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

[joste]

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

Alas, the bottom of the board confirms my suspicions; the chip is a BGA. Nasty to fix if it goes wrong.

I was doing some searching and it seems your particular main board is also used in Polaroid TVs. The power supply seems to be working so it might be worth just replacing that board if you can find it.

In fact, my first thought was to change the mainboard, but where (and how) to find it? I'm not bad myself in searches over the Net, but never managed to find anything about this MB; Can you give me any model or reference to search for? That would be great.

Other thought: since we don't live too far away from each other, I'd be glad to mail this MB for some hot air resoldering if you'd accept?

[joste]

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

Hi Tom66,

I measured the voltage on both coils and yes, you're right about the voltage, but there's a catch: I get 3.3V (1st coil) and 5V (2nd coil) measured against positive power supply rail (+12V). If I measure it properly (against the ground), I get inverted values: -8.8V (1st coil) and -7V (2nd coil)! I'm a bit confused what is going on here?!

Just an hour ago I made it to power on once again, I didn't change a thing, the set responded to RC immediatelly after connected to mains. So, Tom66, you were right about ON/OFF pin - it does send 4.5V to the invertor, so the panel gets its backlight alrght, while ADJ pin had no voltage at all. On the other hand, the ON mode lasted a half of minute, so I didn't have time enough to find the source of 5V and 3.3V. Probably processor is not getting its supply voltage, therefore this malfunction. Yes, while working, processor gets pretty warm in just a few seconds, even working on idle. The EN25B64 (Serial Flash memory) has its 3.3V even in Standby mode.

[tom66]

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

The big coils. Measure the voltage on both ends, while the power is plugged in. One end will measure some random value, maybe even 0. The other should measure 5V or 3.3V.

It is not difficult to repair SMD component faults yourself; all you need is a steady hand, solder wick and fine solder. The last two you can get for about £5 total.

Alas, the bottom of the board confirms my suspicions; the chip is a BGA. Nasty to fix if it goes wrong.

I was doing some searching and it seems your particular main board is also used in Polaroid TVs. The power supply seems to be working so it might be worth just replacing that board if you can find it.

[joste]

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

Diodes around the coils on the left hand side (presumably a SMPS) are looking good both in Standby mode and disconnected. I have a fear that it could be one of SMC ICs just next to them. In that case, there's nothing I could do. Lets hope I'm wrong. Maybe one of these SMD transistors has a current leak?

[joste]

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

Hmm. I think the PSU is good then. Can you post a very high resolution shot (2000x2000 max) of your main board? I am confused at where the 5V and 3.3V is generated from. Maybe it is from what looks to be an SMPS on the left hand side.

Here is the Hi-Res of the MB. Hope it will do, 'cause my camera can't do better than this, sorry

[joste]

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

The pic:

[joste]

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

Here is the printed side of the mainboard. The points marked in red have the nominal voltage in Standby mode. Other voltages are missing in this mode.

I'll take a shot of MB in max resolution, hoping to be as good as needed for you to find what you think the "cause of death" is.

Tom66, thanks again for your help, I do appreciate it

[tom66]

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.

Hmm. I think the PSU is good then. Can you post a very high resolution shot (2000x2000 max) of your main board? I am confused at where the 5V and 3.3V is generated from. Maybe it is from what looks to be an SMPS on the left hand side.

[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.

[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!

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!

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!

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!

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!

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!

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!

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!

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!

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!

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.