Samsung ln46a650a1f won't turn on, 3.15a fuse blows

[rigmaster]

First post, so bear with me if I'm overlooking something simple, or not following board protocol.

I picked up a cheap 46" Samsung model ln46a650a1f . Previous owner bought it new, said it just died one day and they bought a new tv.

Doing some internet research, I read up on all the capacitor problems that were common with these sets, I ordered a bunch of new caps and replaced 11 of them on the power board, along with the 3.15a slow blow fuse that was blown when I got it. None of the caps were visibly bad/swollen, I just replaced them in hopes that it would be a quick and cheap fix. None of the caps that I did not replace seem to be swollen/burst either, no burnt looking components or hot spots on the board that I can see.

Put everything back together and plugged it in, still no power on. 3.15a fuse apparently blew almost immediately after I plugged in the power cord.

Did some more research and saw some people mentioning the rectifier bridge and mosfets. I have not removed any of these from the board, but I did some continuity checks on the 4 terminals of the rectifier bridge. Here are the results (DMM set to 2000 ohms, terminals on the rectifier arbitrarily numbered 1-4 by me, with 1 + 4 being the outer (pos + neg) terminals and 2 + 3 being the center terminals. Meter reads "1" for open circuit.

R= Red probe
B= Black probe

R1/B4= 830
B1/R4= 1
R1/B2= 850
B1/R2= 830
R1/B3= 850
B1/R3= 790
B2/R3= 1
B3/R2= 1
R4/B3= 1
B4/R3= 788
R4/B2= 1
B4/R2= 828


Power supply board part number is BN44-00203A

I am NOT extremely well-versed in electronics, but I know the basics and I'm a pretty good learner. I hate to give up on this thing, yet.

Any and all help/input/advice would be greatly appreciated.


Rig.


EDIT: hope this isn't cheating, I don't have the ability to post good pics of the actual board in question, so here is a link to pics of the EXACT same board courtesy of a previous post from BudM.
http://s807.photobucket.com/user/bud...tml?sort=3&o=3

[williej]

Re: Samsung ln46a650a1f won't turn on, 3.15a fuse blows

Here is the power supply schematic.

[Caleb]

Re: Samsung ln46a650a1f won't turn on, 3.15a fuse blows

Blown bridges are pretty rare, blown mosfets are very common. They are easy to test with the meter in diode mode (which is also how you would test the bridge) you are looking for a solid low resistance that doesnt go away. from the supplied schematic QM803 looks to be the primary fet and the most likely culprit. Make sure you test the secondary output diodes for shorts as well as something else usually causes the fet to fail.

[rigmaster]

Re: Samsung ln46a650a1f won't turn on, 3.15a fuse blows

Thanks for the replies. Bear with me if these are silly questions.

Can I test the fets without removing from.the board?

Do the readings I posted above help determine if the bridge is OK or not? If not, do I just need to retest using the diode function or something else?

Thanks!

[budm]

Re: Samsung ln46a650a1f won't turn on, 3.15a fuse blows

'Can I test the fets without removing from.the board?' Yes, check the resistance between S and D pins, if it shows very low Ohms reading then remove and test it again off the board.

[rigmaster]

Re: Samsung ln46a650a1f won't turn on, 3.15a fuse blows

I admit I can't figure out what's going on. I've done some checks, but I can't make heads or tails of them. I didn't quite understand how Caleb was explaining to test the fets, since there are 3 terminals. I found a clear explanation of which terminal was which (gate, drain, source) but I'm floundering on how to procede.

Do I need to check all of the fets? some have all 3 terminals in line, some have the center (drain) offset, is there a difference in function between the 2 types?

I found a test procedure online where you "charge" the fet using the diode test, but I can't seem to get any of them to act like they're supposed to act per the instructions.

So, in order for this post to contribute at least a little to the process, here are the specs I get while testing the QM803.


005 ohms with probes on D and S (same reading no matter which probe is on which leg) meter set on 2000 ohm range.

"1" (open) with probes on D and screw which mounts fet to heatsink (which should be connected to the D leg, right?).

1605 ohms with red on G and black on D
1215 ohms with black on G and red on D

Sorry to sound borderline incompetent, just having trouble wrapping my brain around this thing.

Once again, thanks to all for the input and the assistance.

Rig



ps- Caleb, can you explain specifically which items I should check (and how to check them) when you suggest to check the secondary output diodes for shorts?

[budm]

Re: Samsung ln46a650a1f won't turn on, 3.15a fuse blows

'005 ohms with probes on D and S (same reading no matter which probe is on which leg) meter set on 2000 ohm range.' We are looking for low resistance between S and D, so set meter to 200 Ohm range an see what you get, it looks bad to me.

[rigmaster]

Re: Samsung ln46a650a1f won't turn on, 3.15a fuse blows

'005 ohms with probes on D and S (same reading no matter which probe is on which leg) meter set on 2000 ohm range.' We are looking for low resistance between S and D, so set meter to 200 Ohm range an see what you get, it looks bad to me.

Thanks budm, that's what I was thinking as well.

The lowest ohm setting on this meter is 2000, I've got another meter that may have a 200 ohm setting, I'll see if I can locate it.

[Caleb]

Re: Samsung ln46a650a1f won't turn on, 3.15a fuse blows

Fets are nothing more than a switch, the gate turns on the connection from source to drain. When they fail usually it is a short between those two but a short can occur between any of the 3 pins and the fet is dead. If it shorts the gate as well then it is a good idea to replace whatever is driving the gate signal as well in your case the icm801a is driving the fet.

When you use diode mode you are pushing a small voltage out of the meter and measuring the amount of voltage lost, when you put it across a short it should give a solid tone, a good diode will typically beep once when the probes are placed across the diode (cathode to black anode to red) then it will give an open reading when placed in the opposite direction. I think you may be mistaking the output diodes for fets, they are labeled on the board as dm852, dm856, dm851 and consist of two diodes in a single package. if you receive a solid tone you will have to desolder one side of the item in question to verify if it is really shorted.

As budm stated 005 ohms means that fet is stuffed.