Toshiba 42XV540U loses backlighting after 4 or 5 seconds

[Spork Schivago]

Hi!

Got a Toshiba 42XV540U that loses backlighting after a few seconds. Sound is there and when it turns on, we can hit source and see the input channels and switch them and everything.

I believe it's the inverter board, but before I replace it, I wanted to make sure. On the logic board, I see a connector that has a GND and a bunch of 24VINV that go to the inverter. I used my DMM and made sure all those 24V's stayed at 24VDC, even after the backlighting went out. They did. On the power supply board, there's a connector that has two labeled 12VLCD and they stayed at 12VDC. There's an POW_LCD on that same connector and the voltage didn't change when the screen was on or off.

I pulled the inverter board and measured the caps, they're good. I pulled the MOSFETs and to the best of my knowledge, they're good. I see a lot of smaller SOT type packages (like SOT-23) and a few look really burnt. I can't send pics tonight, but ShopJimmy has one for 19$.

This inverter is model number: LC420WU 6632L-0482B. That goes for 60$ and they're out of stock, but have a compatible LC420WU 6632L-0482A for 19.00$, free shipping, that I was thinking of grabbing.

It'd be hard to identify what those SOT packages are, because I can't read the markings some are so burnt. The worst one looks like it almost says 5N. Then there's another one that looks like an SMD resistor, but it's it's completely solid black and has no writings that I can read.

Does the symptoms I've described sound like the inverter to you guys and would you try repairing the inverter or just buy a new one?

I'll send pics tomorrow when it gets light out if you want.

Thanks guys.

[R_J]

Re: Toshiba 42XV540U loses backlighting after 4 or 5 seconds

It could be the inverter or bad ccfl, the fact the backlights come on and go off would suggest one of the inverter outputs not working.
If you have a spare ccfl, lay it across all the transformers, it will light up near the transformer, if that output is working.

You could disconnect all but one lamp and see if it turns on, if it turns on and goes off, unplug that lamp and plug in the next one, one at a time trying to see if there is a lamp that does not turn on. If you find a lamp that won't turn on, plug it into one of the other connectors to see if it is the lamp or inverter transformer

[Spork Schivago]

Re: Toshiba 42XV540U loses backlighting after 4 or 5 seconds

It could be the inverter or bad ccfl, the fact the backlights come on and go off would suggest one of the inverter outputs not working.
If you have a spare ccfl, lay it across all the transformers, it will light up near the transformer, if that output is working.

You could disconnect all but one lamp and see if it turns on, if it turns on and goes off, unplug that lamp and plug in the next one, one at a time trying to see if there is a lamp that does not turn on. If you find a lamp that won't turn on, plug it into one of the other connectors to see if it is the lamp or inverter transformer

I got the inverter out, but it's gonna be hard to test that. The inverter board is under the logic board. Those four squares on the inverter board are the transformers, right? I was thinking earlier they probably where transformers. Four transformers on it. But do the CCFL bulbs need to be from a TV or can I use one for the house?

I don't see the bulbs. How hard are they to access on this TV I wonder?

I wish I knew how to test those inverter outputs (transformers, right?) with a DMM or something to verify they're good. On the back side of the board where those transformers would be, it's darker on the one set of two then the other, like discoloured from extra heat, which made me originally think the inverter was bad.

But I think with those SOT23-3L's looking as bad as they do, replacing just the bad transformer might not fix the issue.

[R_J]

Re: Toshiba 42XV540U loses backlighting after 4 or 5 seconds

I looked up the model on shopjimmy, and it showed a different inverter

Yours uses the philips inverters, Yes those T1- T4 are the transformers, If the inverters are bad you will not have the backlights come on at all. So if they flash and go out I suspect one of the ccfls are bad, or the connections for them in the panel are bad.

[Spork Schivago]

Re: Toshiba 42XV540U loses backlighting after 4 or 5 seconds

I looked up the model on shopjimmy, and it showed a different inverter

Yours uses the philips inverters, Yes those T1- T4 are the transformers, If the inverters are bad you will not have the backlights come on at all. So if they flash and go out I suspect one of the ccfls are bad, or the connections for them in the panel are bad.

But no chance a transistor on the inverter board could cause this?

I found a bunch that have the exact same markings, but the DMM reads very different with this specific one. I now have a better DSA meter that can test zener diodes. Using the SMD code lookup page, it's either a transistor or it's a zener diode. I'll pull it and find out later.