Samsung UE46ES8000 Blown fuse but no other signs of short

[jesterace]

Hi I wonder if anyone might be able to steer me in the right direction. I have this Samsung UE46ES8000 TV and it went bang during use. The main fuse on the PSU board is open. I've checked every diode/mosfet/transistor I can see and nothing appears to be short to cause the fuse to go. The DC output of the bridge rectifier is not short either. I've seen online that some of these models are known for having their PFC mosfet or other mosfets shorting and blowing the fuse but i'm not seeing that here. If anyone aware of these models having any issues to cause this? I'm reluctant to just replace the fuse but thought about trying the light-bulb trick to get an idea if its still short.

Is it possible it was just a surge and took out the fuse? Any help as to what the next best step is would be appreciated.

James.

[Davi.p]

Hi, post an bright light photo of the PSB, top and bottom, maybe there's a protection circuit, it's possible that you need only to replace the fuse or also a varistor

[jesterace]

Some photos are attached here.

[Davi.p]

OK, top left part from top view, to the right of the input connector there's a blue button style 2 legs component, it is a varistor, check in ohm its legs, the resistance must be open (or high ohms, don't remember), if it is so replace the fuse ONLY with another same rate one, or take the burnt one and solder on to it two wires, a lamp holder, then put a filament lamp around 80w, detach main board, insert current, if lamp flashes many times, replace lamp with 60w, if it glows once and you have outputs and also backlight then you can remove lamp, put a fuse, connect all and hope..

[jesterace]

varistor reads approx 600k ohms so i assume that is ok. i will try light bulb tomorrow and report back.

[Davi.p]

to be sure, i would desolder one leg of varistor and check it again.. if it's not open i would change it..

[EazyBone]

Fuse is like 30 cents and come in 10 packs. Replace it and plug it in not connected to the tv, if it blows immediately then search harder. Power sure could cause it.. but itd have to be higher then 250v/5a. But the most common is both legs of mosfet and blue caps next to the MOSFETs.

Especially qm801/2 and right over there. Those MOSFETs go alot on boards. Check for solder cracks among them also.

Not much help since you said you did it.

[Davi.p]

Only to specify that i got one brand new unused varistor to test and it is perfectly open

[jesterace]

More info. Varistor tested out of circuit and completely open. Diode DS806 found to be short circuit (and confirmed still short out of circuit). I think the diode might be in line going to transformer to generate 5V STB.

[Davi.p]

I think that DP804, QP805S, QP806S are gone, right?

[jesterace]

They seem to be ok. DP804 measures open one way and about 430 in diode mode the other way. Both Mosfets have no shorts between any pins, and i can measure the built in diode between source/drain on both of them. I've ordered a replacement for DS806 to then try the light bulb.

[jesterace]

I've found that DS814 and DS815 are also shorted, thankfully i have a pack of diodes on the way and they are the same model as DS806 so can replace them all. It seems like the standby power has two feeds, one that comes from rectification of the bridge rectifier and DP804 and another that is rectified by DS814/DS815 but i'm not quite clever enough to understand why it has two paths right now. Unless its because the main bridge only gets activated when the relay closes. I'm learning.

[Davi.p]

it's all right on your last post, you might check the DIP8 chip ICS801S in diode mode between pins 5-8 and pin 3, it is probably shorted, and also surrounding diodes/resistors may have suffered

[jesterace]

I powered up the board with light bulb today after replacing those 3 diodes. Bulb lit fairly brightly and then i slowly saw smoke, turning the board off and scanning it quickly with a thermal camera found that CS809 was burning hot. Nothing else on the board was warm at all. In diode mode in circuit it measured about 400 one way and 600 the other. I am going to take it out to see if its bad. Is it safe to power on the board without this cap to see what happens next? I will check the IC while im at it.

its a 22pf 1KV cap between source and ground on the pwm IC ICS801S.

[Davi.p]

remove the tiny cap, then test the standby IC as per my previous post..

[Diah]

WRONG.. dot move the CS809 at any way for test , replace it and replace ICS801S. as its toasted now.... under side you have short at ZD zener Diode on circuit... search after this short.

[jesterace]

Thanks for the heads up. I have found DS809 to be short on underside. The Zeners on the back under the ICS801S are not short at the moment. Drain to Source on the ICS801S not short but I have ordered replacement IC.

I have also ordered replacement CS809. CS809 is very high voltage, is this for some kind of surge/transient suppression?

[Davi.p]

CS809 is for filtering the spikes of the mosfet switch if i recall, together with the components in parallel to the transformer primary..

[Diah]

Thanks for the heads up. I have found DS809 to be short on underside. The Zeners on the back under the ICS801S are not short at the moment. Drain to Source on the ICS801S not short but I have ordered replacement IC.

I have also ordered replacement CS809. CS809 is very high voltage, is this for some kind of surge/transient suppression?

IKV as original are enough.. as you rook out the cap and D and IC from circuit search after underside all SMT R in question if you have one changed value to higher or open....

[Davi.p]

if the standby ic has no external shunt resistor, typically 0,x ohm 2w, then easily it's internal and the ic is to be replaced, the short in the cap tells so..