Samsung HP-S4253 Rapidly Flashing Screen

[johnboy1313]

Just picked up this Samsung plasma for next to nothing. At the time I went to check it out, it was doing this on the VGA input, but stopped when I changed to the AV input. I brought it home, connected my DVD player to it and watched a few movies with no issue whatsoever. This morning, I turned it on and the screen was flashing on every input. It flashes from super bright to normal about every half second at a steady rate. I haven't opened it up yet. I've got two other plasma sets on the bench right now. I'm just wondering where to start when I do get it opened up, probably later today.

Here's a video
http://s177.beta.photobucket.com/use...E3F3B.mp4.html

[tom66]

Re: Samsung HP-S4253 Rapidly Flashing Screen

When the plasma screen is going very white, it is arcing over internally and most pixels are being forced on (wall charge on each cell is sticking.)

This means one of two things:
- Vs which is normally 180~210V (typically 205V for a Samsung of this generation) is probably exceeding 220V, maybe 230V (possibly just a dirty/dodgy potentiometer); OR
- Yrr/Yfr are set incorrectly (tieny just fixed a similar 50" with a similar problem, except his was arc-bright all the time.)

Since it is regular I suspect a feedback problem with the PSU; it is not able to keep the Vs at the right voltage and the over-voltage or over-power protection (from the too-bright display) is kicking in.

Avoid running it for too long as damage may be dealt to the Y-Main or X-Main. A dim screen may cause Vs to far exceed rated voltage and cause damage due to lack of functioning OPP at low brightnesses.

[johnboy1313]

Re: Samsung HP-S4253 Rapidly Flashing Screen

Avoid running it for too long as damage may be dealt to the Y-Main or X-Main. A dim screen may cause Vs to far exceed rated voltage and cause damage due to lack of functioning OPP at low brightnesses.

How long is too long? I don't want to screw something up just probing for voltages.

[tom66]

Re: Samsung HP-S4253 Rapidly Flashing Screen

No more than 5 minutes at a time. I'm guessing here, as I wouldn't know for certain. Just don't try and leave it on for an hour and see if it will rectify itself...

[johnboy1313]

Re: Samsung HP-S4253 Rapidly Flashing Screen

Ok, with y-sus, or whatever it's called on a Samsung, plugged in I get...

Va = 57 - 65
Vsc = -204 - -217
Vs = 183 - 303
Ve = 107 - 123
Vset = 127 - 163

With ysus unplugged, in the same order...

67
-191
202
113
185-210

The label shows these voltages, in the same order...

65
-185
197
110
195

Just guessing, but I would think my next step would be to disconnect the buffer from the ysus and test the voltages again with the ysus plugged in.

[johnboy1313]

Re: Samsung HP-S4253 Rapidly Flashing Screen

Forgot to add. With zsus disconnected, in the same order as above...

69
-193
208
116
193-215

[tom66]

Re: Samsung HP-S4253 Rapidly Flashing Screen

Vs 303V?! Holy *#@! The PSU and sustains are rated to 250V, by nature of the capacitors alone! The IGBTs and FETs are usually 250V, max!
The panel should be arcing over internally by that point. That doesn't seem right?!

Your PSU is completely unstable but for some reason works better under no load. Dried output caps or bad feedback could cause this issue.

[ben7]

Re: Samsung HP-S4253 Rapidly Flashing Screen

As tom said, yeah, definitely a problem with the PSU! Wow! Sometimes this happens in computer PSUs and the mother board gets toasted

Well, its time for the opening ceremony!

[johnboy1313]

Re: Samsung HP-S4253 Rapidly Flashing Screen

So, I'm thinking my cheap DMM may have got me here. I downloaded the service manual tonight and saw that there are specific test points for the voltages. With everything connected, from the test points I get...

Va = Steady 65V (Normal)
Vsc = Steady -197V (Low, I was able to adjust it up to -186V)
Vs = Steady 197V (Normal)
Ve = Steady 109V (Normal)
Vset = Fluctuating 123V - 168V (Low and all over the place, adjusting doesn't seem to help make any difference))

Even when testing from the harnesses, it does still fluctuate a little, but I still don't see 303V at Vs

Here's the manual if you wanna look at it.


In all of the probing, Vset seems to be my problem. What now?

[tom66]

Re: Samsung HP-S4253 Rapidly Flashing Screen

Yes, Vset can definitely cause this issue.

The little sub-power supply board has a problem on it as that generates the Vset.

Some fluctuation in Vs is tolerable. One standard I saw said allow +/-1V from fully dark to fully bright. Vs is directly proportional to the picture brightness, and the complete discharge of the panel. Low Vs will cause underdiffusion (misfiring or dark stuck pixels.)

[johnboy1313]

Re: Samsung HP-S4253 Rapidly Flashing Screen

Yes, Vset can definitely cause this issue.

The little sub-power supply board has a problem on it as that generates the Vset.

Some fluctuation in Vs is tolerable. One standard I saw said allow +/-1V from fully dark to fully bright. Vs is directly proportional to the picture brightness, and the complete discharge of the panel. Low Vs will cause underdiffusion (misfiring or dark stuck pixels.)

What do you suppose would cause the fluctuation in Vset? There's a 350V 47uf cap just off of the Vset leg of the sub-PSU. I swapped it out with one that I had in my parts box and still get the same fluctuation. These boards are very inexpensive. There's one on the auction site right now for 15 USD shipped. Right now, I think I may be money ahead to just buy this board and replace the whole thing. What do you think?

[tom66]

Re: Samsung HP-S4253 Rapidly Flashing Screen

I would go for it. Someone else had the same problem with an unstable Vset. My guess would be some loss of regulation (feedback problem) or perhaps a bad start up capacitor causing controller instability.

Vset is used in the Yfr circuit to generate the positive ramp waveforms. A fault here could also cause the problem, perhaps due to excessive load. How stable is it with the Y-main disconnected?

[johnboy1313]

Re: Samsung HP-S4253 Rapidly Flashing Screen

~185v-210v

[tom66]

Re: Samsung HP-S4253 Rapidly Flashing Screen

OK, I would first inspect the board for cold joints, try adjusting the Vset potentiometer (a dodgy pot can cause this problem.) If those fail then consider a replacement of the board.

[johnboy1313]

Re: Samsung HP-S4253 Rapidly Flashing Screen

I tried adjusting the pot, it doesn't really seem do do anything when turned either direction. I went back and looked at the board for sale and it was on sale for $11.99 shipped. I went ahead and bought it. It should be here by Monday. I'll let you know how it goes.

[tom66]

Re: Samsung HP-S4253 Rapidly Flashing Screen

I would probably not spend any more than 30 minutes having a look at repairing a fault on a board when I can get it for $12.

[johnboy1313]

Re: Samsung HP-S4253 Rapidly Flashing Screen

Got the new board in the mail and installed. All voltages are steady. Picture doesn't flash anymore. I'm going to do the fine adjustments shown in the service manual and run it for a couple days. Thanks for the help.

[tom66]

Re: Samsung HP-S4253 Rapidly Flashing Screen

Glad to hear you fixed it. Plasma TVs really aren't that hard.

[johnboy1313]

Re: Samsung HP-S4253 Rapidly Flashing Screen

I've fixed 2 of 3. I still don't understand what's going on with the third one. I plan on getting into it again tonight.

[tom66]

Re: Samsung HP-S4253 Rapidly Flashing Screen

Have you got a scope yet?