No backlight on Toshiba 42HL67US LCD

[SurrealMustard]

I'm looking at this 42" Toshiba 42HL67US "Regza" LCD and it appears to power up and produce audio, but the backlight is not coming on. Using the flashlight trick, I can see the outline of the "no signal" box on the screen, so I'm pretty sure we've got video.

What would be the best strategy for getting to the bottom of this? It's one of the older models (circa 2007), so everything is on separate boards and it has what appear to be two inverters instead of one.

[budm]

Re: No backlight on Toshiba 42HL67US LCD

I would find out if you do have 24VDC on the power supply board to run the inverter boards or not, if it does then check to see if the inverter board is getting the BL-ON from the main board or not.
If you look at the back or the front of the TV in the dark room, do you see the backlights eve flash on for seconds?

[SurrealMustard]

Power supply strong

I would find out if you do have 24VDC on the power supply board to run the inverter boards or not, if it does then check to see if the inverter board is getting the BL-ON from the main board or not.
If you look at the back or the front of the TV in the dark room, do you see the backlights eve flash on for seconds?

As far as I can tell, the backlight doesn't come on even for a moment. Looking more closely at the power supply board, I did find a "24v_Inv" on one of the connectors, and it was delivering a strong 24 volts. Would this indicate a possible problem with the inverter?

[ReeceyBurger123]

Re: No backlight on Toshiba 42HL67US LCD

Yes an inverter fault, I have seen these inverters with shorted mosfets. Take the covers of the inverters off and get us some images please.

[SurrealMustard]

Update

Yes an inverter fault, I have seen these inverters with shorted mosfets. Take the covers of the inverters off and get us some images please.

I got the covers off and am attaching images to the post. Is there a way to test the MOSFET on both boards with a multimeter to determine which one is bad?

I always afraid to open any device. I always think, it could be more damage by me. But here seeing all people try to solve opening their device like this.
However, our toshiba tv also stop to work and i cannot solve this.

Feel free to join us for the repair!

[dick_barton]

Re: No backlight on Toshiba 42HL67US LCD

You have a fuse on one of the boards marked F2. You may have more so you need to check them out with your meter set to ohms and ensure that they are short circuit.
The mosfets can also be checked with the meter set to diode mode and check between the 3 pins (1-2,1-3,2,-3) that you have no short circuit.
All checks carried out with set disconnected.

[dick_barton]

Re: No backlight on Toshiba 42HL67US LCD

I think the 4 pin devices with tabs are dual mosfets and you will need to check that there are no short circuits between any of the pins on each of them as ReeceyBurger123 has said.

[budm]

Re: No backlight on Toshiba 42HL67US LCD

Lots of fuses on that board F1~F6.

[orinoco8]

Re: No backlight on Toshiba 42HL67US LCD

Check the input signal of DIM into power board. If this is active, you got a faulty main board that is telling power board not to turn on back light.

You can force backlight to stay on by connecting INV output from main board into DIM input of power board. Can't see them clearly from the first picture you uploaded. Usually it's pin 15 and 16 of the cable between main board and power board.

[budm]

Re: No backlight on Toshiba 42HL67US LCD

We still do not know if we have BL_ON signal or not either.

[SurrealMustard]

Follow-up

I think the 4 pin devices with tabs are dual mosfets and you will need to check that there are no short circuits between any of the pins on each of them as ReeceyBurger123 has said.

All checked out. No shorts.

Lots of fuses on that board F1~F6.

All that were present checked out.

Check the input signal of DIM into power board. If this is active, you got a faulty main board that is telling power board not to turn on back light.

You can force backlight to stay on by connecting INV output from main board into DIM input of power board. Can't see them clearly from the first picture you uploaded. Usually it's pin 15 and 16 of the cable between main board and power board.

I found 24v_INV, but not a DIM. Is it possible that it is identified as something else on this model?

We still do not know if we have BL_ON signal or not either.

Where would the place to test that be?

[dick_barton]

Re: No backlight on Toshiba 42HL67US LCD

Can you post a clear photo of the legend written alongside these two connectors.

[Alastair E]

Re: No backlight on Toshiba 42HL67US LCD

Are these not the sets with the strange switching-regulators on that brown board that give trouble....?

[SurrealMustard]

progress

Can you post a clear photo of the legend written alongside these two connectors.

Yes. It will appear in the attachments section.

Are these not the sets with the strange switching-regulators on that brown board that give trouble....?

Your guess is as good as mine. Can they be tested with a multimeter?

[dick_barton]

Re: No backlight on Toshiba 42HL67US LCD

So the two connectors do not appear to provide any further useful information. Was looking to see if there was a Dim or something similar printed on the board but not on those two connectors.

Maybe it's on the brown circuit board?

Edit
Do you have all the voltages given on the connectors of the brown board or are any missing.

[budm]

Re: No backlight on Toshiba 42HL67US LCD

You have two inverter boards, both of them are the same? It is not likely that both inverters are bad at the same time.

[dick_barton]

Re: No backlight on Toshiba 42HL67US LCD

Are these not the sets with the strange switching-regulators on that brown board that give trouble....?

SurrealMustard, what regulators are on this board which Alastair has mentioned?

[SurrealMustard]

measurements and details

Maybe it's on the brown circuit board?

Edit
Do you have all the voltages given on the connectors of the brown board or are any missing.

A picture of the connectors is attached to this post as reference. The measured voltages are as follows (going from top to bottom):

11.6
ground
6.4
ground
24
ground

5
ground
3.2
ground
29.1
47.7
20.6
ground

They seem in spec for the most part to me. Can you spot anything off about those values?

You have two inverter boards, both of them are the same? It is not likely that both inverters are bad at the same time.

They are very similar, but one is billed as the "master" and the other the "slave". They are different enough to not have the same model number.

SurrealMustard, what regulators are on this board which Alastair has mentioned?

I'm not entirely sure to be honest.

[budm]

Re: No backlight on Toshiba 42HL67US LCD

Check the DCV on those 4 pads of the Master Inverter board when TV is powered on.
BTW, you are 100% sure that ALL the fuses on the master and slave boards are all good?
Pictures provided by shopjimmy.

[dick_barton]

Re: measurements and details

A picture of the connectors is attached to this post as reference. The measured voltages are as follows (going from top to bottom):

11.6
ground
6.4
ground
24
ground


5...........................5.1V
ground...................GND
3.2........................POW_SIG
ground...................Audio_VCC
29.1......................GND_A
47.7......................40V
20.6......................18V
ground...................GND


They seem in spec for the most part to me. Can you spot anything off about those values?

The readings in red do not match up with the connector legend. Audio_VCC and GND_A may be switched.