Samsung LE32A456C2D TV no power

[ezenia]

Hello,

I found a dumped Samsung TV. I decided to take it to see what might be wrong with it.

The TV is lifeless. There's no standby light or anything.

I opened it up to see if there's any clues as to what might be wrong. I took out the power supply board first and immediately noticed a visible burn mark on the TV's metal casing.

It appears a small blue capacitor that sits right next to the massive high voltage capacitor has blown. There's visible burn marks on both the front and back of the power supply board. I took 2 pictures of this which I have attached to this post. The burn marks on the back are in the area where the blown blue cap is.

The fuse on the board also appears to have blown. The PSU board model number is BN44-00214A.

Is this an easily fixable board or will be the repair be more complex? The burn marks on the back where the capacitor should be looks problematic. It seems one of the pads may no longer be any good? I don't see any solder on it.

Thought I'd ask here before wasting time trying to fix it.

[cheeky2]

Re: Samsung LE32A456C2D TV no power

The dreaded brown glue that is used in PSU's that goes conductive over time which has happened in this case. Download the service manual available free online for it and replace the capacitor I think it's a 1KV 2.2nF though could be wrong. Remove all the brown glue from the power supply that are on any transistors high voltage capacitors and if I recall you may have to replace the T3.15A fuse as well. Don't replace the fuse without removing the glue. The Mosfets that are nearby should also be checked for shorts before contemplating powering it up. A decent image of the whole psu would help as well!

[ezenia]

Re: Samsung LE32A456C2D TV no power

I've attached a pic showing the front and back of the power supply board in full. Hopefully the quality is OK. If not, I can try to take some more.

I took off the blue cap, but unfortunately this resulted in the other pad coming off. So now the back of the board where the blue cap should be soldered to has both of its pads missing. This can be seen in the pic I attached. The burn marks that were previously on the back of the board are mostly gone as I used isopropyl alcohol to try and get rid of it.

I put my multi-meter into continuity mode and decided to check QM801 and QM802. If I test QM802, there's continuity between the lone pin and the two pins under it. QM801 doesn't beep when I do the same check. Does this mean QM802 is shorted?

[ezenia]

Re: Samsung LE32A456C2D TV no power

I've also attached a photo of the burn mark on the TV's chassis. Is this mark indicative of damage done to the TV itself, or is it there because of the blown components on the power supply board?

[cheeky2]

Re: Samsung LE32A456C2D TV no power

You need to remove the brown glue from all the components as stated before failure to do so will result in issues powering on and possible damage. See areas in red for glue removal. Remove the components from the board to get glue off as easiest.
The blue area is possibly where the wire links have been damaged common issue.
Clean the mask off the tracks on the pcb so that you can solder to the tracks that originally the 1kV capacitor was connected to.
Deposit of black on last image is the capacitor leg vaporising when shorted.
Remove mosfets from circuit when testing, mosfets normally short out across all three pins.
I have repaired a few of these boards all with similar issues. You haven't stated whether the main fuse has gone either.

If you are not competent in electrics do not attempt this below.
When you have carried out the repair, put a mains 60Watt filament light bulb in series with the mains input so that you can see when you turn on the power supply whether it glows brightly. If it does then you have other issues with the board. If it glows briefly brightly and goes dim then you can safely say the psu is working. Then you can remove the filament bulb from the circuit.
A short on the psu will means the filament bulb will glow brightly.
These psu's are expensive as a result of them typically failing like yours has done.
Good luck

[ezenia]

Re: Samsung LE32A456C2D TV no power

More to do than I wanted/expected, but I'm going to try working on it when I can. It'll be good free practice even if I don't get it working.

For removing the glue, what is used? It seems too hard to just scrape off with something like a flathead screwdriver.

The fuse next to the power connector, marked T6.3AH 250V on the board, is blown.

I'm not too sure about the light bulb test. I probably won't attempt that as I don't know exactly where or how to safely connect the bulb to the circuit. I've briefly tried to look it up, but can't seem to find a video/guide on what exactly needs to be done.

[cheeky2]

Re: Samsung LE32A456C2D TV no power

Video here with comprehensive instructions.
https://www.youtube.com/watch?v=RfMsNqCTfoM
Plenty of videos on 'mains current limiters using bulb'
If you watch the video gives comprehensive instructions.
Basically you are putting a bulb in line (in series) with the electrical device you are testing on the live wire. If you use this then you will not to remove the mosfets to check them. As if they are faulty the bulb will glow brightly.
Scrape brown glue off with stanley blade or penknife.
I would expect around 10-20minutes to repair and complete this job for myself, so not long!

[ezenia]

Re: Samsung LE32A456C2D TV no power

So far I've removed the glue from most of the components. There's just one more that has glue touching the capacitor legs. I'll remove that one later on.

I decided to take out the QM802 component as I was getting continuity on all 3 pins while it was in circuit. When out of the circuit, the same thing happens. So it looks like QM802 is bad?

The two jumper wires next to the blown blue cap still have continuity across both pins. Does this mean they're still good and don't need replacing (just cleaning)?

[cheeky2]

Re: Samsung LE32A456C2D TV no power

Replace both mosfets as one will have been stressed and will likely go very shortly afterwards! If both wire links are good and not blown then leave them alone, though they look rather rough to say the least!
Look at the circuit diagram as there are other components that drive both the mosfets that will need to be checked as these often go at the same time as the mosfet has gone.
If you have other components that have failed then your mosfet will blow again if they haven't been checked.
You will need to do the rest as I have given you enough information to complete the task.

[ezenia]

Re: Samsung LE32A456C2D TV no power

Is it normal for RM812 and RM814 (47Kohm resistors) to be measuring 10.9ohms in circuit? Just wondering if that's normal behaviour or if that signifies an issue somewhere. I tested both of them out of circuit and they read 47Kohm.

EDIT: looks like TM802 was the cause. I desoldered that and now RM812 and RM814 show 47Kohms in circuit.

[ezenia]

So I've been trying to figure out what is wrong with the board and so far this is what I've found (not sure if some of these are considered issues):

• DM804 and DZM805 appear to be bad - diode mode shows OL in both directions even when out of circuit.
• DZM806 may be bad as well - I get OL in one direction (red probe on lone pin) and then 2.2V on one of the pins and then OL on the other (black probe on the lone pin). I got these readings both in-circuit and out of circuit.
• QM802 is shorted on all 3 pins. Guessing this is what took out the diodes. QM801 doesn't appear to be shorted.
• TM802 may be bad. When it's connected to the board, the resistance of RM812 and RM814 drop to about 10ohms. I don't know if this is considered normal. Could TM802 have been damaged by QM802 getting shorted?
• NT811S has continuity between both of its legs when out of circuit. I looked up what this is and it appears to be some sort of thermistor. Is this correct? If so, should both legs be shorted? This was one of the components that had glue covering it.
• The T6.3AH_250V fuse is also blown.
• CM810 had obvious signs of damage, so that was removed long ago.

I'm beginning to think that I may have caused more damage when trying to turn the TV on when I first got it. I do recall seeing the large transformer (TM801S) let out quite a large blue flash and make some pop/bang noise when I was trying to turn it on. This possibly caused the damage seen above. I currently have this transformer out of the board. Is there a way to tell if the transformer is good or if it's damaged?

That's where I'm at so far.

I've attached a link to the circuit diagram I am looking at

[cheeky2]

You need to replace the faulty parts and focus what I have already stated and shown you! Why are you bothering with looking at the transformer or NT881S?? The fuse blew as a result of your glue issues shorting out on CM810 and your mosfet was a result of the glue which would have made loud bang and flash. Read carefully what I have stated together with the images I have uploaded. What experience do you have of repairing power supplies of this type?? None. Do you really wish to waste your time looking at items that have no relevance to the repair? By all means carry on if you wish to waste your time and effort doing so. If you continue in that direction you will be on your own.
Focus on what I have already stated. The diodes and zener diodes in the drivers for the mosfets and sometimes the smd resistors associated with them as well fail. Use common sense if you think the smd diode has failed carefully remove it and test out of circuit. Though considering you will have to order some then pretty pointless as you will have to order more than 4 so just replace all of them in the driver circuit for the power mosfets that have failed.
There are two smd diodes and 4 smd zener diodes in the circuit just replace them! Replace the two mosfets, the fuse the 1kv 2.2nf capacitor and check the resistors in the circuit. That is not difficult in my opinion.
If you don't replace blown parts in the circuit your mosfets will fail again and you will be back to square one! So for the cost of the extra couple of diodes that may test good (but have been highly stressed and likely to fail soon afterwards with the result of repeat failure). I have experienced all of this before with these type of repairs and wasted time in not replacing diodes that test good but will imminently fail shortly afterwards in these type of circuits.
Order your parts, replace them and use the 'mains current limiters using a filament bulb' to test when you have replaced parts.