Samsung Plasma PS50C7790 with black screen but sound

[Axesh]

Hi guys,

I got some trouble with my 3 year old Samsung plasma: it won't display an image (also the OSD) any more... , sound and everything is OK.

So I read some threads and stuff like that about this issue and afterwards I removed the back and checked all fuses and caps, but their all good on all boards (except the logic board, because of the mounted metal plate).

According to the information in the service manual from a different Samsung TV model I was told to measure the Vs and Va voltages from the SMPS. If their not normal without the connected boards the SMPS is the root cause.

So I did that and I measured the following:
- no boards, only SMPS: -> no Power LED on front, TV was not able to start with press on power button -> 0v on Va and 0v on Vs
- Main Board connected -> Power LED on front is working, TV could be started -> -0,1v on Va and 0,4v on Vs
- Main and Logic Board -> same behavior as above -> Va is decreasing rapidly from 55 to 0v, Vs is decreasing very fast from about 200 to 0
- Main, Logic Board and X-Sustain -> same behavior as above, TV takes longer to shut down -> Va is decreasing rapidly from 55 to 0v, Vs is decreasing from about 200 to 6,4v
- Main, Logic Board and Y-Sustain -> no special behavior -> Va is decreasing from 55 to 0v, Vs is decreasing from about 200 to now 10,2v
- Main, Logic Board, X- and Y-Sustain -> no special behavior -> Va is decreasing from 55 to 0v, Vs is decreasing from about 200 to 10,2v

Conclusion:
- with no board connected, no voltage is measurable, Va is always 0v
- with only logic and main board connected, Vs voltage is decreasing from about 200v to 0v
- with all boards connected, Vs voltage is now decreasing down to 10,2v, Va is 0v

According to these measures, I think the SMPS is responsible for the no picture issue, because Va should be 55v and Vs shoud be 211v as normale voltage. Or whats your opinion?
What can I measure to make it more clear?

Any hints how I can check the other boards?
How can I measure the Vsc, Ve an 2Ve voltages?, didn't find any measure points on the SMPS (only Va and Vs)

If only the SMPS is defective, I will exchange that, but I should be clear of that before I buy a new one.
But if the XSUS oder YSUS or the Main or Logic Board is defective, I don't know what to do...

Thanks in advance for your help.

Alex from Germany

PS: please excuse my bumpy english... :-o

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

If the control board detects a fault it may turn off the Vs and Va. The problem is, the fault includes "no board connected"! So you need to locate what fault the control is detecting.

The best way for us to help you with this is for you to show us a picture of the chassis layout of the TV, so we can see how everything is connected.

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

Hei Tom,

thanks for your reply. Here's a picture of the chassis layout of my TV:


From left to right, i think that should be:
Y-Buffer, Y-Sustain, SMPS, Logic Board (below SMPS under the metal plate), Main Board, X-Sustain and X-Buffer

Thanks for your help.

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

Can you show us a closer picture of the control board?

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

Hi Tom,

just arrived back from a short weekend trip, and as requested you will find some closer pictures of the boards seen above. I hope you dont need a picture of the logic board, because for that I have to dismount the metal plate. So, on short notice, here are the pictures of the other boards:

Main Board:
http://abload.de/image.php?img=mainboardzyj4c.jpg

SMPS:
http://abload.de/image.php?img=smpsfxjwc.jpg

X-Sustain:
http://abload.de/image.php?img=x-sustainx1jr1.jpg

Y-Sustain:
http://abload.de/image.php?img=y-sustaincrkxf.jpg

If you need some more, please let me know...

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

...Yeah, sorry, I need the logic / control board, behind the stand!

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

so, here's the logic board:
http://abload.de/image.php?img=logicboardvlpuj.jpg

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

OK, on the right of that board you've got the brown/orange LVDS cable. Unplug this. Then, above it, you have a four-pin header with pins numbered 1 thru 4. Place a wire jumper across pins 3-4. Then turn the TV on. Do you get test patterns?

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

and here a complete overview of the whole schematic:
http://abload.de/image.php?img=completevzrx4.jpg

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

Hi Tom,

No test patterns. Nothing different after disconnecting the lvds cable and set jumper 3/4 on logic board. TV display stays black.
The TV takes it time to start up, about 12 sec until the red led on front side stops flashing. So, same behavior like before... :-(

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

OK, next step. You've got a fuse on the C-boards for Va. Can you measure the DC voltage on each end (red probe on each end of fuse, black on chassis.) TV should be on. It should be Va on both ends, which looks to be 55V on this panel. (Use 200V DC range.)

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

what do you mean with c-boards?
do you mean the fuses with white color, like on Y- and X-Sustain and SMPS
Could you point them on the images or describe it in detail?

Shall I replug the LVDS cable and remove the jumper before measuring the fuses?

Thanks! :-)

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

Oops. I did mean to upload a picture. Let's try again...
TV should be in original state (LVDS in place)

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

Hi Tom,

I measured the fuses on the X-sustain at the top right corner. The green one has 15.3 on each side, the white one 10.3 on each side. The same applies on the Y-sustain board at the top left corner. For the SMPS: the white one on the bottom left (the upper one) is 0 volts, at the other one beneath the voltage is jumping from 0.1 to 0.3. The one next to the cooler near the middle of the SMPS is out of range, I don't get a plausible value, it only shows something like that: "_1__._". Does this help in any case? :-?

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

Sorry, I'm lost -- what does just the fuse I've highlighted read? Also, can you show me a picture of the range your meter is set on?

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

sorry for confusing you... i measured all the way long... :-)

For the marked fuse my meter shows 0.1v DC on both sides
It is set to 200 volts DC, like you requested.
If I switch it to 20 DC it show 0.14v for both sides of the marked fuse.

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

OK, that's definitely bad -- but to be sure, can you quickly measure a fresh 1.5V battery and check the reading on your meter appears good?

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

damn it. So whats wrong with it?
here the test with the battery..., with the same setting I used for the measurments above

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

Well, an important part of all diagnosis is to locate a fault, so in that way, it's good. It gives you information on where to look next to track down the final cause.

I've highlighted a point in RED on a capacitor, please measure the voltage here in the same way as before. Please measure it with the connector in black connected, and then without. Careful: the TV power MUST be off when removing or inserting the connector. Power should be switched on during the test.

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

no difference on this capacitor with or without the highlighted connector:
decreasing voltage from ~55 to nearly 0 volts in about 30-40 seconds