UN55MU8000 Power Supply Help!!

[Storyteller12]

Have this power supply that is bad. Someone else messed with it prior and couldn't fix it. I opened it up and right away there was a mosfet bad and a Diode on the heat sink.
also a little blue cap was bad and so was the fuse.
Bridge rectifier test good

I replaced all bad parts and plugged in on my table. So nothing else connected.

fuse does not blow but voltage fluctuates on primary side from 0-65v. No power on secondary obviously. I don't see anything else that bad. I thought maybe the power ic was and removed it but same problem. I did not replace it though, just removed it

any idea what it could be?

[Davi.p]

i think you found bad QP801 and the right near diode yes? on the source of the qp801 there is a opened big resistor of low value maybe, the reason of fault are maybe the two big el.co. 500v..

[Storyteller12]

Yes I found qp801 mosfet shorted and the diode right next to it. And the little blue cap right there also.

are you talking about the 2 bigger green resistors right below all the way to the left (RP822)?

[Davi.p]

yes, those 2.

[EazyBone]

27.99 on eBay



Let us know if you fix it, I love learning these new things

[Storyteller12]

I did check those 2 resistors and they test good.
not sure what else it could be.

EazyBone, I already bought that board lol but was trying to fix this one as wel. Alsol because all those 50” Samsungs with all in 1 boards has same problem. Primary side voltage fluctuates just like this. I have 3 or 4 with same symptom

[Davi.p]

This sounds very odd, how can the power mosfet burn without discharging at GND, but could have destroyed the oscillating IC to it connected icp801, could you check the big el.caps? Or at least paralleling other ones? Check the 2 smd diodes DP802 & 3, why there are cutted legs of power mosfet+ diodes? havent you replaced them before? checked both blue disk caps? Have you disconnected one leg of the resistors before checking them (there is a zener diode within)?

[EazyBone]

I did check those 2 resistors and they test good.
not sure what else it could be.

EazyBone, I already bought that board lol but was trying to fix this one as wel. Alsol because all those 50” Samsungs with all in 1 boards has same problem. Primary side voltage fluctuates just like this. I have 3 or 4 with same symptom

Yes wtf is that. I can't figure it out on those Samsung all in one. Need to have Nick's TV do a video on it.

I need to spend more time messing around with it.

[Storyteller12]

This sounds very odd, how can the power mosfet burn without discharging at GND, but could have destroyed the oscillating IC to it connected icp801, could you check the big el.caps? Or at least paralleling other ones? Check the 2 smd diodes DP802 & 3, why there are cutted legs of power mosfet+ diodes? havent you replaced them before? checked both blue disk caps? Have you disconnected one leg of the resistors before checking them (there is a zener diode within)?

I was looking at that IC (icp801). I will test that one today and see if I have a replacement. I'll double check the 2 diodes but I'm pretty sure I checked those and they tested good.

I did remove the big 500v caps and they test 68uf but I don't have esr meter to test esr values. Would an lcr meter test the esr value?

also I did replace 1 of those blue disc caps right by the diode and mosfets that were bad.

funny thing, I just tested the diode that was all burned up and black and it tested good. So only the mosfet was bad, but replaced both anyway.

[Diah]

funny thing, I just tested the diode that was all burned up and black and it tested good. So only the mosfet was bad, but replaced both anyway.

what about here the black !!

[Storyteller12]

Diah, I just checked. I can't make out exactly what it is but one side measures 12k ohms while the other side measure 2.2m ohms. I'm not to sure what that means though

[nomoresonys]

what about here the black !!

What is that, a burned out resistor? Looks like some of the solder joints are needing solder, the one right above the red box Diah outlined looks bare.

[Davi.p]

about the green resistors and my question?

[Storyteller12]

about the green resistors and my question?

Sorry, resistors removed and both test good with same reading.
mad for the 2 diodes dp802-3 both test good as well.

I removed that ic also and still have same symptom

[Storyteller12]

What is that, a burned out resistor? Looks like some of the solder joints are needing solder, the one right above the red box Diah outlined looks bare.

I don't see burned resistor. I think it may look that way because of the silicone there. Most of that area is/ was covered in slicone

[nomoresonys]

I see, at first glance couldn't tell what it was, maybe touch up some solders and check the rest of those components related to that bad moset, did it take out ICP801?

[Davi.p]

my suspicion is the PFC coil that has a burnt wire..

[CapLeaker]

Should be easy to fix. Follow the AC from the input socket up going through the 2 filter inductors etc. See where you loose continuity.

[Davi.p]

Removing the mosfet could be ok but removing also the power diode its not good cause the "positivized" ac line cannot flow through those two little smd diodes with the high load of the rest, the two big caps requires too much current for the small diodes and the caps do not fully charge so the voltage is dancing...i'm not sure if you ever fitted all parts also changed.ones..

PS: i think i understad why the Captcha verify, the site has now few available band and the Captcha is to limit the users flow.. am i right?

[Storyteller12]

Just to update and close the thread. New board came in and fixed the problem.
but I did fine the short. It was a tiny transistor next to the 8pin ic. Unfortunately there so tiny and I can see any markings to identify it and replace. Didn't want to remove new boards ic since it all covered in silicone to find out.