Toshiba 49L621U

[jm173]

Re: Toshiba 49L621U

What is weird to me is the voltages I have (+70 v) is different from the baseline voltages that @jostewcrew had mentioned

[jm173]

Re: Toshiba 49L621U

Btw i did try disconnecting the PWM pin 15 and no change.

[jm173]

Re: Toshiba 49L621U

Ya. This thread has been very helpful. Ya i did check that and it was 1.6v which was weird. Someone had mentioned somewhere to short BLON which is 3.5V and the DIM but no help.

[budm]

Re: Toshiba 49L621U

So what Voltage do you have on DIM pin of the power supply board, leave the black probe on the chassis, red probe on DIM pin then turn on the TV while watching the meter, also read post #59 and later.

[jm173]

Re: Toshiba 49L621U

Hello,
I have same problem. My TV does not show any LED backlight. But i know its functioning since the chromecast function is working and I can hear the sound when i cast a video.
Also I checked the voltages.

As I turn on it goes like this

pin 1 vled1- = 35 v to 0.5 v
pin 2 nc
pin 3 vled2- = 35 v to 0.5 v
pin 4 nc
pin 5 vled3- = 35 v to 0.5 v
pin 6 nc
pin 7 nc
pin 8 nc
pin 9 nc
pin 10 nc
pin 11 vled+ = 108 v to 70 v
pin 12 vled+ = 108 v to 70 v
pin 13 vled+ = 108 v to 70 v

Can someone suggest what can i do? The cost of the power supply is a lot now since this is an old model. But I am searching if I can get a good deal.

[jostewcrew]

Re: Toshiba 49L621U

The PWM pin on the power supply side has the pull-up resistor to pull PWM pin solid high when not connected to the main board, which is equal to 100% pulse width which is equal to full backlights level, the switch Transistor on the main board that is connected to the PWM Dim pin is switched on and off with PWM signal from micro controller.
Glad the main board fixed the problem.

great explanation...thank you again

[budm]

Re: Toshiba 49L621U

The PWM pin on the power supply side has the pull-up resistor to pull PWM pin solid high when not connected to the main board, which is equal to 100% pulse width which is equal to full backlights level, the switched Transistor on the main board that is connected to the PWM Dim pin is switched on and off with PWM signal from micro controller.
Glad the main board fixed the problem.

[jostewcrew]

Re: Toshiba 49L621U

so ive read up on pwm and understand quite a bit more...thank you budm and all for the schooling. i still dont know why disconnecting pin 15 (dim solved the issue, other than it bypassed the pwm module...(ic, fet?) dont have the schematic for this to wrap my head around the circuit, but got the main board in based upon your guys' knowledge of the pwm originating from the processor on the board. works like a charm. thanks agian.

[jjanssen7]

Re: Toshiba 49L621U

I've experienced this model to have a common problem with backlight failure. LEDS come on for between 5 seconds and 5 minutes then BL shuts off but image remains on screen. LED strips test OK with LED tester, but fail under normal load. The way to find the one strip that is failing under load is to test each strip one at a time by plugging in a good know strip in the place of the one being tested. Once you find the failed strip, the BL stay on. (This can be a challenging diagnosis because sometimes two strips are drawing too much current and are causing the BL shutdown!)

Only a matter of time until the next strip fails... I suggest starting with a whole new set.

[jostewcrew]

Re: Toshiba 49L621U

ill admit that this is a bit over my head as for now. i will continue to read on this and im sure it will click at my look back at the exapmles given. ty again. last question before i start my own research on all this...... is the pwm hardware driven or software driven? im not sure if im asking this right but is it an operation setting that drives the pwm or is it based on its own creations internal mechanisms?

[budm]

Re: Toshiba 49L621U

See post #75
You need the spec sheet of the panel to know what the LEDs can be driven with.

[jostewcrew]

Re: Toshiba 49L621U

I just search by model number for the spec sheet.

so that number is pulling up led strips for this model. im assuming the types of leds used ant the voltage requirements per bulb/strip/circuit would dictate the variable adjustment by the pwm?

its obvious i need to start reading up. ty for the help and guidance. if it goes out again, ill know where to start. on a side note: im still curious as to why the new main board did not fix the issue being that its on the main board? or is it just controlled there but a part of something else?

[budm]

Re: Toshiba 49L621U

I just search by model number for the spec sheet.
Example of the LCD panel LC500DUE-SFR1 spec sheet.
This one tell you the spec and the requirement for driving the LED.s

[jostewcrew]

Re: Toshiba 49L621U

You have the same LCD panel as the OP, K490WDR1.
I cannot find the spec sheet for this model, the spec sheet will explain what the control signals requirement will be.

no problem....for knowledge...what is the search terms you use to find these and what would it mean to me if i had this information?

[budm]

Re: Toshiba 49L621U

You have the same LCD panel as the OP, K490WDR1.
I cannot find the spec sheet for this model, the spec sheet will explain what the control signals requirement will be.
But at this point you do have reference Voltages of the working condition so just have to make measurement when it stops working.
Also try turning down the backlights level (not brightness) to as low as possible in the USER OSD to see if the TV will stay working longer.

[jostewcrew]

Re: Toshiba 49L621U

Post # 4 show the LCD panel sticker, since there are more than on revision of this TV we do not know what LCD panel is used in your TV.

one sec...going to take a pic it is rev A

[budm]

Re: Toshiba 49L621U

Post # 4 show the LCD panel sticker, since there are more than on revision of this TV we do not know what LCD panel is used in your TV.

[budm]

Re: Toshiba 49L621U

There should be a sticker right on the back of the LCD panel.

[jostewcrew]

Re: Toshiba 49L621U

What is your TV LCD panel model number?

is this the display unit part number itself? if not then the prefixes...(this may be what youre talking about) is: LTM7UCAR

[jostewcrew]

Re: Toshiba 49L621U

pulse width modulator lol....not even close. i guess this is why techs use oscilloscopes? ill research this some more.