Samsung UE40C8790 - defect STD452S MOS-FET in Power Supply

I once had sworn not to test my luck on Samsungs anymore because you have to navigate the boards without proper service manuals. However, this set looked very interesting and its price was ok, too. A defect C-Series does not appear on eBay often. It seems to be generally reliable. On the other hand, there is a D-Series model listed almost every week and they usually have panel failures. Those are really bad. Don't buy a Samsung D!

This TV showed some reaction with its stand-by light, but wouldn't boot. It went click-click-click...

I have to hand it to Samsung, their higher models are looking pretty good:

It also has a fancier remote than the cheaper models.

Considering its young age - about 4y - I did not expect to find any dead capacitors. The seller said that it had intermittent problems and one day it wouldn't start anymore.

In this model Samsung went to extremes to design the device as flat as possible. It is not much thicker than my thumb! The chips on the mainboard use the back cover as cooler. Instead of the normal, large capacitors, they planted nests of tiny ones in parallel. That's not bad in terms of reliability, my only hope is that nobody will have to replace those. Not so easy to find.

The downside is that all connectors besides HDMI and USB need adapters. Even for the aerial antenna. That particular adapter was missing. The original Samsung part is ridiculously expensive. Some shops charge 35€!

Stereo? Who cares! Both speakers share the same cabinet:

A quick measuring of the voltages revealed a dead 12V line. It was dead short and a FET was the culprit. It's the little one in the center next to the coil:

A little weird how they botched the thing onto the board, right across two jumper wires.

An older version of the power board's schematic shows a different 8-pin type.

The solder joints looked terrible. The thing was already wobbling:

So here we have the reason for the intermittent failures. Cold solder joints. They cause instability and probably sparking, which eventually killed the FET. It is a very exotic part, a SamHop STD452S. The data sheet calls it STD4525. Anyway, there was none available on eBay for a reasonable price. I found a Chinese specialist called transistormosfet.com. 3$ a piece and another 8$ shipping. A fair offer. I ordered six pieces. This worked well. After 15 days they arrived as registered mail. I'll keep this shop in mind.

In a forum, people reported the same symptoms. This seems to be a standard failure.

So, what can be done to make the thermal situation of the FET more stable? It is obviously running too hot. First I used leaded solder, which is more elastic than lead-free. Then I decided to use a heat conducting block to attach the FET to the back cover.

First, two little splashes of nail lacquer showed me if and where it is making contact:

I carved out the plastic and did another lacquer check:

So that was looking good. I had the device running without cover and the FET surprisingly didn't run very hot. Anyway, this should prolong its life.

Frau Johansson was very happy with the results:

This TV has a very good picture. It is evenly lit, sharp, with brilliant and bright colors and good contrast. Like all the LED TVs a bit too aggressive in the reds and generally a little artificial for my taste - but I am a plasma man anyhow.

The sound is not bad either. A little inhomogenous, but powerful thanks to the closed resonance cabinet and both speakers working together.

So here we go, one part replaced this time.