Panasonic Plasma TH-50PX75U Test for bad panel?

[tucocaps]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

Well, I think I figured out the neg/pos test points, and did the "short" test. Found both failed boards to be shorted at points along the same connector. Not sure if they were the same pins, but discouraging results all the same.

I guess this TV is for the trash?

[tom66]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

If at same point on SU/SD that does indicate a panel problem.
I have not tried this idea yet, indeed it may be crazy but you have nothing to lose right? If the panel has an electrode short, maybe it's possible to blow up the defective pixel and only lose that pixel? Applying a high current between SC and SS sides ought to do it. Just an idea.

[tucocaps]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

Tom: Wow. LOL. That's what I like about this forum. You guys are really creative. Only problem is I'm just not sure what to do.

Well I haven't gotten so angry as to throw the set out the window yet (it's too heavy for that anyway).

A few things I am curious about. For each line coming out of the SD to the panel, is that actually an entire row of pixels? What if I go back and check and find that different lines on the same connector are shorted on the SD (but still same connector)? I guess I'm trying to understand how I can find this pesky short on the panel and/or do what you just said regarding the electrode.

I'm still confused about this "panel short" as I still can't see it, and definitely did not notice it for the few hours the set was working. You seemed to think a short on the panel wasn't possible if the SD would work for an extended period of time, and then burn out. However, I guess based on this evidence it is possible? Is it like a pixes that shorts when it gets to a certain temperature?

[tucocaps]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

One other side note about this plasma and others. When I picked up the unit, and for some time after, I had the screen lying flat on it's back. I've read that plasmas are only supposed to be standing up properly at all time. Is there any truth to this , or is it simply a myth about plasma screens?

[tom66]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

More or less a myth, as far as I can tell. I've done ~13 plasma repairs all laying down, no failures yet, with one exception, the time I stepped on the 50" G10 Panasonic I was working on. That pissed me off. Panel just went "crack"!

Each line out of SD and SU is one row of pixels. The SS (Sustain-Single?) side is common, all pixels see the same signal, but the SC side (Sustain-sCan?) has the row selectors, which are the buffers SU and SD.

In between the lines of the SC, lets call these SCn (plural), and the common lines of the SSn (all joined together at one point, but only at the SS board), you have the pixels, broken by the Vda address signal. But for now, ignoring the addressing process, assume that between each line SCn and SSn there are 1368 pixels for a 50" 720p. Normally, the pixels are completely open circuit, no conduction. When a high voltage alternating current (+/-200V, 80kHz) is applied to the pixels which have been previously selected, they break down (conduct current) and emit light. There's a lot more physics behind it, but you can think of each pixel basically as a neon light. Look up the breakdown characteristic of such a light.

What apparently happens with these panels is a short develops somewhere along the SCn and SSn lines. Since the pixels are normally open along all lines, applying a voltage (below the gas breakdown voltage of around 130V) to any of SCn and all of SSn will not cause a current to flow, except in any pixels which are shorted. So, you just need to attach a connection across the rough area of the short, without having to locate it precisely.

The trick, and problem, with this method is you need to apply just enough current to blow out the short, without applying enough to damage the whole electrode line itself. The bus lines have to carry the current for all the pixels, which is much larger than any one pixel has to take, so in theory, it shouldn't be too difficult to apply enough to blow out the short whilst not risking damage to all of that line. A plasma display uses pulses of 80 to 150 amps, but the actual continuous current through any line in the display is probably closer to 10mA to 20mA. Therefore, I think if you stay under this limit you shouldn't experience any issues with burning the line out, though a transient of up to 100mA could be OK given the line has to withstand the high current pulses.

[tucocaps]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

Tom,

Thanks again. Still trying to wrap my head around all this information you are passing about the panel and how this all works.

In the meantime, I tried to dig in deeper with the cont checker, and I really think I've found the following. Although the shorts for both shorted SD boards are on the same connector, they aren't on the same pins on that connector. They do seem to "share" shorts on some pins , and the shorts (some of them) are in the same general area, but there are shorts on one board that aren't on the other. I just wonder if I'm chasing my tail here regarding the panel and the shorts. These are all on that second connector (closest to the SD46 white plug connector) on the SD board. I wonder if this is a common plug for shorts, and I just got two boards with a weak chip there, OR if the short(s) on my panel were just working their way over that connector.

The way you describe this "blow out" procedure, it sound like the panel would not be damaged anyway if it were not having a shorted pixel, and if it did, then that pixel would be hopefully "fixed" by getting blown out. Although it all makes sense in theory , I'm again at a loss as to how to practically do this. Not sure what to use for power, how and what to connect to, and what to look for (how to know if a pixel is blown out)?

[tom66]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

Ways to apply the current: you can use a lab constant-current constant-voltage supply, though most people don't have one. Or, you can use a 12V power source with a 120 ohm resistor (5W rated) which limits current to 100mA.

Connect the all of the SC electrodes (of the failed chip) to 12V through the 120 ohm resistor, the SS electrode to ground. All other connections from the panel except the address lines (bottom of panel) left disconnected.. i.e. buffers, SS board disconnected. Then look carefully at the panel. Try to find an easy way to power the battery on and off. Use a voltmeter across the resistor. You should find the voltage drops to zero once the short has been eliminated.

If it does not go you might have to try with 12 ohms / 1 amp briefly though do not do this for longer than 10 seconds.

Sometimes, when one buffer chip has a failure on one output, the failure can spread across the chip randomly due to the sudden extreme heating. I would say that if they share a single point where both have failed, then it could be a bad panel. Now, I'm still surprised that a panel can fail and the buffer can still work. My only guess is the current flowing through the failed pixel is negligible, aside from creating increased heating in the buffer chip, leading to early failure. I would have expected a dim or dark line, for example.

[tucocaps]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

OK, to put this is simpler terms for practice...

I'm going to take that group of rows on the panel (that matches the plug on SD that has shorts), and have to find a way to join/connect the corresponding ribbon cable on both sides of the screen. The SC (SD) side to pos , and the other to neg. I'll do this , with basically everything else on both sides of the panel disconnected.

If the above seems right, I'm still not sure on how to hook on to those ribbon cables properly. They are really thin.

Also, is a 120 ohm resistor a fairly common resistor? I see you are in the UK, but I'm wondering if I can pick one of these up at a common retail store here like Radio Shack, or if I'll have to order.

One more thing as you mention how to check if this has worked. I would assume I should check before I apply the current for voltage across the panel, to test the theory that the panel is indeed shorted?

[tom66]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

You'll have to use some kind of clamp which doesn't damage the connection (it is not replaceable if damaged!) Perhaps some kind of foil and sellotape arrangement could work. Just make sure you don't damage the delicate panel connectors.

Not sure if RadioShack will sell 120 ohm 5W and 12 ohm 25W resistors, but you can get them from DigiKey.

http://www.digikey.com/product-detai...5-12-ND/269944
http://www.digikey.com/product-detai...415-ND/2366277

To tell if it has worked, you measure the voltage drop across the resistor. If the drop is near zero, the panel has no short. Now, this does not tell you if it was not shorted originally, so you may want to try with a very high value, say 1000 ohms, before trying a higher current.

[tucocaps]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

You'll have to use some kind of clamp which doesn't damage the connection (it is not replaceable if damaged!) Perhaps some kind of foil and sellotape arrangement could work. Just make sure you don't damage the delicate panel connectors.

Not sure if RadioShack will sell 120 ohm 5W and 12 ohm 25W resistors, but you can get them from DigiKey.

http://www.digikey.com/product-detai...5-12-ND/269944
http://www.digikey.com/product-detai...415-ND/2366277

To tell if it has worked, you measure the voltage drop across the resistor. If the drop is near zero, the panel has no short. Now, this does not tell you if it was not shorted originally, so you may want to try with a very high value, say 1000 ohms, before trying a higher current.

So, I can't just check the panel for a short with an ohm meter? I'm assuming what you are saying is I'm going to hookup this resistor in line with the positive side of the battery. Then , I'll measure voltage across the resistor when it's all hooked up to the panel. If it reads nearly 12 volts, the panel has no short ... but I can't know if it previously did because the voltage will already be applied. You say if I want to check that, use a much higher resistor so as not to send as much current through, and then I'll see the voltage drop if there is a short?

[tom66]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

If the voltage drop is zero, there is no short. Current I through a resistor is Volts/Resistance so if it had 12V across it, 100mA would be flowing (120 ohm resistor.)

I suppose you could use an ohm-meter before, I didn't consider that.

[R_J]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

You can lay plasma tv's down to work on them without a problem, its only during transportation, mfg's recommend transporting vertical. like the glass people do when they transport plate glass on their trucks, also when they are laid down during transpoprtation the bond between the two pieces of panel glass could leak and no gas, no working panel.

[tucocaps]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

tom: good to hear I had an idea about something here My understanding is quite limited.

Stopped at Radio Shack today, and they didn't have the higher wattage resistors, but I did pickup another variable AC adapter with a 1 watt rating (my guess is that's max?). This one is selectable across a range up to 12 volts. Can these sorts of adapters be used as power sources for stuff like this? If so, can I use a smaller resistor to drop down the amperage? I'm confused about how the amperage works on these cheaper ac adapters. thanks again.

[tucocaps]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

Tom: As an update, I tried to test for the short via ohm meter several ways, and my meter always read an open circuit. My guess is this is because the short problem only occurs after the screen has been powered on and heated up after some time. I tried the lowest to the highest ohm settings, and foiled on plug, then the other. Also tried foiling the ss side and running the probe over every contact on the sc side. I guess I should note that the ss side has 4 ribbon connectors, while the SC has 8. I'm assuming it's a 2:1 ratio starting from the top or bottom.

Do you think this fact affects the theory that we can indeed "blow out" a bad pixel and cause it not to short as the panel warms up? I'm still fuzzy on how applying the current can "blow out" or do such a thing to just that pixel.

[tom66]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

Hmm. I do not know. I cannot see how a short can occur after the panel has warmed up. If such a short did occur, I would expect it to become permanent. It is possible that we are still chasing a wild goose here and your panel is fine, but you have bad SD boards or excessive supply voltages.

The reason the fault may be possible to blow out is because all the pixels normally have no connection between the SC and SS sides. Completely open circuit. The only way current flows is by exceeding the breakdown voltage which is similar to how lightning doesn't conduct continuously as it forms, only when it hits a certain limit. When a faulty pixel forms the theory is there is a short circuit, when all other pixels are open. Passing a high enough current through a thin short between two wires (the electrodes) will damage the path of least resistance, the short. Which should only damage that pixel; in fact it may not even do that, if the short is "parasitic", then it might even clear that pixel and allow that pixel to work again, though I'd consider that unlikely.

An AC adapter which is rated for 1W may well deliver in excess of that 1W nameplate rating; it just might not work properly. You could still use it if you use the resistors, though only at a current of below 80mA, which may not be sufficient.

[tucocaps]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

Tom: You mention "excessive supply voltage". Could this mean what I've been thinking regarding the SC board? Could it in fact be faulty, and somehow burning out the attached SD board, but not the upper buffer board? Also, what about this VDA adjustment? I think I see pots on this SC board. White with a cross head (looks to be for a screwdriver). Could VDA too high be zapping the SD board? Keep in mind, this TV was worked on by an unknown tv "technician" who had previously removed the original SD board ... I assume in an effort to diagnose the original problem which was very likely the 7 blinks I'm chasing now.

The tests I've done regarding the notes from Panasonic, are really just simple bypass and process of elimination tests to see if the set will power on and stay on with the various boards connected. It seems to me, this really doesn't account for a board like the SC which could be defective in such a way that it won't throw a blink code or stop the tv from powering on by itself ... but could be damaging something else?

Visual inspection of the SC board didn't yield anything obvious. I think I could get another one of these SC boards used , but it sure would be nice to be able to test that board in some way. Like I said, I can easily power on the set with all boards except the SD connected - in which case I only have video on the upper half of the panel.

I was thinking about swapping "good" buffer chips from one of the faulty SD boards to the other , in order to hopefully power up the tv again , and perform some kind of test of that SC board (if it's necessary to be powered on with SD attached). What I'm again not sure about is my soldering equipment and skills. I have a 40 watt iron with a inexpensive weller variable heat adjust station (simple pot 1-5). I've read about techniques using solder wick to remove such chips, and resolder, but do you think I'd be able to pull that off without overheating the SD chips?

[tucocaps]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

Tom,

I went back and read your comments, and noticed you said VAD (not VDA). I have located the trim pot for that on SC. What do you think about lowering that and then trying a new or repaired SD board?

[tom66]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

I would try that.
Vad nominally -140V from memory. Lower it to -125V and increase Vsus until cyan tint in middle of panel vanishes (on full white test pattern.) Don't exceed 210V on Vsus.

[tucocaps]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

Tom, What is the best way to adjust these pots? I'm assuming I need to use a meter , and be careful to adjust while the set is on.

Also, do you have any idea on the best way to remove/resolder those buffers? I don't have a desoldering station. The chips are small and so are the pins, and they are pinned on all four sides.

[tucocaps]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

Since I might have to get yet another SD board, can I adjust this VAD with the SD board removed, and then plug in the SD and work on the VSUS? Of course, the goal here would be to reduce the VAD voltage and hopefully stop the SD from burning out again.