Samsung UN55HU8550F - No picture, No sound

[KYBOSH]

Re: Samsung UN55HU8550F - No picture, No sound

I went in and started looking at the power supply again. Measured voltgaes and tbey were all in spec and steady. I measured them with the mainboard removed. When i unplugged the mains and proceeded to plug the harness from the mainbkard i heard the clicking of the relay again where tbere was no clicking while i was testing it in isolation. Something has gots to be up with tbis mainboard.

It also occured to me that, while the backlights i installed seem to be much better than the ones i had in there before, i should have tested them with a backlight tester before install. They came from a donor aet with a cracked dispaly. This was confirmed by a photo of tbe actual TV powered on, displaying and image witb a cracked screen. Even though it worked in the donor TV it may have been on the edge of functionality and could be causing the same error as my old one. Just speculation but i will definitely test them when my BL tester arrives.

I also took a few minutes to apply hot air to the two BGAs there in case some loose solder joints maybe the cause of the issue but no joy.

[nomoresonys]

Re: Samsung UN55HU8550F - No picture, No sound

ok, so with mainboard hooked up do you still have the proper steady voltages on the powerboard connector?

[KYBOSH]

Re: Samsung UN55HU8550F - No picture, No sound

ok, so with mainboard hooked up do you still have the proper steady voltages on the powerboard connector?

Well I went in today looking to take voltage measurements of the the power supply. I measured all the outputs with the mainboard disconnected and they measured true and steady according to the pinout. I then plugged the mainboard back in and took measurements at the mainboard and those always measured true and the voltages were steady as well.

After testing I noticed the backlight were on (not cycling On/Off) SO I lifted to TV to see the TV resting at the Home Menu screen. But there was no response to the joystick control. I restarted the TV by disconnecting the mains and the TV boot looped 3 times then came to the Home Menu again. Again there was no response to the joystick control but there was a response to the remote control for a slight moment. The pointer/cursor moved for a few seconds then froze.

I laid the TV back down and started touching components on the mainboard. Perhaps my fiddling with the voltage readings did something?
When I got to the Panasonic HDMI chips I noticed that one of them was pretty hot to the touch. I took readings with my IR temp sensor and it was pretty hot. 10°C hotter than the twin next to it. There was 1 HDMI port that was hot as well. I tested the HDMI port and surrounding SMDs for shorts. I used a HDMI Plug Breakout Board connector to test individual pins. Everything was shorted on HDMI 1. I didn't see any bent pins or connectors so I decided to remove the HDMI port and the PMIC underneath it (on the other side of the board) in case that was the offending troublemaker. After a hasty removal (I got excited and tore off some pads) I noticed that the shorts were still there. This could only mean that the short was IN the HDMI chip.

I removed the Panasonic IC and the short was gone.

Unfortunately the TV would no power on, do a bootloop or show the Samsung logo without this chip so I decided to use the chip from the spare board that came with the TV. My micro soldering skills are not the best and I definitely need practice but I was able to get the chip on. When I plugged everything up I was greeted to backlights but a blank screen. I took a look at the chip and noticed it was VERY hot! 50°C. I looked for any solder bridges that I might have inadvertently created but didn't see any. Then it occurred to me that THAT chip maybe shorted too. So I pulled the 2nd chip off the spare board and installed it. This time the TV booted up quickly but had several vertical lines on it before it power cycled down. So while I blame my soldering skills for it not powering on properly I noticed the boot up was much quicker and the ICs were nice and cool to the touch. I wasn't able to get the TV to boot back up but I think I am on to something. This could be the reason for the bootloop but it wouldn't explain why the TV doesn't respond to commands or only responds for a few seconds before it freezes.

[KYBOSH]

Re: Samsung UN55HU8550F - No picture, No sound

Just to tidy this up... I was not able to find another working board for this TV. I came across several from TVPartstoday but they all had one unacceptable fault or another. Like not responding to IR commands and things like that. Pretty rare TV to find in the wild and even rarer to find a good mainboard. I eventually gave it to someone to give it a try as it was sitting on my to-do list for about a year.