No backlight on Toshiba 42HL67US LCD

[dick_barton]

Re: measurements and details

I'm not entirely sure to be honest.

Can you read the IC number from the board and then read off what is written on the device. EG IC101 = AM117S etc.

[SurrealMustard]

voltages for the test points on the inverter board

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.

From top to bottom, the voltages are 4.6, 3.4, and 4.3. The one pin on the left appears to be ground.

As for the fuses, I wasn't able to find 6 of them on each board (only 4 if I remember correctly), but all the ones that were there tested good.

[budm]

Re: No backlight on Toshiba 42HL67US LCD

Those 4 pads are the signal receiving pins, they should be BL-ON, DIM, and may be Error pin.

You only found 4 fuses, what are the designators? F1, F2, F3, F4?

[SurrealMustard]

found the culprit!

Those 4 pads are the signal receiving pins, they should be BL-ON, DIM, and may be Error pin.

You only found 4 fuses, what are the designators? F1, F2, F3, F4?

Actually, after going back and taking a closer look, I was able to find all six on both boards. Lo and behold, the last one I tested (F1 on the master board) is indeed wide open.

[budm]

Re: No backlight on Toshiba 42HL67US LCD

That fuse F1 with the 'D' marking on the top is the fuse for running the PWM Controller IC.
You can try 0.50A fuse with leads in its place for testing, you do not have to remove the old one, just solder the wire to each end.
'D' it is supposed to be 0.315A fuse.
DARFON inverter boards suffer from common failure of the small black fuse for the PWM Controller circuit.

[SurrealMustard]

fuze!

Can you read the IC number from the board and then read off what is written on the device. EG IC101 = AM117S etc.

The 9V regulator is a Kia 78009A (PI739) at QE90
The 24v audio is a D2396 AT Q860
The 5v2 is a Sharp 70XP01 at QE52
The 3.3vd has two ICs by it - a B001FFE at QE73 and an FMBG14 at DE70.

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

You are right. Upon closer inspection, it would appear as though I skipped a ground between the 29 and 47 volts, so the order should be as follows:

5
ground
3.2
ground
29.1
ground
47.7
20.6
ground

That fuse F1 with the 'D' marking on the top is the fuse for running the PWM Controller IC.
You can try 0.50A fuse with leads in its place for testing, you do not have to remove the old one, just solder the wire to each end.
'D' it is supposed to be 0.315A fuse.
DARFON inverter boards suffer from common failure of the small black fuse for the PWM Controller circuit.

So you're thinking it's possible that just the fuse has gone bad and not any other component drawing too much on that fuse? If so, would I be best off finding a half-amp surface mount fuse and installing that then?

[budm]

Re: No backlight on Toshiba 42HL67US LCD

You can use any 1/2A fuse for now for testing, more likely just bad fuse like other failed DARFON inverter board.
You can also check the resistance on the output side of the fuse to see if it shows very low resistance with ref to GND.

[SurrealMustard]

diagnosis is looking good!

You can use any 1/2A fuse for now for testing, more likely just bad fuse like other failed DARFON inverter board.
You can also check the resistance on the output side of the fuse to see if it shows very low resistance with ref to GND.

I don't have any half-amp fuzes hanging around, but upon testing resistance to ground using the multimeter, I did have to set it up to a fairly high setting to get it to show anything at all, so it sounds like we're looking at just a bad fuse then. When purchasing one, do I need to find the exact value or should a half-amp surface mounter work fine?

[dick_barton]

Re: No backlight on Toshiba 42HL67US LCD

I believe that both Digi-Key and Mouser sell suitable fuses.

[budm]

Re: No backlight on Toshiba 42HL67US LCD

1/2A will work find, just have to take the Length and width measurement to get the correct size.

[SurrealMustard]

fuze size

I believe that both Digi-Key and Mouser sell suitable fuses.

1/2A will work find, just have to take the Length and width measurement to get the correct size.

I measured the fuse and it appears to be somewhere between 1/8 and 1/6 of an inch long. According to the PDF that was provided, the case size rating of the fuze is 0402, but when looking up those types of fuzes on Mouser, their 0402 fuze is only 1.05mm (.04 inch), which sounds much too small. Is it possible that I measured it wrong? If not, how do I find the correct size rating?

[budm]

Re: No backlight on Toshiba 42HL67US LCD

Are you using digital caliber to measure the part?
https://en.wikipedia.org/wiki/Surface-mount_technology

[SurrealMustard]

the right tool for the job

Are you using digital caliber to measure the part?
https://en.wikipedia.org/wiki/Surface-mount_technology

Tape measure.

[SurrealMustard]

Fixed!

This was indeed the problem. Inverter boards are so inexpensive that I just opted to get a used one and will either sell the broken board or fix it and then sell it.

For now, the replacement inverter solved the problem and the Toshiba Regza lives on.