Samsung PS51D490A1 has sound but no picture

[Ijaz Ahmed]

Hello Everybody

Few weeks ago, I got this PS51D490A1, has sound but no picture. I tested the IGBTs + power diodes on Y-main and X-Main and found few of them shorted. Then ordered the parts from Digikey and at the same time ordered the new boards as well.

The boards and IGBTs arrived and I first repaired the original boards and put them into the TV and power it ON. A spark and two IGBTs got blown on X-Main. So thinking that there might have been other burnt parts (other than those IGBTs which I replaced) on my original boards, I took some courage and tested the new Y-Main and X-Main boards for any previous shorts, found them ok and put them in TV.

The same two IGBTs on the new X-Main got shorted within no time. Then I started doing some research and did the following.

1) Changed the burnt IGBTs again on both X-Main boards.

2) Put the original Y-Main (Y-buffers connected) and measured Vs, Va, Vscan which were OK but SVC_Vs was absent. While the new Y-Main (Y-Buffers connected) had all of these present. So selected the new Y-main for further tests.

3) Tested the TV with new Y-Main and new X-Main connected but brown flat cables (from X-Main to panel) disconnected. No blast, nothing burnt but noticed that logic board LED flashes 5 sec ON and 2 sec OFF. Then I placed some aluminum foil in the output connectors of X-Main (so that feedback to the logic board is simulated) and then the logic board LED changed to 1 sec ON and 1 sec OFF. I also noticed that front Standby RED LED started blinking 5 times when the set is powered ON

Now I am afraid if I re-connect the brown flat cables from X-Main to panel, it may again blow the X-Main.

What is the next step to try ? I have checked Y-Buffer, no holes in the QFP chips, no short from the outputs of ICs to input (which is Y-out on Y-Main). There are no shorts from Y to X brown flat cables (tried to check the shorts inside panel). Also the fuse on the E-F buffer boards is OK. Panel is physically OK, no cracks.

Sorry about the long story, I tried to explain what I have done so far. Any advice would be appreciated. Pictures are attached.

[tom66]

Re: Samsung PS51D490A1 has sound but no picture

Did the IGBTs short immediately on power up with new boards, or only after some running time?

[Ijaz Ahmed]

Re: Samsung PS51D490A1 has sound but no picture

^ Immediately.

The current status is that I have the new Y-Main, repaired original X-Main, X-out flat brown cables disconnected. The green LED on logic board is flashing at 1Hz approx, all voltages are stable now, even Ve on X-Main is generated and Y-out (from Y-Main) along with X-Out from X-Main are generated.

Is it time to connect back the brown X-Out flat cables and see if it works ?

[tom66]

Re: Samsung PS51D490A1 has sound but no picture

Do you happen to have an oscilloscope available?
When replacing IGBTs, did you also replace the little driver chips near them? These can be damaged when the IGBTs fail shorted.

[Ijaz Ahmed]

Re: Samsung PS51D490A1 has sound but no picture

^ Yes, I have an HP digitizing 100MHz scope at hand with a line isolation transformer.

Since the IGBTs were fully shorted on all three pins, so I suspected the driver chips and tested the o/p of the driver chips for a short to either Vcc or Gnd but did not find any. Also the driver chips of the the shorted IGBTs were giving same readings as those of whose IGBTs didn't burn. So I concluded that driver chips were still OK.

BTW: I have never found the schematics of Y-Main or X-Main. Do they exist ? If yes, I would love to have a look.

[tom66]

Re: Samsung PS51D490A1 has sound but no picture

As far as I know, schematics for such boards only exist with Samsung themselves, they are not in any service manual or documentation. However, the general layout of sustain boards is documented well in Panasonic service manuals, which might help you understand. See my signature - interesting sets are the TH-42PZ80, TC-P50G10, TC-P50GT30...

Even though the driver chips may test OK, they can be "walking wounded" and fail as soon as they are put into operation. Unless you check the square wave output directly, there's no way to be sure.

If you post a picture of the Y-main and oscilloscope, then the arrangements for probing the sustain waveform can be made.

[Ijaz Ahmed]

Re: Samsung PS51D490A1 has sound but no picture

Thanks a bunch for the Panasonic service manual tip. Now I understand what X and Y main are actually doing.

Sorry but got busy with some other work, yet have to fix the plasma TVs. I have ordered the boards + some SMT components for my faulty plasma TVs. As the boards arrive, I will get back to my threads here.

[tom66]

Re: Samsung PS51D490A1 has sound but no picture

It also might help to understand exactly how a plasma TV works. The operation hasn't changed significantly since they were first demonstrated. From 2001, explains the operation of the whole process.

[Ijaz Ahmed]

Re: Samsung PS51D490A1 has sound but no picture

I have ordered the parts X-Main, Y-Main and Y-Buffers for this PS51D490A1. In the meanwhile, I kept working on this TV from time to time and have checked the following.

With X-Main FPC (brown flat cables) disconnected,

1) Upon power up, front Standby LED flashes 5 times. Not sure what it is telling.

2) VS, Ve, Vscan, Va V(ERC) are all normal and stable.

3) Green LED flashes approximately at 1 Hz on Logic board.

4) Checked the Y-Buffer for shorts (output to input) and also tested the panel FPCs (from every X-FPC to every connection of Y-FPCs) and there is no short between scan and sustain electrodes on all combinations. Did not check them with data electrodes, since fuse on F and G buffers are OK.

5) Checked the Y-out and X-out waveforms, they look OK.


As soon as I connect the X side FPC cables to X-Main and power on the TV, the two IGBTs in XG and XR section blows up. (picture attached and IGBTs marked).

Now I am just wondering why those IGBTs keepl failing. Before connecting the X FPCs I had tested the gates to verify the IGBT drivers. One had gate drive waveform at about 15V while other was having it at around 70V (this looks suspicious but tested it on another X-Main which I have, it was same).

(i) Could this be bad Logic board issuing drive pulses in some non-synchronized fashion which is causing collision ?

(ii) Can a bad Y-Buffer cause X-Main failures ? I didn't change it yet.

(iii) Could this be a shorted panel ?

Any help will be appreciated. The new set of boards is on the way, will get in next 2-3 days.

[tom66]

Re: Samsung PS51D490A1 has sound but no picture

Well it is sounding like a panel short.

All xmain electrodes are connected. The ymain scan side electrodes are separate. So, to rule out bad plasma panel you could try running TV with only one ybuffer ribbon installed. Or even none at all. If it still fails the panel should be OK.

[Ijaz Ahmed]

Re: Samsung PS51D490A1 has sound but no picture

Well it is sounding like a panel short.

All xmain electrodes are connected. The ymain scan side electrodes are separate. So, to rule out bad plasma panel you could try running TV with only one ybuffer ribbon installed. Or even none at all. If it still fails the panel should be OK.

This model has one single Y-Buffer.

You mean I should connect all X-main electrodes (brown FPCs) and remove all Y-Side flat cables from Y-Buffer, if does not fail then start attaching the Y-FPCs one by one and see where it fails ?

I did a complete short check from all X-Electrodes to all Y-Electrodes (each connection on each Y-Cable) but found nothing.

[tom66]

Re: Samsung PS51D490A1 has sound but no picture

That's the idea. Now, I don't think the panel is shorted, and anyway, it would normally blow Y-buffer in that case, as that's a lot more sensitive to over current. However, we've had some Panasonic PZ70 plasmas with intermittent panel shorts - oddly the panel will blow the SU/SD buffer board after warming up, and you can see a faint mark on the panel where it failed. And ohm tests pass. But they normally last around 30 minutes before failing.

I think it's more likely some problem with the control board sending incorrect drive waveforms or something on the X-main that is damaged but not obvious.

Have to say most of the problems with these models are the 43" sets and quite a few bad X-main boards, figure that when the panel cracks potentially there's a chance of X-main failure... alternatively, people just get frustrated with their TV after it dies and smash it.

[Ijaz Ahmed]

Re: Samsung PS51D490A1 has sound but no picture

Got the new Y-Buffer, Y-Main and X-Main as a set.

Replaced Y-Buffer just as a precaution and used my previous X-Main with Y-Electrodes disconnected and X-Electrodes all connected. No smoke, though I could hear some arcing noise from X-Electrodes side which was not continuous and would occur and disappear. All the critical voltages were found OK.

Then disconnected the LVDS and placed a jumper on control board for test patterns and connected the top Y-Electrode to Y-Main. I was able to see some white random pixels on the top of the screen though I was expecting some regular pattern.

Connected the 2nd Y-Electrode and more area of top portion of screen started showing the irregular white random pixels, and arcing noise increased.

Then tried to see the arcing. Most likely it is occurring at the edge of the glass panel where X-Electrode leaves the glass panel.

Does it look like panel problem or it is due to un-connected Y-Electrodes ? The arcing is randomly occurring at the edge of glass and looks like it travels between adjacent electrodes at the places where they terminate on X-Side.

[Ijaz Ahmed]

Re: Samsung PS51D490A1 has sound but no picture

With all X-Electrodes connected and only One Y-Electrode connected, I was able to get some picture (attached). The picture is noisy, colorless showing a channel name 'ATV' and horizontal lines are not even continuous.

Main issues now.

1) The horizontal lines are discontiuous and no color, even in test pattern mode.
2) However in test pattern mode, these bright pixels change color R, G, B etc.
3) If I add one more Y-Electrode, then even this picture disappears.
4) Adding even more Y-Electrodes invites arcing problem at the panel Edge.


I think it is faulty Logic Board. Any advice or more testing before I start looking for a logic board ?

[Ijaz Ahmed]

Re: Samsung PS51D490A1 has sound but no picture

Bump..

Any expert opinion ? My conclusion is going towards a bad panel because in color test mode, some pixels don't light up in all R, G, B and white colors.

Or there is still some possibility of electronic problem ?

[shawn76]

Re: Samsung PS51D490A1 has sound but no picture

It is amazing I found this forum. Great reads. I have a RCA 42pa30rq that the picture went black. Sound works perfectly along with being able to maneuver around with the remote from input to input and channel to channel. I turned it off and turned it back on and saw a little faint snow the first couple times but not now. Still everything else works. Pulled the back off and visually can't see anything burned, melted or the obvious to the lamen. I don't have the sensor tool or anything but should I just replace both x and y boards. I only know that much because of reading a lot here. Help would be much appreciated .