Samsung LN46D-550SK1FX- No Power, No LED- Blows AC Fuse

[bstevens82]

I have a Samsung LN46D-550SK1FX LCD TV that recently died randomly while watching it. There is no POWER and no LED light. I pulled it off the wall to find the AC fuse was blown. I replaced the fuse and it immediately blew again. I unplugged the two other boards and it still blows the AC fuse immediately upon being plugged in. There is no visual damage to the board (burn marks etc.) and no bulging caps. I am a bit novice when it comes to testing components but with a little reading I checked the caps and each is able to load and discharge via multimeter. I checked the diodes and they are all working. I also went through and checked the transistors/MOSFETS/bridge rectifier. From the attached picture the ones with a green mark on the heatsink checked out ok and the black mark on two of these showed shorted across the terminals(one mosfet and the bridge rectifier). Also it appears the white film capacitor is bad next to bridge rectifier. Would these cause the AC fuse issue? I will attempt to de-solder and find part numbers. Is there anything else I should be looking for? Do other parts usually go if these go? Any help would be much appreciated. I am learning as I go but willing to put the work in. Please see the attached pictures of the board (BN44-00441A).

[selldoor]

Re: Samsung LN46D-550SK1FX- No Power, No LED- Blows AC Fuse

Nice description of problem and action taken - pictures are just a little grainy.

You say the Bridge rectifier shows as being shorted? - well that is very likely to blow the fuse. - Take it out and test it again. Also test the pads for circuit shorts shorts when it is removed.

You may like to google "dim bulb test" if you can make up one of those it may help you to
prevent damage to new parts.

[bstevens82]

Re: Samsung LN46D-550SK1FX- No Power, No LED- Blows AC Fuse

Ok finally had some time to pull out what I thought might be bad and it looks like one of the mosfets is bad. I have uploaded a couple more pictures pointing out the culprit. Upon closer inspection the solder had appeared bubbled. I tested the bridge rectifier and it appears to be fine. Not sure exactly what is meant by "check the pads" but I did look for shorts across where the removed parts were connected and do not see anything. Anything else outside of replacing this mosfet I should test? Trying to minimize my parts orders. Thanks for the help.

[budm]

Re: Samsung LN46D-550SK1FX- No Power, No LED- Blows AC Fuse

That is the PFC Voltage booster section, you should replace that 8-pin SMD PFC Controller IC, also check all the resistors, diodes, in that section real well, especially the ones connected to the Gate of the MOSFETs.

[bstevens82]

Re: Samsung LN46D-550SK1FX- No Power, No LED- Blows AC Fuse

Ok we are making progress. I replaced both mosfets and the bridge rectifier and ic controller( tough to solder that tiny chip). It stopped blowing the fuse right away and i get the red standby light and it blinks when i hit power but nothing comes on screen or sound wise. What do i check next? When plugged in i think i am getting proper voltage on main board side so dont think it is that. Not sure where to go from here...Any help would be appreciated.

[budm]

Re: Samsung LN46D-550SK1FX- No Power, No LED- Blows AC Fuse

OK, so you are now getting the standby Voltage.
So what DC Voltage do you get on all those pin of the CN802?
It is double-row setup so it will be easy to get to all the pins.
We need to find out if you are getting the PS_ON to turn on the PFC Voltage booster circuit, if it does then you should have >360VDC at the two legs of the main filter cap.

[bstevens82]

Re: Samsung LN46D-550SK1FX- No Power, No LED- Blows AC Fuse

Ok dcv is starting at pin 1 and going across top: 13.17,13.17,13.17,13.17,0,5.2,5.2 next going across bottom from 2 : 5.02,0.00,0.01,0.00,5.20,3.08. On the large caps each measures 174 and on cni801 from pin 12: 12.28,13.15,12.44,0.00,0.00. So not getting 360 on either cap.

[freakaftr8]

Re: Samsung LN46D-550SK1FX- No Power, No LED- Blows AC Fuse

Your not getting PFC on or p_s on. Either jump p_s on to 5v or p_s on to ground and recheck voltage at main cap. Use a 100 ohm resistor for the jumper.

[bstevens82]

Re: Samsung LN46D-550SK1FX- No Power, No LED- Blows AC Fuse

Ok jumpered both ways and get no change in caps. Both still at 174. Could I have screwed up the soldering of the controller IC and get this or is it something unrelated?

[budm]

Re: Samsung LN46D-550SK1FX- No Power, No LED- Blows AC Fuse

Did you carefully check the Gate drive resistors, Source resistor, VCC at the PFC Controller IC?

[bstevens82]

Re: Samsung LN46D-550SK1FX- No Power, No LED- Blows AC Fuse

Couple questions- which resistors are these? The set near the main board lead (RM817) or the set to the right of bridge(CX8015)? Or am I completely wrong in my guess? Also how do you check VCC at PFC? It is pin 8 from the data sheet and I have the voltage range on the sheet. Do i just check to see that it is 13-20 v when plugged in?

[budm]

Re: Samsung LN46D-550SK1FX- No Power, No LED- Blows AC Fuse

Vcc is checked against the Gnd pin of the IC or use the negative leg of the main filter cap as the Gnd ref for the primary side of the circuit.
If you look at the Output pin of the PFC IC, is should be connected to the Gate of the MOSFET through a resistor, sometime there will be Diode/resistor network connection also.
You will be working in the primary deadly side of the circuit so you must be real careful not to touch anything with your hands or body, it is best to plug the TV into GFCI outlet at least.