Sharp LC-40LE550U no backlight

[Markpixley]

Re: Sharp LC-40LE550U no backlight

ok from what I can tell...the positive lead of the cap goes to that string of resistors...

Starts with "3323" (332KoHms)>>>3323>>>3323>>>1204 (1.2MoHms)>>>3903 (390k)...

then it goes to a 1002 (10k) and splits into a trace that looks simply like a test point (no connection to anything) and the other side to a smd cap that measures .756nF>>>then into the far right leg of the UL1032 (looks to be an AS431)...which if I am correct is the cathode leg of a shunt regulator...

The middle leg of said regulator (anode) goes through the "0" jumpers all the way back up to pin #1 which is the FB of the Ricktek chip...

(AS431 https://cdn.badcaps-static.com/pdfs/...13bcd4cdd8.pdf)

[budm]

Re: Sharp LC-40LE550U no backlight

OK, if you look at the spec sheet of the 431, you will see APPS CIRCUIT that there are two resistors on the REF pin 1 (Left pin is 1 looking at it with the Flat side facing you and the Legs pointing downward, one will be Ref to GND, and another should go that 5 332K resistor string.
You can also check the DCV between th negative leg of the filter cap C115 and the ref pin 1 to see if it shows 2.5VDC.

[Markpixley]

Re: Sharp LC-40LE550U no backlight

Ok upon closer inspection I am wrong...the last part of that resistor chain does not go "end to end"...the 332k ties into the top end of the 1204 (1.2Mohm) but then the trace swings around it and connects to the bottom end of the 390k which goes on to the top of itself to the 10k>>>to the cap .756nf...

A continuity test between the bottom end of the 1.2M and the top of the 390k indicates they ARE connected...so they are connected in parallel...and their end point goes on to the 10k...

[Markpixley]

Re: Sharp LC-40LE550U no backlight

By the way thanks for your input...I am learning...slow to process this stuff though...I appreciate your advice.

[budm]

Re: Sharp LC-40LE550U no backlight

I just look through my gallery of my repair works and found the LC-40LE550U with the DCV reading of 348VDC for the cap that I repaired in Feb due to bad connector on the LED strip, you are getting 340V so it is really close to what you have, so can you verify for sure that you are getting 340V.
https://www.badcaps.net/forum/showthread.php?t=57110

[Markpixley]

Re: Sharp LC-40LE550U no backlight

OK, if you look at the spec sheet of the 431, you will see APPS CIRCUIT that there are two resistors on the REF pin 1 (Left pin is 1 looking at it with the Flat side facing you and the Legs pointing downward, one will be Ref to GND, and another should go that 5 332K resistor string.
You can also check the DCV between th negative leg of the filter cap C115 and the ref pin 1 to see if it shows 2.5VDC.

Testing the negative of C115 and pin 1 I get 2.45...

[Markpixley]

Re: Sharp LC-40LE550U no backlight

I just look through my gallery of my repair works and found the LC-40LE550U with the DCV reading of 348VDC for the cap that I repaired in Feb due to bad connector on the LED strip, you are getting 340V so it is really close to what you have, so can you verify for sure that you are getting 340V.
https://www.badcaps.net/forum/showthread.php?t=57110

Yes 340.8...I had the same issue with the LED solder strip but I'm fairly certain it is fixed....just for testing last night I shorted pin 13 to ground popped a USB stick in with a movie and watched a little to see what it would do...the back light seems on full with no option to change it (remote changes nothing)...my point is the screen looks to be full back light so I'm fairly sure the connection issue with the light strings is gone...but what do I know?

[Markpixley]

Re: Sharp LC-40LE550U no backlight

Testing the negative of C115 and pin 1 I get 2.45...

I'm wrong if you are looking down on transistor the way the data sheet shows the cathode would be pin 3...which is where I get 2.45v

Pin 1 I get 4.34v (if pin is grd ref)

[Markpixley]

Re: Sharp LC-40LE550U no backlight

So wait...help me understand...if I have 340 volts does that mean its 8 volts to low and I have an open in the LED string still?

By shorting to ground am I simply by-passing the open short somewhere?

[Markpixley]

Re: Sharp LC-40LE550U no backlight

I just look through my gallery of my repair works and found the LC-40LE550U with the DCV reading of 348VDC for the cap that I repaired in Feb due to bad connector on the LED strip, you are getting 340V so it is really close to what you have, so can you verify for sure that you are getting 340V.
https://www.badcaps.net/forum/showthread.php?t=57110

This looks identical to my TV (which I picked up for $25.00 on CL) but mine is a Sharp...rebranded ...China will get our money one way or another...

[budm]

Re: Sharp LC-40LE550U no backlight

So wait...help me understand...if I have 340 volts does that mean its 8 volts to low and I have an open in the LED string still?

By shorting to ground am I simply by-passing the open short somewhere?

You are able to get the backlights to come on when you grounded out the LED return pin on the LED controller IC so it is not the LED string problem at this point, the one I worked out has no backlights due to bad connector soldering problem.
It is only 8V difference, so I am not sure if that is significant or not.
BTW, when you get the backlights to come on, were they bright enough to see good clear pictures on the screen? You said it is bright, super bright?
By the way that 348V reading is open circuit, no LED load due to bad connector, I did not take the reading after the connection is fixed, now I wish I did take the reading.

[Markpixley]

Re: Sharp LC-40LE550U no backlight

You are able to get the backlights to come on when you grounded out the LED return pin on the LED controller IC so it is not the LED string problem at this point, the one I worked out has no backlights due to bad connector soldering problem.
It is only 8V difference, so I am not sure if that is significant or not.
BTW, when you get the backlights to come on, were they bright enough to see good clear pictures on the screen? You said it is bright, super bright?
By the way that 348V reading is open circuit, no LED load due to bad connector, I did not take the reading after the connection is fixed, now I wish I did take the reading.

Yes very bright and distinct picture...something a little weird though (could be related to a mad mojo connection I was using to short pin 13) was any time the volume went up loud the screen would dim...mind you the back is off and things are not all connected solidly, but it might be related to a power draw...

Other than that the screen looked really good but changing the backlight in the menu did nothing...+/- had no affect...

[budm]

Re: Sharp LC-40LE550U no backlight

I am beginning to think that there is problem with the IC.
You no longer have the control due to the IC is bypassed through Ground instead of through the Internal MOSFET, I think the internal MOSFET is not being turned on.

[Markpixley]

Re: Sharp LC-40LE550U no backlight

I am beginning to think that there is problem with the IC.
You no longer have the control due to the IC is bypassed through Ground instead of through the Internal MOSFET, I think the internal MOSFET is not being turned on.

That seems to be the one common place all the traces in question land at...I will see if I can source one and replace it...I think Digikey handles richtek stuff...