Sanyo dp26649 LCD tv no backlight

[boardjunkie1]

Got one of these given to my in a "dead" state. Pwr supply checks out
OK, but there is nothing at the BL_ON pin when powered up. What
conditions must be met before the proper voltage appears at the BL_ON
line?

I don't normally work on consumer stuff like this so my theory on LCD
stuff is basically non existant. Display controller issue maybe?

 

[boardjunkie1]

On Dec 26 2012, 11:32 am, boardjunkie1 <boardjunk...@gmail.com> wrote:

Got one of these given to my in a "dead" state. Pwr supply checks out
OK, but there is nothing at the BL_ON pin when powered up. What
conditions must be met before the proper voltage appears at the BL_ON
line?

I don't normally work on consumer stuff like this so my theory on LCD
stuff is basically non existant. Display controller issue maybe?

Just an update on this. I removed all of the inverter transformers and
found one with a bad primary winding that wouldn't ring. I put one of
the good ones back in and still get no BL_ON voltage (I'm assuming
this should be a 5v logic hi). Sounds like the driver circuit for the
failed transformer was damaged and still throws it into error mode.
Sound right?

 

[John-Del]

A bad inverter transformer won't cause a problem until the backlight starts, then causing an error shutdown. Some inverter transformers (particularly the domino type) are easily damaged when removed from the board, so maybe you damaged the xfr when you pulled it. Any chance you're actually getting a very brief backlight on command?

When off, you should be getting a standby voltage of typically 5 or even 3.3 volts from the power supply to the main. When you hit the power button, you should get an "on" signal from the main back to the power supply to turn on the rest of the secondaries (such as 24 for the back light inverter board). At some point, you'll get a backlight on signal for the inverter as well. Depending on manufacturers strategy, different things need to be accomplished by the main before the backlight on command is sent.

You can always jump the on command line to the 5v with a 470 ohm resistor and see what happens. YMMV...

That's how the majority of these things are configured. I guess in a weird situation, a shorted inverter transformer, used in a TV that runs it's 24V from the smps all the time could cause a problem on plug in, but I've never seen one configured that way.


On Wednesday, January 2, 2013 5:51:22 PM UTC-5, boardjunkie1 wrote:

On Dec 26 2012, 11:32 am, boardjunkie1 <boardjunk...@gmail.com> wrote:

Got one of these given to my in a "dead" state. Pwr supply checks out

OK, but there is nothing at the BL_ON pin when powered up. What

conditions must be met before the proper voltage appears at the BL_ON

line?



I don't normally work on consumer stuff like this so my theory on LCD

stuff is basically non existant. Display controller issue maybe?



Just an update on this. I removed all of the inverter transformers and

found one with a bad primary winding that wouldn't ring. I put one of

the good ones back in and still get no BL_ON voltage (I'm assuming

this should be a 5v logic hi). Sounds like the driver circuit for the

failed transformer was damaged and still throws it into error mode.

Sound right?

 

[boardjunkie1]

On Jan 3, 7:31 am, John-Del <ohge...@aol.com> wrote:

A bad inverter transformer won't cause a problem until the backlight starts, then causing an error shutdown.  Some inverter transformers (particularly the domino type) are easily damaged when removed from the board, so maybe you damaged the xfr when you pulled it.  Any chance you're actually getting a very brief backlight on command?

When off, you should be getting a standby voltage of typically 5 or even 3.3 volts from the power supply to the main.  When you hit the power button, you should get an "on" signal from the main back to the power supply to turn on the rest of the secondaries (such as 24 for the back light inverter board).  At some point, you'll get a backlight on signal for the inverter as well.  Depending on manufacturers strategy, different things need to be accomplished by the main before the backlight on command is sent.

You can always jump the on command line to the 5v with a 470 ohm resistor and see what happens.  YMMV...

That's how the majority of these things are configured.  I guess in a weird situation, a shorted inverter transformer, used in a TV that runs it's 24V from the smps all the time could cause a problem on plug in, but I've never seen one configured that way.







On Wednesday, January 2, 2013 5:51:22 PM UTC-5, boardjunkie1 wrote:
On Dec 26 2012, 11:32 am, boardjunkie1 <boardjunk...@gmail.com> wrote:

Got one of these given to my in a "dead" state. Pwr supply checks out

OK, but there is nothing at the BL_ON pin when powered up. What

conditions must be met before the proper voltage appears at the BL_ON

line?

I don't normally work on consumer stuff like this so my theory on LCD

stuff is basically non existant. Display controller issue maybe?

Just an update on this. I removed all of the inverter transformers and

found one with a bad primary winding that wouldn't ring. I put one of

the good ones back in and still get no BL_ON voltage (I'm assuming

this should be a 5v logic hi). Sounds like the driver circuit for the

failed transformer was damaged and still throws it into error mode.

Sound right?

No, there's nothing at the enable line at power up. Since a primary
winding was bad I believe the driver IC for that section is damaged
causing the feedback to disable the backlight. No transformers were
damaged when removed, I checked them all with my Sencore LC77...thats
how I found the bad one. I can verify this by scoping the primary side
drive when forced on.

All other voltages are there. 24v,12v, 5v, and 5v stby.

 

On Thu, 3 Jan 2013 11:01:40 -0800 (PST), boardjunkie1
<boardjunkie1@gmail.com> wrote:

On Jan 3, 7:31 am, John-Del <ohge...@aol.com> wrote:
A bad inverter transformer won't cause a problem until the backlight starts, then causing an error shutdown.  Some inverter transformers (particularly the domino type) are easily damaged when removed from the board, so maybe you damaged the xfr when you pulled it.  Any chance you're actually getting a very brief backlight on command?

When off, you should be getting a standby voltage of typically 5 or even 3.3 volts from the power supply to the main.  When you hit the power button, you should get an "on" signal from the main back to the power supply to turn on the rest of the secondaries (such as 24 for the back light inverter board).  At some point, you'll get a backlight on signal for the inverter as well.  Depending on manufacturers strategy, different things need to be accomplished by the main before the backlight on command is sent.

You can always jump the on command line to the 5v with a 470 ohm resistor and see what happens.  YMMV...

That's how the majority of these things are configured.  I guess in a weird situation, a shorted inverter transformer, used in a TV that runs it's 24V from the smps all the time could cause a problem on plug in, but I've never seen one configured that way.







On Wednesday, January 2, 2013 5:51:22 PM UTC-5, boardjunkie1 wrote:
On Dec 26 2012, 11:32 am, boardjunkie1 <boardjunk...@gmail.com> wrote:

Got one of these given to my in a "dead" state. Pwr supply checks out

OK, but there is nothing at the BL_ON pin when powered up. What

conditions must be met before the proper voltage appears at the BL_ON

line?

I don't normally work on consumer stuff like this so my theory on LCD

stuff is basically non existant. Display controller issue maybe?

Just an update on this. I removed all of the inverter transformers and

found one with a bad primary winding that wouldn't ring. I put one of

the good ones back in and still get no BL_ON voltage (I'm assuming

this should be a 5v logic hi). Sounds like the driver circuit for the

failed transformer was damaged and still throws it into error mode.

Sound right?

No, there's nothing at the enable line at power up. Since a primary
winding was bad I believe the driver IC for that section is damaged
causing the feedback to disable the backlight. No transformers were
damaged when removed, I checked them all with my Sencore LC77...thats
how I found the bad one. I can verify this by scoping the primary side
drive when forced on.

All other voltages are there. 24v,12v, 5v, and 5v stby.
I don't have an encyclopedic knowledge of all LCD TVs, but in my

experience there usually was no feedback from the inverter to the main
video board. The usual sequence goes something like this:

When the TV is plugged in the standby power supply (5v stby) comes up,
which starts the standby controller (often a part of the main control
IC). This may then briefly turn on the main power supply, check for
problems, then turn off the main power supply. The standby controller
then monitors the remote sensor and control buttons for an 'ON'
command. When one is received the main power supply is turned on, a
self test is run and if everything is OK the inverter enable line is
driven high. If the self test fails typically an LED is flashed to
indicate an error.

Typically the inverter is supplied by the manufacturer of the LCD
panel, and several versions may be found on a given model. A quick
search on eBay shows at least three different models of inverters,
including one that was notorious for blowing drivers, and another that
was notorious for failures of the transformers.

An additional point - some models of Sanyo are known to have a 'lock
up' mode. If a failure is detected, they will shut dwon and refuse to
restart until a particular reset sequence is entered via the remote.

One point you did not mention - does the power button on either the
remote or the control panel switch the main power supply off and on?
If it does, one technique I use is to hook a signal source (video
game, DVD player) up to an input, turn the TV on, then use the input
button to cycle through the inputs to see if sound is heard.

PlainBill