Sharp LC40LE431U No picture or sound

[chillrich]

Hey Guys

First LCD repair maybe you guys dould help shed some light on this for me.

Initial symptom;
Loss of video but audio kept working until. After power cycling there was no picture or sound but indicator LED seemed normal.

Troubleshooting;
Power supply checked good, +5, SB+5, +12, +24. Was able to verify video with flashlight test, and verified no audio as well. The backlight inverter voltage is labelled LED+/LED- and comes down from ~+50VDC to +24VDC, this seems odd to me, they are 2 sets of very light gauge wires going into the top of the display. I checked the cap is in value and the zener measures about a .3 voltage drop in one direction(infinite in other polarity). I desoldered a leg of the AOD446 LCD FET to measure it and it fell off!

Questions;
Could this be the single point of failure?
Are the light gauge wires powering the backlight? There isn't a discrete board for backlight inverter so it must be on the PS right? Can the backlight be tested with a benchtop PS?
One of the vids I watched said "try to remember if there was audio just after you lost your picture", but then didn't explain why this is important. This is exactly what happened in our case, why is it important?

Thanks in advance
Craig

[budm]

Re: Sharp LC40LE431U No picture or sound

How about the pictures of the whole back side of the TV so we can see how all the boards are connected together?

[chillrich]

Re: Sharp LC40LE431U No picture or sound

Here it is

[budm]

Re: Sharp LC40LE431U No picture or sound

There are two pairs of the LED connectors that go to the panel. Take the DCV reading on each pin with ref to GND when the TV is turned on, you will have to leave the read probe on the pin so we can see if the Voltage does jump up and then come back down or not, you will have to do that 4 times on all 4 pins.

[chillrich]

Re: Sharp LC40LE431U No picture or sound

Ok, I will have to wait until the replacement AOD446 gets here.

It looks to me like there is only 2 connections to these 4 wires, Red on right is labelled LED+ on PS board and Black on left is labelled LED- on PS board. The LED+ was the one that started around +50VDC and lowered to +24VDC, I didn't catch where the LED- had for voltage initially but seemed to end up around +3VDC. Does this help?

[budm]

Re: Sharp LC40LE431U No picture or sound

There are two LED strings (2 connectors), the two strings are connected in series to form one big long string. If you look at the bottom side you will see how they are connected in series.
What is the rating of the electrolytic cap laying on it side next to the LED connectors?
http://www.shopjimmy.com/megmeet-mp1...pply-unit.htm#
Sharp MP123-24SP Power Supply for LC-40LE431U / LC-40LE433U.
pictures provided by shopjimmy.

[chillrich]

Re: Sharp LC40LE431U No picture or sound

100uf/35V it measures 94.5uf
The one beside the diode is rated 47uf/100V and measures 45uf

Both measurements out of circuit

[chillrich]

Re: Sharp LC40LE431U No picture or sound

Purchased and installed new power supply board

VDC readings to ground on the 2 connectors from right to left are as follows;
Con 6 red 24.8v
Con 6 blk 1.8v
con 7 red 1.8v
con 7 blk 0v

Looks to be an open circuit in the LED strings, I swapped the the connectors between con 6 & 7, no change. Does this mean both LED strings are faulty? Is the next step to disassemble the display?

[budm]

Re: Sharp LC40LE431U No picture or sound

'The one beside the diode is rated 47uf/100V and measures 45uf' What Voltage do you get between the two legs of this cap?

[budm]

Re: Sharp LC40LE431U No picture or sound

Purchased and installed new power supply board

VDC readings to ground on the 2 connectors from right to left are as follows;
Con 6 red 24.8v
Con 6 blk 1.8v
con 7 red 1.8v
con 7 blk 0v

Looks to be an open circuit in the LED strings, I swapped the the connectors between con 6 & 7, no change. Does this mean both LED strings are faulty? Is the next step to disassemble the display?

24Vdc does not look right, too low for the whole LED strings. White LED needs about 3~3.5V, so 24V will be about 8 LEDs, I am sure this Tv has more than 8 LEDs per strip and the strips are more likely to be connected in series. Are you keep the probe on CON6 red while turning the TV on?

[chillrich]

Re: Sharp LC40LE431U No picture or sound

I opened up the display and found the right side connector to backlight to be faulty. Soldered the wires directly to circuit board and the backlight works now. I will reassemble the set and see what happens. Don't understand why there is no audio though. Is audio in series with backlight?

[budm]

Re: Sharp LC40LE431U No picture or sound

I opened up the display and found the right side connector to backlight to be faulty. Soldered the wires directly to circuit board and the backlight works now. I will reassemble the set and see what happens. Don't understand why there is no audio though. Is audio in series with backlight?

Great! So what Voltage do you get on CON 6 Red wire?
Sound is not related to backlights. Did you check the user menu to see if the internal speakers are turned on.
BTW, how many LED strips are there and how many LEDs per strip?