Samsung Plasma PS50C7790 with black screen but sound

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

Did you measure the decreasing voltage on the fuse end too?

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

yes, same decreasing voltage on the fuse you highlighted for me.
From about 55 to nearly 0, on both ends. It stops a 0.1v after ~40 seconds

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

any advice from the experts on this issue?
Can I measure anything else to determine the faulty board?

Thanks in advance.

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

(Sorry, missed your post.)

I am thinking the logic board is shutting down the power supply. What do you measure on VSON?

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

I measured the voltage on the connector pins on the SMPS:
VSON is always on 0v ("standby" and "on")
PSON (standby) is 3.3v
PSON (on) is 0.2v

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

So even after the Vs/Va collapse, VSON is 0V? It should be high (>2V) then drop to 0V if the logic board detects a fault. If PSON drops out, it might be a main board fault.

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

your right, VSON jumps shortly after start up to ~3,3v for a second, then goes down to 0v
Any chance to identify the fault that the logic board is detecting?

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

I don't know for sure. Sorry. I am not familiar with these plasmas. You would need an oscilloscope to test the boards.

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

i can get one, thats no problem, but I'm not familiar with an oscilloscope. So it would possibly be a challenge to test the boards with your advice... :-o
Or what do you think?

My understanding is now: SMPS is not the reason
The logic board detects a fault on start up, and in this case it shuts down Vs to 15v and Va to 0v.
Each of the connected boards could be the reason for the fault, including the logic board.
Am I right?, or whats your conclusion?

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

I'm perfectly fine with helping you set up a scope to test your boards -- that won't be a problem.

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

ok, I will try to get one tomorrow... *thumpsup* :-)

I just checked the VSCAN and VE voltage:
VSCAN is also decreasing from the normal -195v to 0v
and VE is decreasing from the normal 89v to 14.8v

Damn... :-(

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

Vs is used to generate both Vscan and Ve, so the real problem is Vs-ON, which turns on/off the Vs voltage.

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

according to this video (http://www.youtube.com/watch?v=JjrwTZC5nVg), the fault could be possibly be located in Y-Main, Y-Buffer or X-Main board, because the LED on the logic board is blinking in a 5s rythm (5s on, 2s off, ...). PSON behaves normal, Standby is always on 5v, only VSON is not like it should be. But according to the video, the logic board should be okay, because VSON gets up to 3.3v for a second, and did not always stay on 0v.

So, what I have to set up to test these boards with the scope?
Did I need any special equipment?

Thank you very much for your support!

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

The scope needs to be rated to 400V and you need a 10X probe, that should be all.

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

Hi Tom,

the scope and 10x probe is ready for testing...
Here's a picture of the scope.
Its a Createc SC 01, with only max. input voltage 220 VDC/VAC
Does this also fit?

Please give me a "call" when you have some time to get me through this...

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

I don't know if I would trust that scope. It might be damaged by the high voltage a plasma TV uses. Do you have the manual for that?

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

Hi Tom,

no, I don't have a manual for that, and even didn't find one on the internet. But now I got another scope: HAMEG HM203-4, which is specified for 500V DC and peak AC. Can we work with that?

I attached a screenshot from the manual, which you can find here.

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

Hei Tom,

do you got some time to check the specs of the other scope?
Is it helpful for our test?

Thanks in advance...

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

The Hameg is a far superior instrument. I would recommend it.

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

so, do you have any advice what to measure on my TV?
Please keep in mind that I'm not familiar with that type of equipment... :-o