Fixing a Samsung LE40B652T4W power board (BN44-00264A)

[tobi1]

Hi all,

I've been procrastinating about this project for a while now but I've finally got round to trying to fix it. I'm not suceeding so any help would be greatly appreciated (Thank you!). I'm armed with a soldering iron and a multimeter.

I've done the following so far:

• Replaced the fuses
• Replaced the STRW6252 chip

Tried powering it up and blew the 5 amp fuse in the kettle lead. Tried again with a 13 amp fuse but it didn't work (the red standby light on the front didn't come on).

I'm thinking a 5 amp should work though (P=VI, P=200W, V=220-240V) so I don't know what the issue there is.

I've attached photos. I've shown my soldering job of the new STRW6525. It's an okay job i.e. I've not got a short circuit there.

A few legs on the board look like they may have a but of 'rust' near the solders. Could this suggest anything? Also, none of the capacitors visibly show a bubbling up of their tops so I'm presuming none have blown (amateur fixer/upper here so I have no idea if this is true).

BN44-00264A Board


My soldering job


Potential rust marks? DP805


Back of TV setup


Back of power board

[downunder]

Suspect you have a short in the primary of the power supply. The fuses you replaced, were they blown? Inspect those small blue ceramic
capacitors. They have a habit of burning out and shorting, and check the bridge rectifier for shorts (at the top of the longer heatsink).

Discharge the two large capacitors that are mounted on their sides and measure between +ve and -ve of one of them with the meter on ohms.
You should get a diode-type reading, low one way and quite high with the meter leads reversed. Low both ways indicates a short in the primary side. 99% chance it won't be those two capacitors though.                      Bruce

[bonesy]

First thing to always check with these boards,if fuse on input has blown,is the bridge rectifier.Quite often it starts off with the solder cracking around the legs,but at some stage they go short circuit.

[zomikula]

Also check the diodes on that model
there were some isues with them.

[tobi1]

Thank you all!
I'll be checking tonight all the suggestions and I'll let you know how I get on.
Zomikula, do you know which diodes? If so may you describe them to me please as there's no schematic for the circuit board.

 

[zomikula]

Just the one on the psu
check for any leaky ones.

 MUR360  was defo an issue on the 37"
may have carried on to the 40" as well

[tobi1]

Hey all,

well it was a good attempt.

I found cracking at the Bridge rectifier legs. I also found a few other solders that looked suspicious so I decided to re-solder those.

I also tested a couple of the resistors and diodes. Zomikula, I couldn't find a DP805. There's DP804, DP806 & DP810 and all these were fine (constant reading only in one direction).

I cleaned the board (IPA) and put it back together and then BOOM.

So the fuse popped again and this time the resistor NT8125 (photos below) decided to explode too!

I presume if I replaced this resistor then the same explosion would happen again because of a problem somewhere else on the board? I don't like giving in so if any of you may know where I've faultered please let me know. Otherwise I'll concede defeat.

Thank you all for your help up to now as well!

Toby

Potential cracking


What went boom

[Turnip]

Hi Tobi 1

Suspect exploded thing is a resistor, where when it heats up, it's resistance falls - Sort'a soft start.

Also suspect you gave it a hard start with a short in the P/S primary.

One useful way is to use maybe a 100 watt bulb in series with the mains input - If it glows brightly and doesn't go out after a few seconds, you've got a short.

Some shorts are only evident with voltage applied so you can measure around in 'relative safety' if you can't suss it cold - Chris.