Sharp lc-55n620cu no sound

[linksys1127]

Re: Sharp lc-55n620cu no sound

I checked the two small caps for the PVDD pins and they were .1uf. Removed two from a donor board that were the same just slightly larger so easier to install. They were .1uf and 50v rated according to the schematic for the donor board. I installed a new MOSFET N104 and replaced C127 and C128 with two tantalum 106 caps. Checked C127 the good cap and it was reading 10uf. Verified my connections with my meter. Everything looked good. Plugged tv in and powered it up. No sound but it powered up fine. Reading 21.6v on all 4 of the PVDD pins going to the AMP IC just no sound. The short is definitely fixed thank goodness. Just following up on my progress. Guess I have more work to do. Thank you

[linksys1127]

Re: Sharp lc-55n620cu no sound

I do have a cap meter. I’ll check them and replace accordingly. I’ll report back once I get it back together and let you know if it all works. Thanks budm!

[budm]

Re: Sharp lc-55n620cu no sound

Those small cap may be 0.1uf caps. Do you have cap meter to check the capacitance? The Voltage rating should be 35V.
c127/128 (probably 22uF) you may be able to get away with low ESR lytics type cap 47uF 35V rating.

[linksys1127]

Re: Sharp lc-55n620cu no sound

What should I replace the tiny SMD caps for the PVDD pins for the AMP IC. C128 won’t be a problem. It’s larger and easier to replace from a donor board. I’ll remove C127 and verify capacitance and replace it with the same. Would electrolytic be an option? I’m glad it wasn’t the IC too. That would have probably been beyond my abilities if I could have even got a replacement! Thank you

[budm]

Re: Sharp lc-55n620cu no sound

I am glad it is not the IC. Hopefully when you put in the new MOSFET sound will work, then you can remove C127 to check the capacitance which I believe to be the same as C128.
BTW, it is also a big help when you are able to follow the given instructions which makes much easier for me to help.

[linksys1127]

Re: Sharp lc-55n620cu no sound

THANK YOU to budm and budwich for all your help!

[linksys1127]

Re: Sharp lc-55n620cu no sound

So the problem was a burned MOSFET N104 near plug XP1 on the top of the mainboard and a bad SMD cap C128 on the bottom of the mainboard near screw ground S1.

[linksys1127]

Re: Sharp lc-55n620cu no sound

C128 was the culprit. No more shorts to ground after removal!! I took a look at it after I took a picture with a magnifier and noticed a slight defect. Removed it first and now no more shorts! Thank you so much for helping me through this budm!

[linksys1127]

Re: Sharp lc-55n620cu no sound

I do not have a short on MOSFET drain now. I have previously removed C121 and C141 caps output of PVDD pins and still had a short. On the bottom of the board I have two caps C127 and C128 that show short. I'm going to remove those and see if the short goes away. Divide and conquer! I'm including a picture of the area. The dotted square is under the AMP IC. Hopefully that will be the culprit. Thanks for hanging in there budm!

[budm]

Re: Sharp lc-55n620cu no sound

OK, so if you check the resistance on the burnt out MOSFET Drain, the shorts should now be gone.
I have you done this test to verify for sure that the shorts is in the Amp section. I would remove the caps first, removing the IC will be harder because it has exposed pad that is solder the copper plane to use it as heatsink.

[linksys1127]

Re: Sharp lc-55n620cu no sound

Removed L37 and had a dead short on both pads. Removed L108 and only had a dead short on the two L37 and L108 pads on the AMP IC side, closest to the large SMD cap C2. Thank you budm.

[budm]

Re: Sharp lc-55n620cu no sound

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

Remove L37 first and then check both empty pads of L37 to see if shorts goes away, if not then remove L107 and test the empty pads of both L37 and L108 to see if shorts will be present or not.

[linksys1127]

Re: Sharp lc-55n620cu no sound

Those caps for the PVDD pins were so incredibly small. Seriously doubt I have the skills to replace. Can I go back with a electrolytic making sure I have polarity correct? Thank you

[linksys1127]

Re: Sharp lc-55n620cu no sound

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

[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

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

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

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

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

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.