Samsung Plasma PS50C7790 with black screen but sound

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

Do you have a probe with a 1X/10X switch?

Can you post a close picture of the Y-main, on the connector which goes to the buffer boards (long boards along the left side, from the rear.)

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

yep, I got one of these 1/10x probes, and here's the picture of the Y-Buffer Board

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

OK, please follow instructions completely.

• Turn instrument on.
  
  
• A trace - a horizontal line - should appear on the display.
  
  
• Set the INTENS knob so the trace is about mid brightness.
  
  
• Set the FOCUS knob so the trace is as clear as possible.
  
  
• Turn AMPL I to 10V/div (*not* 10mV/div)
  
  
• Move the Y POS I so the horizontal line is two vertical divisions from the bottom of the screen. The display is divided into 8 vertical divisions approx 1cm each.
  
  
• Make sure INVERT is not depressed.
  
  
• Make sure X MAG 10 is not depressed.
  
  
• Make sure COMPONENT TESTER is not depressed.
  
  
• Make sure DUAL/CHOP is not depressed.
  
  
• Under AMPL I there is an AC/DC/GND control, set this to DC.
  
  
• On AMPL I turn CAL arrow so it locks in the CAL position.
  
  
• Set TIME/DIV to 2ms/div
  
  
• On TIME/DIV turn CAL arrow so it locks in the CAL position.
  
  
• Set TRIG to TV:H and DC.
  
  
• With TV off, connect hook end of probe to red point (of the blue capacitor, the exposed lead) and black connector to the screw highlighted.
  
  
• Turn the TV on and post what you see.

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

everythings set, except "Set TRIG to TV:H and DC"
I put the trigger selector to DC, but theres no TV:H...

What do you mean with "turn CAL arrow so it locks in the CAL position"
Is that the furthermost left position, on 9 o'clock?, that the only position where the knob is "locking"

My 10x probe doesnt have a hook, only a thin point.
So the black one you described is my one with the crocodile clamp?

But with this setting, a doesnt get a peak on the scope...
Any easy setup to test the scope?

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

Your scope is in the CAL position.

It's a little different to the photo I worked off, so instead set TRIG to ~ and do not use the TV:H.

You need to get a probe with a hook, you can touch it against the capacitor lead but it will be very hard to take a picture at the same time while not being electrocuted. Hook probe is "hook and forget", allowing you to easily take a picture.

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

do you mean AC with ~ for the Trigger Selector?

I got a probe with a hook, but its only a x1 probe.

I put it in Channel 1, but still no peaks on the scope... :-(
Is there an easy setup to verify that the scope is working correctly?

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

Set volt/div to 500mV/div, timebase to 200us/div, and connect to the CAL 2V point.
Do you see a waveform?

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

Edit...

with the calibrator I got something like this (the dottet line is a complete line, like the bottom line):
----_____-----_____

but what should i have to set on the trigger selector? ac, dc, hf, lf or line?
or does it matter?

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

Doesn't matter for now, just that proves the scope is working. Go back to original settings except set the TRIGGER to LINE.

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

with the original setting from you and the x1 probe with the hook (the other one doesnt work with the calibrator) the flat line jumps up a little bit (1/10th of a div) with clicking of the relais by start and shut down of the TV.
When i measure that with the scope meter, it displays a voltage of 0.39 on start up and an -0.39 on shut down

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

OK, it sounds like the Y-main is not working. Can you post a picture of the X-main now -- the opposite board to the Y-main.

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

here you go...

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

Before you do the X-main test please tell me if you see a brief wave (even just a flicker) appear on the Y-main output or is it continuously dead?

For X-main test, same settings as before, but test as indicated in the picture.

[Axesh]

Re: Samsung Plasma PS50C7790 with black screen but sound

no, with the posted setting for the Y-sustain I only got a litte jump from the horizontal line of the oszi when I start up the TV from standby. No wave or rectangle impulse at all. Can I try a different setting at the scope?

On the X-Sustain its compareable, but the jump is 1 division up. After a cold start, the jump goes fast to 3/5ths of the next division and then goes slow to reach the end of this division. But also no wave or increasing line on the display of the scope: just the whole line goes up and down after shutting down the TV.
In volts I measure 10.5v with the meter on this position.

Does this makes any sense?

[tom66]

Re: Samsung Plasma PS50C7790 with black screen but sound

OK. This means it's unlikely to be something related to the X-main or Y-main, and likely to be on the logic board or main board. Can you repeat the Y-main test one further time, but with the Y-buffer completely disconnected?

[DonLawrence]

Re: Samsung Plasma PS50C7790 with black screen but sound

Hi community,

I use this thread, because I have the same problem with my Samsung since yesterday, and this thread contains all the necessary pictures already...

I am electronics engineer and have measurement equipment available but I must admit that Plasmas are not really my field of expertise.

Without schematics it is hard understand the details of this device but this is what I know so far:

The TV switches on and I hear the welcome sound but there is no picture. There are no obvious bulgy Caps on any boards. The Logic Board seems to try to enable Vs right after switch-on (I measure the 3.3V level for about 0.5s after switch-on at the respective SMPS input pin but it withdraws the signal after 0.5s)

During this short time I see a 211V-level at Vs, which slowly decays to 0V after Vs_EN is reset.

This safety- behaviour is normal but I do not know how to identify the error source, since all other PCBs are connected to the logic board and a missing board is also a valid error condition obviously...

Since Vs toggles, I do not think that the logic board itself is the source of error.

Who can aid me with further procedures to hunt down the faulty board?

Best Regards,

DonLawrence

[DonLawrence]

Re: Samsung Plasma PS50C7790 with black screen but sound

OK, I did not watch the video before I posted and now I followed the procedure of tom66 as well.

My TV also shows the 5s/2s On/Off scheme from the Samsung video!

I measured the Y-Main and X-Main at the respective Capacitor pins and I see positive going pulses and a slight movement of the baseline during the short presence of Vs before safety- shutdown on my scope with both boards. I did not test Y-Main without Y-Buffer, since it is hard to detach it from Y-Mains, even after removing all panel flexes and all 4 mounting screws...

At least this seems to be different from the original thread where no pulses were observed...unfortunately the thread stops at this point.

Can anybody tell me what should be observed at these points?

[attainteddragon]

Re: Samsung Plasma PS50C7790 with black screen but sound

i would try to run the test patterns. disconnect the lvds cable from the control board and place a jumper between pin 3 and 4 of the auto/ext/gen.

[DonLawrence]

Re: Samsung Plasma PS50C7790 with black screen but sound

That does not work. The TV shows the same behaviour and I expected it because the test patterns can only be displayed with intact panel circuitry. The Samsung video says that the reason for the Logic Board LED to toggle in a 5s/2s timing is either Y-Main or X-Main or Y-Buffer. Of course I can try to check each component on each board but that is a very tedious work and I prefer measuring at the right points to track down the problem.

Does anyone know a good procedure to do this?

[attainteddragon]

Re: Samsung Plasma PS50C7790 with black screen but sound

does va rise to it's respective level when first powered on?

what year is the manufacture of this set?