Samsung LA32M51BS

[edharvey]

Hi guys

I have a 32" samsung LCD that when switched on, the power light will switch on and off in time with a clicking noise in the back of the set.
I have opened the TV up and found two burn marks on the power board.

As seen in the pictures there is a burnt out component that looks like a resistor, however I believe is actually a fuse (labelled FD802S)
There is also a burn mark near the transformer (also pictured)
All the capacitors on the board look fine (no bulging etc)

Just wondering if anyone else has had a similar problem and what the best direction is to go from here, I assume that just replacing the fuse won't help as there is probably something that caused it.

Thanks for your help
Ed

[edharvey]

Re: Samsung LA32M51BS

I have just had a look on the internet and located the powerboard on Ebay for this TV.
It is part number BN94-00699D interestingly enough it is from a New Zealand supplier which is where I live, however it is still $120US which I would like to avoid paying if possible!


http://cgi.ebay.com/BRAND-NEW-SAMSUN...efaultDomain_0

[PlainBill]

Re: Samsung LA32M51BS

Hi guys

I have a 32" samsung LCD that when switched on, the power light will switch on and off in time with a clicking noise in the back of the set.
I have opened the TV up and found two burn marks on the power board.

As seen in the pictures there is a burnt out component that looks like a resistor, however I believe is actually a fuse (labelled FD802S)
There is also a burn mark near the transformer (also pictured)
All the capacitors on the board look fine (no bulging etc)

Just wondering if anyone else has had a similar problem and what the best direction is to go from here, I assume that just replacing the fuse won't help as there is probably something that caused it.

Thanks for your help
Ed

It would be interesting to know the history of this set - as in has anyone been inside it before you?

Looking over the board, the power flow is fairly obvious. Power comes in, is rectified, appears to go through a PFC circuit, and then to STR1, a combination SMPS controller and power FET in a single package. You were close with your interpretation of FD802S; it's a fusible resistor - see the symbol on the bottom of the board - a combination of a resistor and a fuse.
If you follow the traces, you see that one end of FD802S goes to the burnt pin on the power transformer.

And that's where history comes in. There are two possible explanations for a burnt pin like that. Scenario one is a bad solder joint caused overheating, degradation of the solder, and eventually an arc. At some point the main power FET shorted, blowing FD802S. Assuming availability of parts, that is certainly repairable. The flaw in this conjecture is the solder joints on the other transformer pins appear to be perfect.

This leads us to the other possibility. Someone was troubleshooting and got careless, shorting that pin to ground. The arc burnt the pin, blew the fusible resistor, and possibly did additional damage. While this repair would be possible, that leave open the question of what was the original fault?

PlainBill

[edharvey]

Re: Samsung LA32M51BS

Thanks for your reply Bill, very detailed!

I am not sure of the history of the set, I bought it cheap from an auction company which is responsible for selling goods from insurance claims etc. A couple of screw pads on the plastic back cover have broken through, however this would suggest that someone has hit the side of the set rather than opened it.
Would you recommend, as a first step that I find a replacement for this resistive fuse and see what happens? I am in my second year of an Electrical engineering degree, so I know a bit about this stuff but my knowledge is still quite limited in some areas.

Thanks

Ed

[PlainBill]

Re: Samsung LA32M51BS

Thanks for your reply Bill, very detailed!

I am not sure of the history of the set, I bought it cheap from an auction company which is responsible for selling goods from insurance claims etc. A couple of screw pads on the plastic back cover have broken through, however this would suggest that someone has hit the side of the set rather than opened it.
Would you recommend, as a first step that I find a replacement for this resistive fuse and see what happens? I am in my second year of an Electrical engineering degree, so I know a bit about this stuff but my knowledge is still quite limited in some areas.

Thanks

Ed

Just replacing parts to see what happens is usually an expensive way to damage a piece of electronics beyond repair. I'd suggest the following steps.

1. Examine the burnt pin carefully. Is the end of the pin pitted like an arc was occurring there, or is the damage at the point where it passes through the board? If the damage was at the end of the pin it would indicate TWO problems with the TV.

2. Check the power FET (STR1) for shorts. If it is shorted, replace that also.

3. Check the diodes on the secondary side of the transformer for shorts (unlikely, but it's a sensible precaution.

4. Look for any other problems - shorted semiconductors, bad solder joints on transformers, etc.

5. Replace the fusible resistor. I don't have more than a vague guess as to it's value.

PlainBill

[edharvey]

Re: Samsung LA32M51BS

Ok, will do. Does anyone know where I might be able to source a schematic for this circuit in order to find the value of the fusible resistor?

[PlainBill]

Re: Samsung LA32M51BS

Ok, will do. Does anyone know where I might be able to source a schematic for this circuit in order to find the value of the fusible resistor?

You could try Googling the part number of the power supply and schematic.

PlainBill

[edharvey]

Re: Samsung LA32M51BS

Yes, this was obviously my first attempt however I was not able to find anything. Are there any websites that specialise in schematics or should I contact Samsung and ask for it?

[hoadle]

Re: Samsung LA32M51BS

Here the parts for samsung around $85
https://www.encompassparts.com/webwi...number=6461772

[PowerAmpFreak]

Re: Samsung LA32M51BS

I think the fusible resistor is your least problem. Better focus on what may have blown it. When you find out, you may use a temporary fusible resistor like 1ohm for testing purposes. When the psu appears to be good, you'll have to use the correct type of fusible resistor of course.