Samsung UN58H5202AF - killing mosfet.. no power

[Doc38343]

OK I read the other posts and found same burned board and Q9101 shorted as budm
So I replaced the Q9101 (ordered from mouser) anyway it worked great for 3 hours and blacked out so i pulled the board and same mosfet Q9101 is shorted again.. there are 3 very brown spots on this board over the two in the middle QM801cs and qm802cs also brown at the 4 diodes at very top of cold section DM841-844 C they seem to be getting hot also... i bought 5 mosfets but don't want to waste another without trying to isolate the cause.. a long shot could be a defective new part but i doubt that seeing the heat marks...
What would cause that mosfet to fail??

After checking all the diodes and other components on the board and they seem good.. so i put another new Mosfet on it Q9101.. this time i added some of the silicone heat sink to help cool the hot devices. I haven't run the set yet, but overnight two more things occurred to me..
1. could the capacitors cause this? they seem good but i did not pull them out of circuit.
2. Could the LED backlight brightness be too high causing this?

Perhaps experience could enlighten me.... Thanks

[Nick's Tvs]

Re: Samsung UN58H5202AF - killing mosfet.. no power

Hello,

We had the exact same set come into our shop, and the exact same transistor burnt out. We started off by doing exactly the same thing and replaced the transistor to have the exact same failure as you did but only within 10 minutes. From there, we decided to just purchase a power supply online to replace it. That power supply lasted about 4 days before yet again the exact same failure happened. After that we decided to purchase the entire kit from shopjimmy and proceeded to replace all of the boards. Once that was done, we yet again had another failure within two days. We gave up and refunded our customer at that point. The only parts not replaced were the LED strips, maybe somehow they failed and were causing the power supply to go out. All I can tell you is that our customer was pissed and that entire process was a pure nightmare.

[budm]

Re: Samsung UN58H5202AF - killing mosfet.. no power

What happen if you turn down the backlights level to 50% or less, also check the LED current by checking the Vdrops on the LED current sensing resistor.
That MOSFET is for the boost converter so may be the Boost converter inductor may be bad or the IC is not driving the MOSFET at the correct frequency. I would get the spec sheet of the LED driver IC and probe the circuit with scope.

[Doc38343]

Re: Samsung UN58H5202AF - killing mosfet.. no power

What happen if you turn down the backlights level to 50% or less, also check the LED current by checking the Vdrops on the LED current sensing resistor.
That MOSFET is for the boost converter so may be the Boost converter inductor may be bad or the IC is not driving the MOSFET at the correct frequency. I would get the spec sheet of the LED driver IC and probe the circuit with scope.

First thanks to both of you guys for helping.

What i did was pulled all the diodes and tested. tested caps.. they are ok also.
I put it all back and put the new mosfet on. Then went with the silicone solution to cool the 3 mosfet that are getting hot under the board. I also tested the backlights with the cheap little tester (did not pull the panels) the top pair pulled 98.6 from the tester.. the bottom pair pulled 68.7v.
I re-connected and powered on and the LED rail voltage (read from chassis to the top diode and the horizontal jumper near the jack) was 233.6v
The actual readings on the led connector were 101.4 at top pair and 86.9 at bottom pair.
I went into the menu and cut the LED level from 90% down to 50% it is still plenty bright, I think that was partially the problem. I could not find the service manual for this one and i am not paying $14.99 for it so I don't really have the optimum settings/voltages beyond the board markings. It has run 5 hours yesterday evening and about an hour so far today.. I will run it all day and test the temperatures with laser thermometer to see how it holds.
So far so good. Cooling may be the answer and undoubtedly the leds are getting older and drawing more current than when new. I think it was designed to fail in a calculable amount of time by Samsung design.

[funkmasterfrank]

Re: Samsung UN58H5202AF - killing mosfet.. no power

Does anyone have another replacement part number other than the original Samsung part number? I would like to order from Digikey or mouser

[jjanssen7]

Re: Samsung UN58H5202AF - killing mosfet.. no power

This model is worst than most at burning out LEDS. On each TV of the 8 I've seem that mosfet is burned up and when I open up the TV many of the leds are burnt out. just sayin'

[jetscreamer]

Re: Samsung UN58H5202AF - killing mosfet.. no power

Are you saying that bad led strips are causing q9101 to fail?

I've replaced the one I am working on twice now.... first time lasted a few hours. 2nd time I dimmed the leds to 50% and it has been on for 24hrs so far (thanks budm). My board is burnt in the same sport just like the original posters. Wish I had the testing voltages for the PSU board.



Used:
https://www.digikey.ca/products/en?k...EAUMA1CT-ND%20

[petehall347]

Re: Samsung UN58H5202AF - killing mosfet.. no power

as far as see the led.s are turned up way too high in default settings ..i always turn them down whatever tv or monitor it is .

[nomoresonys]

Re: Samsung UN58H5202AF - killing mosfet.. no power

Looks like this model needs a re-design.

[budm]

Re: Samsung UN58H5202AF - killing mosfet.. no power

Were some of the bad LED's shorted out?

[jjanssen7]

Re: Samsung UN58H5202AF - killing mosfet.. no power

I have repaired about 10-12 of this specific model. In EACH case I replaced some/most of the LEDS which were burnt out. Then, in EVERY case the power-supply needed to be replaced/repaired as well before backlights would turn on. I would make sure with testing and visual inspection that all LEDS are working before repairing the power supply. Doing things this way I've had a 0% return rate within a two year window.

Hard to say if the power supply is burning out that LEDS and then taking itself out or the other way around, but there seems to be a connection.

[shmasood]

Re: Samsung UN58H5202AF - killing mosfet.. no power

Where did you get power supplies from. Also any instructions on accessing backlight lrds since I could not find service manual or youtube videos for this model.

[sputnikoff]

Re: Samsung UN58H5202AF - killing mosfet.. no power

I have same issue with this set.There are 6 strips of leds inside at least 3-4 and some 5 leds gone on each row.

[sputnikoff]

Re: Samsung UN58H5202AF - killing mosfet.. no power

power supply wouldn't fire up,even with cable to main board off.Q9101 was failed.Replaced and unit up and running.

[farog]

Re: Samsung UN58H5202AF - killing mosfet.. no power

Hi,

Please i need help. I have the QM801CS and QM802CS transistors/mofset who is burn on this power supply (BN44-00787A) so i can't read what is write on it. Can you help me by giving me the references of these transistors?

Thanks

[jaysonfp]

Re: Samsung UN58H5202AF - killing mosfet.. no power

Hello,

We had the exact same set come into our shop, and the exact same transistor burnt out. We started off by doing exactly the same thing and replaced the transistor to have the exact same failure as you did but only within 10 minutes. From there, we decided to just purchase a power supply online to replace it. That power supply lasted about 4 days before yet again the exact same failure happened. After that we decided to purchase the entire kit from shopjimmy and proceeded to replace all of the boards. Once that was done, we yet again had another failure within two days. We gave up and refunded our customer at that point. The only parts not replaced were the LED strips, maybe somehow they failed and were causing the power supply to go out. All I can tell you is that our customer was pissed and that entire process was a pure nightmare.

Sounds like the LED strips were bad.

[skiinggord]

Re: Samsung UN58H5202AF - killing mosfet.. no power

14nm50n mosfets are QM801/802

[Nick's Tvs]

Re: Samsung UN58H5202AF - killing mosfet.. no power

Just an update, but as it has been mentioned by jjanssen7 replacing the LED strips is essential. We have always replaced all LED strips with a brand new remanufactured sets from shopjimmy in addition to replacing the faulty mosfet since our last post and have been succesful with ever repair after.

[alfatv]

Re: Samsung UN58H5202AF - killing mosfet.. no power

The way I see it, when too many LEDs are shorted, voltage drop across all LEDs is smaller, then source-to-drain voltage is higher. Multiplied by current, mosfet power dissipation goes up. If too much, it overheats and blows up.
Unless you change backlights, I advice not to fix or sell these boards alone.