No backlight on Sharp LC-32LB261U

[lkvee]

Red LED blinks 8 times before turning steady blue, but I see the "No Signal" image without backlighting.

At CN9101, I get these readings on steady red LED (standby):

NC gives 0 volts
+12V gives 12.59V
+12V gives 12.59V
+12VAU gives 12.47V
GND gives 0V
GND gives 0V
NC gives 0.057V
DIM ON/OFF gives 0V

At CN9101, I get these readings on steady blue LED (ON):

NC gives 0V
+12V gives 12.58V
+12V gives 12.58V
+12VAU gives 12.33V
GND gives 0V
GND gives 0V
NC gives 3.35V
DIM ON/OFF gives 2.964V

At CN8101, I get these readings during steady blue LED (ON):

VLED+ gives 12.38V
VLED+ gives 12.38V
N/A gives 0V
N/A gives 0V
LED1 gives 0V
LED1 gives 0V

Three filter caps spec'ed for 56uF and 450V give these readings:

C9103, C9102, and C9101 give 0V when power is disconnected
C9103, C9102, and C9101 give 166V when LED is steady red (standby)
C9103, C9102, and C9101 give 166V when LED is steady blue (ON)

Thinking LED1 and main cap readings are off, but not sure what to examine and/or replace next. Gratuitous picture T-Con board included.

[cliff_hootarski]

Re: No backlight on Sharp LC-32LB261U

Connection CN9101 is a 16 pin connector. We need to know if the main board is telling the power supply to come ON. What is the voltage reading of pin 2 ON/OFF?

[dskall]

Re: No backlight on Sharp LC-32LB261U

What are you getting when first powering on at VLED

[budm]

Re: No backlight on Sharp LC-32LB261U

"DIM ON/OFF gives 2.964V" Do you mean that the Dim pin = 2.964v, and also the ON/OFF (Bl-on) pin is also 2.964V? Those are two separate pins.

[lkvee]

Re: No backlight on Sharp LC-32LB261U

Can't find the button/link to edit my original post, but the original post was for Pins 15 to 1 in descending order.

For Pins 16 to 2, I got these readings during standby (steady red LED):

NC 0.23V
+12V 12.6V
+12V 12.6V
+12VAU 12.48V
GND 0V
GND 0V
NC 0.16V
DIM ON/OFF 0.14V

For Pins 16 to 2, I got these readings during ON state (steady blue LED):

NC 0.23V
+12V 12.59V
+12V 12.59V
+12VAU 12.34V
GND 0V
GND 0V
NC 0.16V
DIM ON/OFF 2.964V

I put a probe on Pin 2 and then plugged in the TV. Voltage went to 2.964V and descended to near zero until the 4th (out of 8) red LED blink and then went back to 2.964V.

Replicated on Pin 1.

Have yet to test VLED but want to do so and measure from the minute I plug in the TV also.

[budm]

Re: No backlight on Sharp LC-32LB261U

So you are saying Pin 1 (DIM) has exact same Voltage reading as Pin 2 (ON/OFF)?
Also do this: Put black probe on the chassis and keep red probe on the VLED+ then turn on the TV to see how high the DCV goes up to and wait for seconds to see what it goes down to, it is going to happen fast so keep your eyes on the meter.

[lkvee]

Re: No backlight on Sharp LC-32LB261U

Black lead was on the metal chassis. Red one was on one of the two VLED+ pins. I plugged in the TV and observed voltage.

Immediately went to 12.39V and then went to "OL" (around the fourth out of eight red LED blinks), then to a range of 36-37 volts and back to 12.39V

I'm using a Fluke meter with autorange.

Not sure if what I saw was a consistent "surge".

Both VLED+ pins behaved this way.

[budm]

Re: No backlight on Sharp LC-32LB261U

Since the meter went into OL (Over range) mode, you should set it to 200V manual mode.
It sounds to me that you have open circuit int he LED strings.

[lkvee]

Re: No backlight on Sharp LC-32LB261U

TV changed behavior. LED cycles with two RED blinks, but fails to turn blue. Using the remote fails to turn it on, too. Eventually, that blinking red LED goes steady but I still can't turn it on.

I do remember being less than careful enough with the needle probe on CN9101. I saw a blue spark on three occasions.

Here's what I found for CN9101 on the outer pins:

NC 0V
+12V cycles between 12.5V and 0V
+12V cycles between 12.5V and 0V
+12VAU steady at 12.4V
GND 0V
GND 0V
NC cycles between 3.3V and 0.5V
DIM ON/OFF cycles between 2.9V and 0V

Readings are also similar for the inner pins but I failed to get anything other than 0V on Pin 2 (DIM ON/OFF).

Forgot to measure those capacitors.

[lkvee]

Re: No backlight on Sharp LC-32LB261U

Just acquired the service manual from Elektro Tanya. It's the same manual for the next two larger size TV's, too.

There's a decision flowchart, and one of the decision diamonds ask whether I am getting 12V AND 19.5V from the power board. If not, replace it.

I see the 19.5V at C9100 but nowhere else. I can say with confidence I never got 19.5V on any pin during any state.

My schematic literacy is barely >0 but I can't read which pin's supposed to generate 19.5V