Toshiba 55HT1U LCD No backlight in section. Has sound and image.

[Innovator]

Re: Toshiba 55HT1U LCD No backlight in section. Has sound and image.

Is the video pretty much worthless? It was very hard to capture any light from the screen, its just a black image. Also, the start attempts (flashing) are much quicker with the two top master/slave lamp connectors plugged in, so fast that I couldn't pick up some of the flashes at 30fps. With the two top M/S lamps connectors unplugged, the flash duration is a few 10ths of a second, much longer.

Does anyone even have a wild guess at what MIGHT be going on? More pictures needed?

Thanks.

[dick_barton]

Re: Toshiba 55HT1U LCD No backlight in section. Has sound and image.

Can you post sharp photo's of the power supply to see if there is a way of bringing up the backlights as you can on other sets without the main board connected. Is there a model number on the power supply?

[Innovator]

Re: Toshiba 55HT1U LCD No backlight in section. Has sound and image.

Here are pictures of the power board. I confirmed 24V output, but not much more than that.

[dick_barton]

Re: Toshiba 55HT1U LCD No backlight in section. Has sound and image.

Can you list the voltages no matter what they are alongside P802 & P803
eg
P803
Pin 1 GND = 0V
Pin 2 GND = 0V
Pin 3 24Vaud =
etc

Photo Courtesy Shopjimmy

[Innovator]

Re: Toshiba 55HT1U LCD No backlight in section. Has sound and image.

Also, per the attached below, during attempted starts I am getting ~150VAC across red arrows and ~120VAC across purple arrows. I am assuming each of these white connectors (white and tan cables) attaches to lamp pairs if they span the entire length of the screen?

[budm]

Re: Toshiba 55HT1U LCD No backlight in section. Has sound and image.

Ohh, do not try to measure that high frequency high Voltage outputs, the start up Voltage can be as high as 2000V and running Voltage around 1000V or more, you will damage your meter.

[dick_barton]

Re: Toshiba 55HT1U LCD No backlight in section. Has sound and image.

I'm out of my depth here but if the backlights are tubes(CCFL) then I would be expecting a much higher voltage to bring them on.
What also confuses me is that in some spec's on this set I have just looked at it said the set had automatic backlight adjust for better black levels dependant on the image so this could be another area that could cause problems if not working properly.

[Innovator]

Re: Toshiba 55HT1U LCD No backlight in section. Has sound and image.

Can you list the voltages no matter what they are alongside P802 & P803
eg
P803
Pin 1 GND = 0V
Pin 2 GND = 0V
Pin 3 24Vaud =
etc

Photo Courtesy Shopjimmy

I am getting all of the marked voltages at every pin reference to GND. 24V, 12V, 5V.

[budm]

Re: Toshiba 55HT1U LCD No backlight in section. Has sound and image.

The Inverter transformers has 3 pins on the bottom row and one pin on the top row which goes to the lamp socket, Please check the resistance between the top pin and the center pin of each transformers to see if they all within 3% from each other, and also please inspect all the male pins of the lamp socket. You did already inspect the jumper connector between the top and bottom inverter board real well to make there is not sign of heat, or bad connections, right?

[Innovator]

Re: Toshiba 55HT1U LCD No backlight in section. Has sound and image.

Ohh, do not try to measure that high frequency high Voltage outputs, the start up Voltage can be as high as 2000V and running Voltage around 1000V or more, you will damage your meter.

Too late, but thanks for the warning. Meter (Fluke) is rated for 600V and I thought that was the highest voltage for these lamps, did not realize start voltage could reach that high. Whatever the case I did about 20-30 reads and the voltages mentioned above are what I got consistently red to red, purple to purple, arrows that is.

Why am I not seeing higher start voltages? Seems that might explain why the screen wont light!

[dick_barton]

Re: Toshiba 55HT1U LCD No backlight in section. Has sound and image.

How about Dim, Dim_S & INV ?

[Innovator]

Re: Toshiba 55HT1U LCD No backlight in section. Has sound and image.

My meter is only able to resolve decaohms, they all read 0.2ohm. I have high precision meters at work that work well in low range. Do you think its worth bringing them in and confirming the 3% tolerance?

[Innovator]

Re: Toshiba 55HT1U LCD No backlight in section. Has sound and image.

How about Dim, Dim_S & INV ?

Dim: 3.3
Dim_S: 5
INV: 4.5

[budm]

Re: Toshiba 55HT1U LCD No backlight in section. Has sound and image.

My meter is only able to resolve decaohms, they all read 0.2ohm. I have high precision meters at work that work well in low range. Do you think its worth bringing them in and confirming the 3% tolerance?

We just want to eliminate the possibility of bad transformers so we will be chasing tail, I expect the reading to be 500~2K Ohms range so you need lower Ohm scale.

[Innovator]

Re: Toshiba 55HT1U LCD No backlight in section. Has sound and image.

We just want to eliminate the possibility of bad transformers so we will be chasing tail, I expect the reading to be 500~2K Ohms range so you need lower Ohm scale.

I misread your other post on which pins to test.

1 transformer read 12kohm!!! The rest were within 1% of each other @ 625ohm.

Anything I should know or do before replacing it?

[budm]

Re: Toshiba 55HT1U LCD No backlight in section. Has sound and image.

So of ALL 36 transformers (for 18 lamps) only one has high resistance? I would order more than one since one went out (can be due to worn out lamp which will draw lots of current but not enough to trip the protection circuits).
So get the transformer and then go from there.

[Innovator]

Re: Toshiba 55HT1U LCD No backlight in section. Has sound and image.

So of ALL 36 transformers (for 18 lamps) only one has high resistance? I would order more than one since one went out (can be due to worn out lamp which will draw lots of current but not enough to trip the protection circuits).
So get the transformer and then go from there.

Bud, that is correct, just 1. Transformers were $6/ea. from China so instead I just bought a set of the inverter boards (4) for $18 on AliExpress! Granted its on the slow boat here, but I should have spare parts to last a while!

[budm]

Re: Toshiba 55HT1U LCD No backlight in section. Has sound and image.

When you get the new boards, I would check the resistance of those transformers to make sure you did not get bad boards.