Seeltech ST2468DHD 24" LCD HD TV Standby only - help needed!

[joste]

Hello all!

Yesterday my Seeltech ST2468DHD 24" LCD HD TV has stopped working. It was on and after few minutes it went to Standby all by itself, then two seconds later came on again. It didn't take too long to repeat this and then I decided to stop it, unplugged power cord, waited a bit and plugged it back again. But, this time TV didn't want to turn on, it was constantly in Standby mode with red LED on.

I have changed all electrolytic caps on its Power/Invertor board (producer is AOUYAN, model LP220401 REV1.8.) but it didn't make any change. I still have stable 12.08V voltage and red LED on. The set doesn't respond neither to IR remote, nor to its keys.

So, I wonder if you guys would have any advice on this, I will appreciate it very much. If you'd need more details, please let me know. For now, I'll hang on the photo of the power/invertor board, maybe it'll bring some memories back.

Thanks!

[joste]

Re: Seeltech ST2468DHD 24" LCD HD TV Standby only - help needed!

No ideas?

[tom66]

Re: Seeltech ST2468DHD 24" LCD HD TV Standby only - help needed!

Can you photograph the top and bottom of the board around the connector which goes to the main input board? From this position it looks like it only has one or maybe two power supply rails; this will make it easier.

[joste]

Re: Seeltech ST2468DHD 24" LCD HD TV Standby only - help needed!

@Tom66,

Thanks for your reply. Here are the pics, and I can tell you that I've tested all diodes (even SMD ones) and they seem to be OK, even Schottky. I even managed to turn it ON once (just by being stubborn), but it lasted for 30 sec and TV went to Sdby all by itself. Since then, no luck at all. The whole Power/invertor board has been properly recapped, still the same.

Yes, you'll notice just two 12V rails, one ON/OFF and one ADJ. The rest is GND.
So, the set is always in Standby, no way to make it turn ON.

I feel it's some dumb thing, but I can't find it and it is driving me a bit crazy (the TV is my only 1080p TV screen).
Let me know if you'd need any additional info, I'll be glad to provide it asap.

Thanks and looking forward to your thoughts.

[tom66]

Re: Seeltech ST2468DHD 24" LCD HD TV Standby only - help needed!

@Tom66,

Thanks for your reply. Here are the pics, and I can tell you that I've tested all diodes (even SMD ones) and they seem to be OK, even Schottky. I even managed to turn it ON once (just by being stubborn), but it lasted for 30 sec and TV went to Sdby all by itself. Since then, no luck at all. The whole Power/invertor board has been properly recapped, still the same.

Yes, you'll notice just two 12V rails, one ON/OFF and one ADJ. The rest is GND.
So, the set is always in Standby, no way to make it turn ON.

I feel it's some dumb thing, but I can't find it and it is driving me a bit crazy (the TV is my only 1080p TV screen).
Let me know if you'd need any additional info, I'll be glad to provide it asap.

Thanks and looking forward to your thoughts.

That's odd. It appears that there is no standby bus in the TV, am I correct? So I think we can rule out the power supply itself as being at fault. Can you attach a picture of the main board?

Then again, I see the ON/OFF pin. Perhaps this is only for the backlight...

Are there any part numbers on the power supply board which might be helpful in tracking down a service manual? e.g. on one power supply I have here there is a sticker saying "DPS-182BPB", which brings up hits for Philips 32" TVs. (Which is correct.)

Which capacitor brand and series did you use to do the recap? They look like Jamicon, which are general purpose capacitors and these will probably not work so well.

As you were able to get it to turn on once, see if you can do that again, and measure the power supply rails: if they are too low then the TV could be shutting down because of this.

[joste]

Re: Seeltech ST2468DHD 24" LCD HD TV Standby only - help needed!

Power/Invertor board producer is AOUYAN, model LP220401 REV1.8. and so far I haven't found anything on it, except for one single forum in Italian where people have had the same board in some other Sansui TV set, but there the problem was clearly burnt active semiconductors (MOSFET and the rest) and after all they used an external 12V PSU to power up the set as invertor worked well with it.

Capacitors used were really Jamicon 1000uF/25V 105°C and I have used them to repair other sets (mostly Samsungs) and it worked well, so I don't see why they wouldn't work here as well?

I'll try to be stubborn again and make it power on again, measuring stability of these 12V and what happens with pin ON/OFF.

The mainboard is some CV209L, no hits at all. the board has many unused connectors for a DVD player as an option (for other models). I don't find anything else to be so significant about it. And yes, the CPU is covered with an aluminum cooler, so I don't know what it is. All I know is that it used to work very, very well until two days ago and then - poof!

Thanks again Tom66, please let me know your thoughts.

[tom66]

Re: Seeltech ST2468DHD 24" LCD HD TV Standby only - help needed!

Jamicon are okay - the set should power up with them. But it will probably die in a year or so using those. Regardless, I don't think those are the main problem.

It's pretty common to see the main processor having a heatsink. That chip integrates a CPU, video scaler, and audio decoder all-in-one to reduce the cost. Probably also has a DVD player decoder for the DVD models.

I've highlighted what I think are some voltage regulators. Using a digital voltmeter, measure the voltage at all three exposed pins and the metal tab for every one and report back. If one pin doesn't connect to the board (like with the big regulator, bottom left), no need to measure it. Also, report back the model number on the chip, it will be something like "AM1117-33", for a 3.3V regulator of the AM1117 series. To measure, use the red positive lead to probe a pin, and connect the black lead to the metal case of the TV, or the back of the LCD panel itself.

Also, could you take a higher resolution picture of that board? I'm struggling to read the component IDs (U9, R82 etc.) on it.

[joste]

Re: Seeltech ST2468DHD 24" LCD HD TV Standby only - help needed!

Thanks, Tom!

Well, I agree with you regarding caps, and it also has crossed my mind to search for these voltage regulators on mainboard. The biggest one (first one on the far left corner of the mainboard) is BM1084, probably giving supply voltage (5V) to CPU or even USB port. I was about to test them all, but I need to have a snack first, I'm starving :p

I'll be back with readings asap, thanks!

[tom66]

Re: Seeltech ST2468DHD 24" LCD HD TV Standby only - help needed!

Thanks, Tom!

Well, I agree with you regarding caps, and it also has crossed my mind to search for these voltage regulators on mainboard. The biggest one (first one on the far left corner of the mainboard) is BM1084, probably giving supply voltage (5V) to CPU or even USB port. I was about to test them all, but I need to have a snack first, I'm starving :p

I'll be back with readings asap, thanks!

The CPU is likely to use a 1.2 - 1.8V core voltage, at the very most 3.3V. One of those regulators probably provides 5V to the USB as you say.

[joste]

Re: Seeltech ST2468DHD 24" LCD HD TV Standby only - help needed!

There are 3 jumpers around (3.3V, 5V and 12V), currently set to 5V; wondering what it was for?

Yes, this regulator is for USB, but didn't I say that the problem occured while watching some mpeg-coded content from my USB Stick and continued while using HDMI-source? Would this be relevant?

Right now can't turn it on, I keep pressing buttons full steam, but no luck yet.
I'll keep you posted.

Thanks

[tom66]

Re: Seeltech ST2468DHD 24" LCD HD TV Standby only - help needed!

There are 3 jumpers around (3.3V, 5V and 12V), currently set to 5V; wondering what it was for?

Yes, this regulator is for USB, but didn't I say that the problem occured while watching some mpeg-coded content from my USB Stick and continued while using HDMI-source? Would this be relevant?

Right now can't turn it on, I keep pressing buttons full steam, but no luck yet.
I'll keep you posted.

Thanks

I don't think it's the cause. Unless that voltage regulator's output is monitored (so it can show an error if you are drawing too much current), which is a possibility, but it's more likely one of the other regulators has failed, or you have another problem at hand. The fact that you got it to power up once tells me it's intermittent; I'm just hoping it's a simple bad capacitor or cold joint.

[joste]

Re: Seeltech ST2468DHD 24" LCD HD TV Standby only - help needed!

Tom, should (would) it power up without LCD panel and lamps connected? The cables are short so they wouldn't allow me a bit of space to conduct measurements while open (hopefully once it powers on again).

Tested for cold joints, no luck there neither.

[tom66]

Re: Seeltech ST2468DHD 24" LCD HD TV Standby only - help needed!

Tom, should (would) it power up without LCD panel and lamps connected? The cables are short so they wouldn't allow me a bit of space to conduct measurements while open (hopefully once it powers on again).

Tested for cold joints, no luck there neither.

Ideally, at least the lamps should be connected. I think you can go without the panel.

Cold joints could also be in the processor area. As it has a heatsink it gets hot. Remember Xbox 360 and BGA joints cracking? Not sure if it is a BGA or QFP. Anyway...

[joste]

Re: Seeltech ST2468DHD 24" LCD HD TV Standby only - help needed!

Yes, Xbox was really nasty one... OK, I go pressing "Power" button again

[tom66]

Re: Seeltech ST2468DHD 24" LCD HD TV Standby only - help needed!

You can try measuring the LDOs with and without power. Both will be useful. A little trick to try: press the button, pull out the power, and *just* before the LED goes out plug it back in and see if it starts.

[joste]

Re: Seeltech ST2468DHD 24" LCD HD TV Standby only - help needed!

Nope, can't make it ON no matter what I do. LDOs seem to be OK measured without power.
I can tell you I got a bit tired, I'm on it since this morning (I'm in Europe).
Tom, I would like to thank you for being with me all day long and I'll try to continue after a nap.

See you tomorrow, bye!

[tom66]

Re: Seeltech ST2468DHD 24" LCD HD TV Standby only - help needed!

Nope, can't make it ON no matter what I do. LDOs seem to be OK measured without power.
I can tell you I got a bit tired, I'm on it since this morning (I'm in Europe).
Tom, I would like to thank you for being with me all day long and I'll try to continue after a nap.

See you tomorrow, bye!

I'm in the UK so it's normal time here. It would help if you posted the list of measurements anyway, I am trying to think of all the things that cause this problem and I have heard LDOs are a big problem.

[joste]

Re: Seeltech ST2468DHD 24" LCD HD TV Standby only - help needed!

Here is the list of measurements for LDOs from left to right on the picture above:
BM1084 (the USB guy) has 0.73V between pins 1 and 3 (GND and Vin). All other LDOs have plain zeros. Measurement perfomed in Stby mode, so no wonder. What I really wonder about is what component should be in charge of turning the set from Stby to ON mode? Should it be the CPU and some other component as well? I don't manage to figure it out. I've noticed that IR sensor 5V voltage drops to 4V for a second every time I press Power button on the RC, but nothing else happens.

[tom66]

Re: Seeltech ST2468DHD 24" LCD HD TV Standby only - help needed!

Here is the list of measurements for LDOs from left to right on the picture above:
BM1084 (the USB guy) has 0.73V between pins 1 and 3 (GND and Vin). All other LDOs have plain zeros. Measurement perfomed in Stby mode, so no wonder. What I really wonder about is what component should be in charge of turning the set from Stby to ON mode? Should it be the CPU and some other component as well? I don't manage to figure it out. I've noticed that IR sensor 5V voltage drops to 4V for a second every time I press Power button on the RC, but nothing else happens.

Probably one of the blue or white wires tells the PSU to come on. But then again it can't work without a standby supply. Are you getting 12V on the positive wires?

[joste]

Re: Seeltech ST2468DHD 24" LCD HD TV Standby only - help needed!

Yes, 12V (12,08V) come to the mainboard, but I couldn't find where do they exactly disappear. What would you suggest to look for in this case?

Tomorrow I'll post a picture of the printed side of the mainboard where 5V and 3.3V are present in certain test-points while in Standby. Again, in other TP they're absent.

Another remark: on the top lefthand corner of the MB there is a placement for external power supply (again these famous 12V) where this voltage is present in Standby mode.