Samsung LE40A536 - constantly restarting

[Scenic]

Got a 40" Samsung LCD TV with an odd problem

Samsung LE40A536 40" Full HD LCD TV
PSU/inverter board: IP-211135A / BN44-00199A
Mainboard: BN41-01145A / BN94-02459 (also silkscreened as "530_NORMAL_idTV").

Stickers on the component side of the mainboard say
"TULIP 40 NEW 584 A" and "AMBER 40 DTV 636B".

All PSU caps have been replaced with low-esr Panasonic/Matsushita and a few Rubycon, and some horrible soldering from a TV tech (?) before me has been fixed up. I even replaced every single tiny 10uF 50V cap (i was in there anyways.. so why not..)

TV turns on fine, picture is perfect, but it automatically turns off / goes into standby after 15 or so secs and restarts immediately.
Selected source doesn't seem to matter.
You can turn the set off (standby) within the ~15sec timeframe when it works normally.

In short: everything seems to work fine (functions wise), but it constantly reboots itself.

Read about the EEPROM trick, and went hunting..
little 8pin sop chip on the solder side of the mainboard where the CI card slot is.

Chip markings Read:

ATMLH0834
2EB 1

google search lead me to this
https://cdn.badcaps-static.com/pdfs/...4bbddfe6a9.pdf

Part marking scheme fits perfectly, so it's a AT24C256B Two-wire Serial EEPROM. bingo.
Soldered 2 wires to pin 5 and 6, reinstalled the whole mess, twisted wires together, plugged the set in and as expected, it didn't turn on. Removed the short on the two wires, got the TV out of standby aaaand.. reboots after 15secs

I then put the whole mess back together (needed room), and I've left the 2 wires soldered to the EEPROM dangling out at the side of the TV case.

Randomly playing around with them somehow got the TV to turn on and actually stay on in DVB-T/DTV/Freeview/whatever it's called mode, and it kept running for 6+ hours without a hiccup. Picture, sound, menus.. everything worked like it's supposed to. I then put it into standby and tried powering it up again.. and the same 15sec. fault came back

The EEPROM reset did something, but not what i was expecting. When i go into the service menu now, the Power On timer was reset to zero from the roughly 5900 hours it showed before.
Thinking that the EEPROM might be bad or corrupted, i bought a 5pack of new ones on ebay (cost about the same as one of them in the local electronics store). They arrived after more than 1 week of waiting (from a seller in germany.. ridiculous)

They're new cut-off-tape EEPROMs, supposedly being STmicroelectronics ST24C256B. However, I can't find any form of datasheet whatsoever on the net. Searching for the markings on the chip ("4256BWP") gives no useful results. Let alone datasheets.
So either i got something else than a 256KBit 2-wire serial EEPROM, or ST sucks about as bad as Atmel for cryptic labelling of their SMD devices.

Anyways.. soldered a new one in, slapped everything back together aaand.. it doesn't work at all

After plugging it in, only the standby LED is on (doesn't turn on by itself as it normally would). Trying to power it up with the power button in the front does nothing at all (LED stays solid, no reaction).
When you try it with the remote, the LED turns off (as if it's about to turn on), nothing happens (no backlight, no sound), and it goes back into standby (solid red LED) after ~10-20 seconds.

I've tried it multiple times, and let it sit there for a while just in case it was still doing something (re-writing the EEPROM perhaps).. but no. Same thing. In fact, it acts 100% the same as if there was no EEPROM at all (yes.. i've tried that too).

I'm completely lost now.
Other than trying to source an Atmel AT24C256B hoping it's just a compatibility issue with the above ST variant, i have no clue what else to try.

The whole thing has been a total waste of time and money for me so far..

Oh and yeah.. replacement mainboards aren't available anywhere, and very similar mainboards from other TV series (which would fit physically, have the same connectors in the same orientations and positions and matching screen resolution) are expensive as hell, and there's obviously no guarantee that it's going to work at all.

Crappy youtube video, showing the 15sec. problem
http://www.youtube.com/watch?v=xlfDREBcevs

Any ideas?
or can i finally bash this thing into pieces for being so frustrating and a general waste of money..?

PS: no service manuals available. At least I couldn't find anything except part of the full manual on ET that only explains how to take this piece of shit apart, which i found after i already took it apart.

Attached is a pic of the back of the mainboard. If you need more pics, just say something.

[randtek]

Re: Samsung LE40A536 - constantly restarting

I'm thinking your new EEPROM is clean-not programmed. When you change an EEPROM, it must be programmed with the basic data that the TV is expecting or it will not work. If you did not get from the manufacturer of the TV, it is unlikely that the EEPROM will be programmed for your circuit board.

[Scenic]

Re: Samsung LE40A536 - constantly restarting

The EEPROM i changed is just for the user settings (channels, brighness settings, bass, etc.). Not the main firmware EEPROM.

Blank ones will normally get re-programmed on first startup (which takes a while), but in this case here it doesn't seem to want to do that. Or the chips i got aren't EEPROMs.. can't tell cause i can't find a datasheet for a "ST 4256BWP" (sold as ST 24C256).
All i can try now is to source the same EEPROM that was originally in there (Atmel AT24C256B), hoping it's a compatibility issue or something.

Otherwise this thing is pretty much ready to be scrapped. New mainboard costs about as much as i paid for the damn thing.

If even Plainbill and the other TV gurus don't post in here, this seems to be pretty much a lost case i think

[graham01]

Re: Samsung LE40A536 - constantly restarting

That is a very strange fault, I have done a few 40" TV's that have had cap problems, and after replacing them, if they still dont work, resetting by shorting out the EEPROM pins fixes it, if not, a new EEPROM does. (Strangely only the R88 models have required a new eeprom in my experience) I have never had to go further than replacing the EEPROM, but I have also never had that problem with restarting after 15 seconds. It seems to long to be a safety device tripping too.

If you disconnect the LVDS does it still do that? What is the model number of the mainboard? I have 3 here, it is a long shot but if I have it you are welcome to take it off me.

[tom66]

Re: Samsung LE40A536 - constantly restarting

ST 4256BWP is not a 24C EEPROM. All 24C series eeproms begin with "24" and most with "24C". You were probably sent the wrong part.

[Scenic]

Re: Samsung LE40A536 - constantly restarting

If you disconnect the LVDS does it still do that? What is the model number of the mainboard? I have 3 here, it is a long shot but if I have it you are welcome to take it off me.

I haven't tried to disconnect the LVDS cable yet. Gonna do that tomorrow. It did work perfectly fine for hours when i somehow got it to stay on once though.
I know these TVs have a watchdog timer (the LExxA/B/C/Dxxx series apparently run linux), so that might be part of the reason.

Motherboard model is BN41-01145A / BN94-02459
It's also silkscreened as "530_NORMAL_idTV" and on the component side, there are 2 stickers saying "TULIP 40 NEW 584 A" and "AMBER 40 DTV 636B"

The service menu identifies it as a "TULIP" (can be seen in the youtube video)

The front of the board looks almost identical to these two
Those are not identical to my board (different part number). I just post these to give you an idea of what it looks like, as I currently don't have a photo of mine. Gonna take a couple pics tomorrow when i take it apart again

http://shop.sjstv.co.uk/ekmps/shops/...72x-2250-p.jpg
http://shop.sjstv.co.uk/ekmps/shops/...672d-614-p.jpg

The only visual difference is that mine uses a pin-header for the LVDS cable instead of a flex cable connector seen in the pics (top left, you can see where the solder pads for the pin header are)

edit:

ST 4256BWP is not a 24C EEPROM. All 24C series eeproms begin with "24" and most with "24C". You were probably sent the wrong part.

I feared something like that. But WTH is it then?
I mean.. the original Atmel EEPROM was labelled

ATMLH0834
2EB 1

and it took a lot of google searching to find out that it's a AT24C256B .. i actually found this via a post on a russian forum, including the link to the datasheet, where in the part marking section, it says that 2EB 1 means 24C256B. Who the hell thinks of crap like that?
You need the datasheet to find out that 2EB 1 means 24C256, but you can't find the datasheet cause it's not called what's written on the chip. WTF

I'm gonna go ahead and buy a real 24C256B. Seems to be the next-cheapest try before considering a new/old mainboard.

[graham01]

Re: Samsung LE40A536 - constantly restarting

Sorry, don't have that board good luck with it!

[PlainBill]

Re: Samsung LE40A536 - constantly restarting

What is the history of this TV? It certainly sounds like corrupted firmware. Is an updated firmware available?

PlainBill

[Scenic]

Re: Samsung LE40A536 - constantly restarting

Previous owner bought it new, and after about 2 years, it started having trouble to turn on out of standby (bad caps ; referred to as the clicking problem on those Samsung LCD TV's that have a relay.. this TV doesn't have one).
TV shop replaced some of the caps (and tried replacing some SMD electrolytics on the mainboard, ripping ground pads off.. i fixed that up). TV turned on no problem, but this weird 15 second problem appeared.

The shop told the previous owner she'd need a new PSU, quoting 200euro costs (~ 270usd), which is what made her sell it dirt cheap.

Ive replaced the rest of the PSU caps (Samwha XC) that the TV repair shop didn't touch, down to all the tiny 10uF 50V caps. PSU is pretty much ruled out. Couldn't find any problems with it.
So the only thing left is the mainboard unfortunately

Coudln't find any firmware updates for it. In fact, i have a hard time finding anything for this model..
Samsung only has it in the Product archive (in german), and the downloads section just lists 3 manuals.
http://www.samsung.com/at/consumer/t...LE40A536T1FXXC

edit: interestingly, only Samsung Austria lists this model. Samsung Germany doesn't have it on their page at all.

[PlainBill]

Re: Samsung LE40A536 - constantly restarting

The best idea I had is based on the fact that there are two possible scenarios.

The first is the main board is controlling the activity via the PS_ON line. After 15 seconds it switches the main power supply off, then the main board switches the main supply back on. The other is PS_ON stays active; the power supply drops out momentarily.

As a test to determine which is happening, monitor or control the PS_ON line to take control away from the main board.

PlainBill

[Scenic]

Re: Samsung LE40A536 - constantly restarting

Okay.. back to this thing.

Unplugging the LVDS connector didn't change anything (apart from me not being able to see what's going on)

All PSU voltages seem to be normal. They're dropping noticeably under load (13.5V vs. 12.7V for example), but no sudden collapse that would cause it to restart.
Then i went checking the PS-ON signal and indeed, it drops from about 3.3V to 0V when it's restarting and comes back up.

Taking control away from the motherboard would require injecting 3.3V in there.. but from where? 5Vsb via resistor with the pin going to the mainboard disconnected (pulled out of the plug) ?

Anyways.. I've shot a bunch of pics, some of which turned out suprisingly good, while others turned out just to be a multi-colored pixel-salad.
If you need any detail shots, just let me know.
Original resolution is 4mp (2300 x something). I hope the ones I've uploaded are good enough.

mainboard2, 3 and 4 show the various model designations (as if it wasn't confusing enough already), mainboard5 shows the LVDS connector (pin header instead of flex cable connector), PSU3 shows the 2 connectors going to the mainboard.
For some reason, the pin number labelling is flipped horizontally (on every single connector on the board) compared to the rest of the text, so i added them the right way around.

edit: forgot to mention: I didn't rip off the ground pads for the SMD caps on the mainboard. That was the previous "TV repair shop" (I kinda doubt that it actually was one).
I did replace the general purpose 85°C Jamicons that they "soldered" in there (just barely) with Chemicon LXF though.

[retiredcaps]

Re: Samsung LE40A536 - constantly restarting

Taking control away from the motherboard would require injecting 3.3V in there.. but from where?

I haven't thought it out, but use an ATX power supply to supply 3.3 V DC somehow?

[Scenic]

Re: Samsung LE40A536 - constantly restarting

ooookay.. now this ended up weird.

I soldered wires to the back of the PSU board, going to ground and the PS-ON pin (clearly labelled in the last pic in post #11).
Then i popped the wire out of the connector going to the PSU (it's the kind that's held in the connector plastic with a metal tab) and isolated it.
I was planning on using 2x AA batteries in a battery holder as a substitute, as i had this stuff at hand.
Connected everything up, had the battery holder ready (no batteries, so nothing going to the PSU) and plugged the TV in. To my suprise, the darn thing turned on without the 3.xV being present at the PS-ON pin.. wtf?

And it shut itself off after 15secs and restarted... all without any PS-ON connection to the mainboard. what the hell

[PlainBill]

Re: Samsung LE40A536 - constantly restarting

ooookay.. now this ended up weird.

I soldered wires to the back of the PSU board, going to ground and the PS-ON pin (clearly labelled in the last pic in post #11).
Then i popped the wire out of the connector going to the PSU (it's the kind that's held in the connector plastic with a metal tab) and isolated it.
I was planning on using 2x AA batteries in a battery holder as a substitute, as i had this stuff at hand.
Connected everything up, had the battery holder ready (no batteries, so nothing going to the PSU) and plugged the TV in. To my suprise, the darn thing turned on without the 3.xV being present at the PS-ON pin.. wtf?

And it shut itself off after 15secs and restarted... all without any PS-ON connection to the mainboard. what the hell

Some designers have done weird things with the PS_ON line. One I've seen is a pull-up resistor on the PS_ON pin so the power supply will turn on if the connector to the main board is disconnected.

Now you have established that the TV continues to cycle. Have you checked the output voltages of the power supply? Do they change too?

PlainBill

[Scenic]

Re: Samsung LE40A536 - constantly restarting

The voltages are normal so far (just dipping down a bit under load). They don't suddenly dip down to less than what they're labelled as. (i.e. 13V doesn't go down to 10V or less causing it to shut down)
The moment it restarts, they're pretty much immediately at 0.x V and back to nominal within 1 second.

This is kinda hard for me to explain (not a native english speaker), but it looks like the PSU tries to turn itself off normally, rather than shutting itself off because it detected a fault (a voltage going out of spec for example)

Sort of like a PC motherboard trying to turn the ATX PSU off, but it can't because something's pulling PS-ON to ground. (just as an example of what i mean)


I've just tried something..
With the PSU board completely disconnected from the mainboard, all voltages (multiple 12.5-13V lines and 5V) are constantly present, i.e. none of them ever turns off, which seems strange.. aren't they supposed to only come on when there's a PS-ON signal coming from the mainboard (with the exception of 5Vsb of course) ?!

Following the PS-ON trace, there are a few resistors in its way. It goes off to 2 resistors, a transistor and then to one of the optocouplers, the other trace goes over 2 jumpers (red) to a resistor, which then goes to a pad with a resistor and capacitor to ground and yet another tiny transistor.. from there on it gets kinda hard to follow, but it looks like it ends up controlling the TO252 device that's half cut off at the top of the photo.

I'm gonna try to take a better photo of that part of the board.

[Scenic]

Re: Samsung LE40A536 - constantly restarting

Hmm... I've tried checking that TO252 component at the top center in the pic above.
Was very hard to read what's engraved, but i managed to read 40P03-20, which according to alldatasheet seems to be a 40P03 P-channel power MOSFET.
Either I'm measuring this wrong (diode/beep test) or this thing is actually shorted on all possible pin combinations (at least in circuit)

[Scenic]

Re: Samsung LE40A536 - constantly restarting

Got it desoldered to test it out of circuit. After heating the solder up for about 5secs. it started to smell funny. you know.. the something-got-awfully-hot smell. not a good sign.

Tried checking it according to the simple method posted here
In this datasheet, there's a block diagram showing a diode between drain and source, so i expected to get a reading one way but not the other.
Unless I'm too dumb to measure this, it looks like there's a short between drain and source, as no matter which way around I connect the leads of my DMM, i always get 0.00 ohm.

So.. bad MOSFET after all?!
Seeing how it's connected to the PS-ON line via a tiny driver transistor and a bazillion of resistors, I think this would explain why all output voltages were present all the time. The PSU simply couldn't turn itself off.. (?!)

I'm kinda clueless atm

[Scenic]

Re: Samsung LE40A536 - constantly restarting

Bought a new-old PSU board (IP-211135A / BN44-00199A) on ebay rather cheap. Waiting for that to arrive.
Also bought a bunch of Atmel AT24C256B EEPROMS, not some substitutes like the weirdo ones I've bought earlier (those mystery ST 4256BWP).
Waiting for these too at the moment.

We'll see how it goes.
This TV has been a massive waste of time and money so far. I'm kinda tempted to look for / buy a cheap 40" Full HD Samsung with a cracked screen, but otherwise working and just swap the panels, as I know that my panel and inverter are both OK.
Seems to be the easiest solution apart from selling this TV for (or in) parts, with the only definitely OK part (the panel) being hard to ship, and without shipping (local pickup only), I won't really get anything for it.

[Scenic]

Re: Samsung LE40A536 - constantly restarting

• Replacement PSU board didn't change anything (big suprise)
• Replacing the EEPROM made it rewrite its contents, and then do the same ~15sec. restart bullshit.

Bad mainboard. Perhaps corrupted firmware or something, or some component hiding somewhere that's bad, but doesn't show any visible signs of failure.

Bit the bullet and bought a tested and working mainboard on ebay for 50eur w. free shipping out of a working TV with a crack in the panel. It took over 3 months for one of them to pop up on ebay at all, so I grabbed it while I could.

Should arrive in a few days.

Still puzzled as to what causes this..
I might be able to dump the EEPROMs (main firmware and others) and rewrite the ones on my dud board. I have a feeling that the main firmware somehow got corrupted or something (messed up OTA update!?) and something's not working right, causing a watchdog timer of some sort to reset/restart the TV.
Background: The mainboards of the A/B/C/D series (LE40A536) Samsung LCD TVs apparently run a flavor of linux internally, with a serial debug port in the form of a 3.5mm headphone service jack on the back. Can be seen in the attached photo.

The one time I got it to stay on I either somehow "made the failing part work" temporarily (which I doubt), or caused the watchdog timer not to take action for some reason (which seems more plausible IMHO)
At least that's my best guess as to what the hell is/was going on..

[tom66]

Re: Samsung LE40A536 - constantly restarting

I thought only the newer and higher end Samsung plasmas and LCD TVs used Linux. The older ones use an ARM processor but a custom operating system. That being said, wouldn't be the first time I've seen Linux in a TV (including one Samsung plasma I dealt with)...