LE40M86BD switching off/on

[Dodgexander]

Hi all. I own a LE40M86BD which in the US is internally comparable to the 65f series and externally to the 61f.

It's been turning itself off, then on, then off, then on for a while but it's now been getting a lot worse. It starts to happen about 5-10 minutes after its switched on. This on/off cycle happens now for as much as an hour and then stops on its own. After the TV has stopped turning itself off then on it works completely fine!

I've read online similar cases and unfortunately they all lead to the same blown capacitor problem. See my pictures in the album but there are no signs of bad caps, the only thing i can see is discoloration on the back of the main board. I just can't see anything in particular wrong.

What could the problem be? When the screen turns off the TV clicks. You see the TV turn off just the same as If you unplugged the cord then power is restored and it comes back on for a few seconds, the picture is shown fine and then once again turns off.

I've tried following another thread on here from someone with the 46 inch version and replaced two commonly at fault SMD caps on the motherboard. I've also desoldered and tested the usual culprit caps on the power board and they all test fine.

The dc voltages supplied to the main board seem fine too. I tested them with everything plugged in while the set is switching itself off/on and apart from the loss of power they all read fine.

I've been trying to run the tv without the motherboard connected by shorting the 5.2v trigger and power on/off pins on the dc voltage cable going to the motherboard but it makes a terrible scratching noise and the screen doesn't turn on. So I can't test if the fault is purely down to the motherboard. (I can get a replacement mb for the tv for £32) does anyone know why this doesn't work?

I also tried to measure the AC input voltage but I can't for the life of me get a reading. I'm not sure why because I can read the dc voltages just fine. I can fit the probes into the connector but just can't get a reading.

Photo album : https://goo.gl/photos/sAt5NG6tQ9wNpJum7

Right now I just want to make sure it's the motherboard at fault and the power supply is okay before buying a replacement board. I'm not sure how to diagnose the existing motherboard so am happy to replace the whole thing.

Thanks for any help.

Dodge

[budm]

Re: LE40M86BD switching off/on

'See my pictures in the album but there are no signs of bad caps,'
Bad caps can look normal, you need to check the ESR and the capacitance.

[Moreno83]

Re: LE40M86BD switching off/on

When the tv is cold , turn the tv on and use a hairdryer to heat up the power supply. Try to aim just at the power supply ( capacitors location ) and see if the tv stays on. Tv tries to turn on but its missing that little bit of power to turn on.

If heating up the power supply does not work then aim the hairdryer at the metal shield on the mainboard. I see you already removed the shield? Those small caps underneath that shield on the board get hot , they fail easily. I see that a lot on those lcd tv's. My bets are on the mainboard!

[Dodgexander]

Re: LE40M86BD switching off/on

'See my pictures in the album but there are no signs of bad caps,'
Bad caps can look normal, you need to check the ESR and the capacitance.

I tested all the caps on the power board in this picture:

and I replaced the two in this picture:

When the tv is cold , turn the tv on and use a hairdryer to heat up the power supply. Try to aim just at the power supply ( capacitors location ) and see if the tv stays on. Tv tries to turn on but its missing that little bit of power to turn on.

If heating up the power supply does not work then aim the hairdryer at the metal shield on the mainboard. I see you already removed the shield? Those small caps underneath that shield on the board get hot , they fail easily. I see that a lot on those lcd tv's. My bets are on the mainboard!

Thanks, I gave this a try and it doesn't do anything. I was hoping it would show a particular spot was at fault but the TV just keeps switching off and on.

Is there a way I can get the power supply working without the motherboard so I can be sure a motherboard replacement will work?
I tried to short the 5.2v standby/detect and power pins on the voltage cable to the motherboard and the detect and power pins on the inverter cable but I can't get it to start up without the motherboard. The power supply just makes a fuzzy sound, I think coming from the large transistor.

I've tested the voltages and they seem fine but I'm not sure if that is enough to know if a replacement motherboard will fix the switching off/on.

[vinceroger69]

Re: LE40M86BD switching off/on

try heating the mainboard too as it might be a bad bga solder joint etc

[Dodgexander]

Re: LE40M86BD switching off/on

try heating the mainboard too as it might be a bad bga solder joint etc

Thanks vince, I'm guessing I'll need a heatgun or reflow station to do this?

[vinceroger69]

Re: LE40M86BD switching off/on

just use a hair dryer heat the board for a few ninutes just see if it makes a diffrence or not some times mainboards do fail due to bad capacitors/regulators/bga chips etc

[Dodgexander]

Re: LE40M86BD switching off/on

The set isn't turning off/on today so it's hard to test. I have tried before with a hairdryer though and it hasn't helped. Need to wait for it to start turning off/on to test. It's so intermittent.

[Dodgexander]

Re: LE40M86BD switching off/on

Okay so I left it in standby for a while and it switched itself on which I've noticed from time to time.

Nonetheless after I had it off for a while it started the turning off/on clicking loop and I tried warming things up with the hairdryer. Sadly it had no effect, by the end the motherboard was warm and the heatsink almost too hot to touch. I tried all around the board but no place stopped the power cycling. I tried even warming the power board too but it didn't help.

Shall I risk trying a new motherboard?

[CapLeaker]

Re: LE40M86BD switching off/on

Shall I risk trying a new motherboard?

I would... there is water damage on the backside of the main board.

[vinceroger69]

Re: LE40M86BD switching off/on

I would... there is water damage on the backside of the main board.

I did not see the link too the pictures in post 1 looking now yes certainly looks water damaged so if was my set i would try a mainboard too.

[Dodgexander]

Re: LE40M86BD switching off/on

I would... there is water damage on the backside of the main board.

Whoah water damage? Really? Wow I didn't expect that. Where has it come from it's been mounted on our living room wall!

[comptek]

Re: LE40M86BD switching off/on

i have often seen that on boards i think its what they clean boards with during production and considering the gap between the board and the back of the screen its unlikely to be water damage imo

[CapLeaker]

Re: LE40M86BD switching off/on

Whoah water damage? Really? Wow I didn't expect that. Where has it come from it's been mounted on our living room wall!

I dunno. Given, the pics aren't the best, but it certainly looks like the back of the main board was wet at some point of its time.
Clean it off with 99% alcohol and a toothbrush etc. dry it, install it and try it again.

Also what does STBY voltage, PSon and BLon pins do on the power supply when the set goes into the cycle? If you loose PSon during that cycle, you have a main board problem.

What is underneath the metal shield on the front of that main board? Any caps in there?

[Dodgexander]

Re: LE40M86BD switching off/on

Can I buy a different # board from say the 46" version of the same tv?

[CapLeaker]

Re: LE40M86BD switching off/on

It should be the same board #, but some others are compatible. I wouldn't risk that unless I would be 100% sure.

[Dodgexander]

Re: LE40M86BD switching off/on

It's strange because I've seen boards on eBay from the same tv model but they are a different board number. The one I found for a good price is for the 46 inch version of the tv but the number is different.

Edit# this is a different number but is for exactly the same model as my tv. http://pages.ebay.com/link/?nav=item...218263&alt=web

I've had the set since new and all that's ever happened to it is the firmware was updated by a Samsung engineer. This is the first time the set has been opened! How can mine have a different motherboard sticker to others! How can I know if other boards will work?
My board is BN94-01264b and I can't find any with that number.
The one on eBay which is from the 46 inch version is BN94-01490e

[vinceroger69]

Re: LE40M86BD switching off/on

I wonder if budm or one of the members abroad will look on shopjimmy for you and see what they list as substitutes for your BN94-01264b board (as shopjimmys blocked uk ip address etc so its hard for us too search now) you also mention you have had the firmware updated by samsung in the past i wonder if you cant find a board if its worth trying a new eeprom chip if you can get one maybe a more experienced member will know if a bad eeprom chip can cause your issue your having. It may also be advisable if you have any ipa etc just clean the back of the board of etc it may not be water damaged but ive not saw one with that much staining on etc before.

[CapLeaker]

Re: LE40M86BD switching off/on

SJ doesn't list that board... Isn't there something equivalent to SJ in the UK?

[multimeter]

Re: LE40M86BD switching off/on

sounds like a cold joint on the power supply to me,try flexing the power suppy with a insulated plastic rod or simular,does it make the fault re-occur ?