Sharp lc-55n620cu no sound

[linksys1127]

Had this fairly new smart tv given to me. Thought I would try and repair it with my limited knowledge. They said the volume just quit working. Plugged tv in and powered up just fine. Great picture! Pulled up the apps. They all seem to work fine. The volume just doesn't work. Pulled the back off the tv and this is what I found.

It fried the mainboard but I was able to remove the remaining pieces and clean the board up and scrape the traces and patch a new mosfet, Q4459, in. The tv wouldn't power on. The standby light flashed once every second which I assume is showing a fault and not allowing the tv to power up. Then noticed, as you probably could guess, the new mosfet was smoking. Removed the new mosfet and the tv again powered up fine just no sound. All of the voltages check good from the power supply board to the mainboard with the exception of 22v instead of 18v from pin VCC_A which leads to the melting down fet. I've searched and can't find a service manual for this tv and no mention of this model searching on this site. Any help would be greatly appreciated.

[linksys1127]

Re: Sharp lc-55n620cu no sound

Here's the data sheet on the fet

[budwich]

Re: Sharp lc-55n620cu no sound

look on the power and see where / if any voltages in the "neighborhood" are labelled / found there. Perhaps they overloaded the audio circuit causes the supply regular to "burp"... or if you get lucky enough, you can find the regulator and what sets the voltage... probably a feed resistor or otherwise that may be bad that is causing the regular to "over voltage" on the output.

IF you look on the power board, you can only see 18v outputs, so it would appear that the regulator / supply on that board is not very good. Most mfrs do not provide power boards schematics... your mission should you accept it is to slowly trace thru the board and find which regulator (follow from the 18v label pin) is of interest. People will likely be able to help you if you can narrow that down... actually there are a few here that will be able to trace the board on-line and likely tell you where to look...

PS. I just saw mission impossible fallout... :-)

[budwich]

Re: Sharp lc-55n620cu no sound

Since you can run the set with the fet removed, you help your tracing by careful metering where the 18v/22v is coming from on the power board.... that should identify the ic quickly. Once you got that number, people may help you with its troubleshooting it.

[linksys1127]

Re: Sharp lc-55n620cu no sound

I do accept the challenge with my limited abilities! Will follow up with my findings. Thank you for your advice!

[budm]

Re: Sharp lc-55n620cu no sound

Since you remove the power MOSFET, then you should check the resistance between circuit GND and Source pad, circuit GND and Drain pad to see which one shows really low resistance.
Those MOSFETs look to be for switched power supplies.

[linksys1127]

Re: Sharp lc-55n620cu no sound

I've marked the areas tested. First I went from the board ground screw to the source side of the fet trace. Showed open or OL. Then I tried board ground screw to drain and showed direct short. I marked the pictures where the drain trace leads to through the back of the board and it appears to go to board ground. I'm sure I'm looking at it wrong. Am I testing it wrong? Thank you folks for your help.

[budwich]

Re: Sharp lc-55n620cu no sound

I think that you are probably testing OK... but maybe I don't understand. You are looking at the "burned up fet circuitry"... right? But from your first post, you indicated that the voltage showing up at that fet was 24V and not 18v. IF that is true, then my "gut" is telling me, your problem is not the "burned up fet" on the main board, it is some kind of issue on the power board that is providing the wrong voltage which is overdriving that FET and burning it up... maybe. You need to be looking at the power board.

[linksys1127]

Re: Sharp lc-55n620cu no sound

I had the mainboard already removed so I checked the traces to ground to provide those findings to Budm. Tomorrow I’m going to install mainboard and power the tv up and try to trace the 22v back and try to find where it’s source is. The mission starts after work tomorrow. Thank you for you and Budm’s assistance.

[budm]

Re: Sharp lc-55n620cu no sound

That 18vdc is not tightly regulated at all due to 12V/18V power supply feedback only sensing the 12V out but not the 18V output, so the output will be higher due to no load and no feedback.
The 18V is more than likely to be the power supply for the Class D power amp IC on the main board, the 18V is fed to the Source pin of that switched POWER MOSFET, Drain is the Voltage output to feed the amplifier. Since you read dead short on the Drain pin so you need to check the resistance between VCC pin of the audio power amplifiers IC N60? (close to the speaker jack, NF is marked on the top of the IC but I cannot read the P/N) pin against GND to see if it shows dead short or not.
http://www.shopjimmy.com/catalogsear...harp+lc-55n620

[linksys1127]

Re: Sharp lc-55n620cu no sound

Took a quick picture before going to work of the amplifier IC and the data sheet page with the pin outs. So I check VCCS to ground for short to test amplify circuit in the chip? Thank you

[budwich]

Re: Sharp lc-55n620cu no sound

maybe... maybe trace via resistance / continuity check from the output of the "removed fet" to see exactly where that output heads on the board... assuming it goes to one of the "v lines" in the chip. To me "VCCS" is actually "VCC5" when I read your picture... which likely means 5v VCC input. Hopefully that is coming from the fet in question but maybe not. The VCC5 should still show a relatively good impedance measurement to ground but you can confirm that either way. I would think the 18v supply is not driving the VCC5 directly. Perhaps one of the pins maybe.

However, without a pin description, its tough (unless one has prior experience with the ic / design in question)... hence the suggested trace to see where the output shows up going.

[budm]

Re: Sharp lc-55n620cu no sound

Lets see if the MOSFETs in the output stages are shorted or not by first remove the speaker cable connector and then check the resistance of each speaker connector's pin 9you have 4 pins) on the board against GND to see if any one of them show low resistance or not.

[linksys1127]

Re: Sharp lc-55n620cu no sound

Thank you for the clarification. It is VCC5. That's what I get for trying to read that first thing in the morning. I'll check from the fet trace and try to identify on what pin on the amplifier chip it's supplying voltage. I'll also check the pins to ground for the speaker connections to see if they show a short or low ohm reading. Thank you folks for all your help. Will report back soon.

[budm]

Re: Sharp lc-55n620cu no sound

The VCC5 power supply pin is for running the logic section of the IC.
There are many power pins for that IC, I.E xxVDDxx.

[linksys1127]

Re: Sharp lc-55n620cu no sound

Didn't have much time to look at this but here's what I found. Found no direct connection with my multimeter from drain trace of the burned FET to any pin on the AMP IC. Took readings from all four pin outs to the left and right speakers to ground. All read around 4.7k ohms. Then I took readings from all pin outs on the AMP IC to ground. I included a picture of what I found. PVDD1A, PVDD1B, PVDD2A and PVDD2B all read around .4ohms. Does this mean that the AMP IC has failed internally? Thank you for all your time and advice!

[budm]

Re: Sharp lc-55n620cu no sound

Since the Drain pin is basically shorted to GND then it will hard to trace using meter to see if it has direct connection to the IC.
PVDDx's are more likely to be the Drain pin of the upper POWER MOSFETs of the output stages where the 18V is fed to. PVDD is the common name pin for the power supply pin used in the Class D amplifier.

Look for filter caps connected to those PVDD pin, hopefully it will be shorted out MLC Cap type and not the IC.
Look at the attached PDF you can see what I am talking about with regard to the PVDD pin and the MOSFETs.
If I were you I would start removing the filter caps connected to PVDD pin one at a time to see if the shorts will go away or not. Removing the IC will be harder.

[linksys1127]

Re: Sharp lc-55n620cu no sound

Well I removed both MLC Caps on the output of the PVDD pins. Still have a direct short on the PVDD pins to ground. Guess I've got a bad AMP IC. Looks like either I've got to source a new mainboard or try to find the AMP IC and invest in a hot air station. Thanks to budm and budwich for all your help and advice!

[budm]

Re: Sharp lc-55n620cu no sound

https://www.badcaps.net/forum/attach...5&d=1533818916

Can you check the resistance between GND and either end of L37, L108 which is right next to C2?

[linksys1127]

Re: Sharp lc-55n620cu no sound

Both sides of L37 and L108 show a short to ground. What next? Thanks budm!