@jostewcrew

Just wondering if the Standby voltage is sufficient for the mainboard.

I think that the ACO (pin 11) might be the Standby voltage lead and was wondering if 3.54V was good enough and that it should be 5V instead.

What is the board number of the power board? There may be a schematic out there for it although Toshiba are not known for releasing information. — jayeff

Toshiba Television

Toshiba 50L1450U Won't Power on, Power Board seems ok. No light.

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Update (07/09/2020)

Update (07/08/2020)

Update (07/17/2020)

Update (07/22/2020)

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