Toshiba 40XV645U shutting itself off

[Mr Bill]

I picked this one up a few days ago. It had sound but no backlight (not even a flash). Putting my brightest flashlight on the screen showed an image. I thought this would be an easy deal. The inverter board that's in the tv is a samsung LJ97-02098A.

Now i have a cracked screen 40" proscan in my basement with a LJ97-02098B. Shop Jimmy shows these 2 inverter boards to be compatible substitutes. I tested the proscan a month ago when i got by letting it run for several hours non stop and it worked perfectly. So i pulled the inverter and swapped them out. The tv fired up backlights and all. I thought done....Wrong

When the tv is cold (off for several hours) The tv will turn on for about 3 minutes. But after that 3 minutes its on for 10 seconds then fully power off then turn back on. It keeps recycling until i hit the power button. If i unplug the inverter board the tv will stay on. There is still an image so the rest of the tv is working.

The inverter has a 3-4 wire plug that goes to the tcon. Could an arcing bulb cause the tcon to tell the main board to shut down or do you think its a heat issue. I know this inverter is good but i can always put it back in the proscan and see if it's still working correctly.

The panel is a LTA400HF02. I got the datasheet but i don't know if it has circuit protection built in. Just wondering which direction i should go with this.

[tom66]

Re: Toshiba 40XV645U shutting itself off

A few things to check:
- Does audio remain? Any distortion or crackling sound to it?
- Checked the +24V? (inverter end)
- Checked BLON/DIM? (inverter end)

[Mr Bill]

Re: Toshiba 40XV645U shutting itself off

A few things to check:
- Does audio remain? Any distortion or crackling sound to it?
- Checked the +24V? (inverter end)
- Checked BLON/DIM? (inverter end)

Tom,
I checked the +24 volt at the fuse on the inverter and it read 23.75v solid until the tv shut off. A few seconds later when the tv turned back it read again 23.75v.

I got to thinking...What if the main board was heating up while proccessing the video from a source causing the shut down. I unplugged the inverter board and ran my xbox through a hdmi input. The tv has been running for over a hour (sound only). It's not shutting down. So that rules out the main board. I can still see an image in the dark.

So either the board i have is now deffective or shop jimmy is wrong stating the inverter boards are compatible. I'm going to order the exact replacement and hope this will end this dilema. This inverter board is cheap ($20-30). I did forget to mention above the original inverter board was cooked pretty good. One fuse was blown and 2 Mosfets. You could see pretty clear the large brown stains on the bottom of the board.

I'm going to put my inverter board back in the proscan and see if i get the same symptoms. I'll update this thread in a few days with results.

Thanks

[tom66]

Re: Toshiba 40XV645U shutting itself off

Your 24V is dropping out and you are blaming the inverter for not working?


Wouldn't always rule out main board even if you have video. Sometimes the video process is separate from the sub micom (controls backlight, power on/off, LED, IR etc.) However a check of BLON/DIM will verify if it is the main board or not.

[Mr Bill]

Re: Toshiba 40XV645U shutting itself off

Wouldn't the backlight just turn off but the tv continue to run if the 24v drops out?

[Mr Bill]

Re: Toshiba 40XV645U shutting itself off

With the tv running a source inverter connected i took a measurment in the 10 second window the backlights & tv were on.
24V - I get 23.03v
BLon - I get 3.281v
Both measurments were dead solid, no fluctuation.

Disconnected the inverter, still running a source: (tv stays on)
24v - I get 23.77v (measured at PSU)
BLon - I get 3.305v (measured at the main board)
Again the measurment were dead solid.

The panel datasheet shows the +24v needs to be 22-26v and the BLon should be 2.4-5.5v

so it's in spec.
opinions?

[budm]

Re: Toshiba 40XV645U shutting itself off

What kind of connection on the inverter board, does it use for dimming function? the PWM pin for backlights dimming or it uses Dim pin for dimming.

[Mr Bill]

Re: Toshiba 40XV645U shutting itself off

The dimming pin is #13 of the large connector on the inverter board. The spec on the datasheet says its 0v-3.3v. Should i measure this while it's shutting down?

[budm]

Re: Toshiba 40XV645U shutting itself off

Yes, since the Inverter board is getting the 24v and the BL_ON signals OK.
By the way, did you do any comparison reading between this board and the original board?

[Mr Bill]

Re: Toshiba 40XV645U shutting itself off

The original board was burnt. 1 fuse and 2 mosfets were shorted. It had some obvious browning on the back side of the board where the 2 mosfets sat.

With the tv on and a source connected i'm getting 3.339v
Turn the source off (backlights still on) i'm getting .100v

The spec states 3.3v MAX. Could this be our culprit? Would turning the backlight brightness down make the Dim voltage go down also?

[budm]

Re: Toshiba 40XV645U shutting itself off

If it is analog dimming signal, at 3.3V = max brightness, at 0V = min brightness, so try turning down the brightness and see if the Dim voltage will go down.
Notes: I just looked at the spec of the panel you posted, 3.3V will give 100% PWM for MAX brightness, 0v = 20% PWM, so it looks like it uses DC voltage for controlling the PWM circuits of the inverter.

[Mr Bill]

Re: Toshiba 40XV645U shutting itself off

I turned down the backlight to 80 which was 2.6v and it still shut off after about 10 seconds. I then turned the backlight down to 40 (1.4v) and it didn't shut off for about 3 minutes. So it seems the lower the Dimming voltage the longer the time it takes to power off.

Would disconnecting the Dim pin be the next step or is there something else to try?

[budm]

Re: Toshiba 40XV645U shutting itself off

That means may be sone kind of lamps over current sensing may be happening, try going down as low as possible to see if it will stay on longer.
I need good close up of the whole Inverter board and also close up in 1/4 section so I can see how the current sensing circuits are setup.

[Mr Bill]

Re: Toshiba 40XV645U shutting itself off

If i turn the backlight all the way down to 0 the tv stayed on for over an hour. I bumped the backlight up to 5, 10, & 15 in 10 minute intervals and the tv continued to stay on. So the tv has been on for about 90 minutes straight and still going. I tried taking pictures with my camera and they came out pretty grainy. I "borrowed" photos from shop jimmy. It is the exact board that is in the tv now. They are much better quality.

[budm]

Re: Toshiba 40XV645U shutting itself off

I am beginning to wonder the problem may be one or more lamps are drawing too much current.One way to find out is to use the IR Thermometer to check the temperature of the inverter transformers to see which one is hotter than the other.

[Mr Bill]

Re: Toshiba 40XV645U shutting itself off

It's funny you mention one of the transformers may be getting too hot from a lamp drawing too much current. Here is a picture of the original board. You can clearly see that T103 has a nice browing on the backside of the board. I don't have an IR thermometer but i can pick one up the next time i drive by harbor freight.

[budm]

Re: Toshiba 40XV645U shutting itself off

I got mine from HF for $19.99 on sale.

[Mr Bill]

Re: Toshiba 40XV645U shutting itself off

Bud
All 4 transformers are about the same temp.
I pulled another inverter from a working 40" dynex and the tv still shuts off.
I even went as far as removing the lcd and swapping the backlights since i have a broken screen 40" that uses the same inverter and still the same result.
If the inverter senses a fault does it tell the tcon board via the small 2 wire plug to shut down?

[budm]

Re: Toshiba 40XV645U shutting itself off

I am not sure what that small connector on the Inverter board is for since it does not show in the LCD panel spec. One pin is SYNC, GND, the other two pins are not used. Not sure what This connection between the T-CON and the inverter board is for. What happen if you do not have that connected?

[Mr Bill]

Re: Toshiba 40XV645U shutting itself off

I unplugged that little connector going to the tcon and the tv still showed a picture with backlights so i am not sure what it's even used for.

I went back and started over checking the +24v
With the tv on (no source) the +24 was reading 23.77v
Tv on, source hooked up, and backlights at 100 the +24 dropped to 22.81v

The dimmer i made the backlights the 22.81v would rise. So i found a used PSU on ebay for $25.
The new PSU +24v reads 23.76v no source
With a source connected and the backlights at 100 the +24 read 23.70

I am not sure if the voltage drop was causing the tv to shut down and start rebooting or if it was a thermal issue but the replacement psu did the trick. I played a 2 hour movie with the backlights at 100 and the tv stayed on. I rechecked the +24v after the 2 hours and it was still reading 23.70v.

I must admit i am surprised that if the 22.81v on the +24 was causing the rebooting it would have just killed the backlights but the tv would continue to stay on. I guess that wasn't the case for this one.