Panasonic Plasma TV TX-P50X50B - Shorted power supply

[tom66]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

Immediately? Or after some time? This is a very strange fault. Did you apply thermal paste to the devices?

[Kairi]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

It blew immediately. I have thermal paste but I highly doubt it's the heat. More likely too much current through the fets as heat failure will take place after some time, not instantaneously. I replaced the fets with the LAST set I have and did some detective work...

With the AC connected to the power board only, everything was normal, I connected a 25w bulb to the dim bulb tester and it exhibited the standard behaviour of it being bright at first and then dimming slightly once the cap is charged up. I connected the two ribbons to the right of the power board and the cable to the y sustain board and it was still ok. I measured the voltages across the main cap which was 309V. I was unable to check the current draw on the AC side as my multimeter doesn't support that, only DC. The fets does not even get warm.

As soon as I connected the AV logic board to power board, the light bulb flashed accompanied with the clicking sound which I have identified was coming from the fets itself. I then removed all other connections except the AV board and it was still exhibiting this behaviour. It is now just the power supply and the AV board. The fets get really hot after a little while.

I shall also mention that before I plugged it in, I tested it with bulbs with combined wattage of 300W (I wired it in parallel) and it was still flashing.

[mmartell]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

So on standby the Main board is shorting out the supply ? Any continuity on the Main board between the 5v standby input and ground ?

[Kairi]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

I don't think the TV is ever in standby. According to the manual, the standby LED at the front is red but I get a green flashing LED, which means as soon as I plug it in, it tries to power up the entire thing. I tried to use the remote to put it into stand by but it doesn't work.

I'll provide a picture of the back of the board if it helps...

http://*************/a/img51/9193/ff3g.jpg

[tw2005]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

I don't think the TV is ever in standby. According to the manual, the standby LED at the front is red but I get a green flashing LED, which means as soon as I plug it in, it tries to power up the entire thing. I tried to use the remote to put it into stand by but it doesn't work.

I'll provide a picture of the back of the board if it helps...

http://*************/a/img51/9193/ff3g.jpg

that's because in the settings on power loss it is set to return to its last state which presumably was on when the PSU failed. Same as in a blackout when power comes back on it automatically starts.

[tom66]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

Do you think those FETs are maybe for the VSUS and there's a short or other fault on them?

This got me thinking, I have an -untested- 42" PX50B PSU if you'd like it... could be good for parts... I say untested because it gave no standby light and came from a cracked screen Panny. Though IIRC, the fuse was good. Send it to you for a fair price? I also have the A board, again, untested, and the SN board, which I believe works due to some testing, but I am not certain about.

[Kairi]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

The V sustain board is linked to the AV board and I've already disconnected the ribbon cable, I highly doubt those fets are for any other thing but the switching of the power supply. They are on it's own heatsink and the PCB traces connects to the main cap and the primary side of the main transformer. I may be wrong though as I couldn't find a schematic for this particular board and Panasonic's service manual stated that the PSS board is not servicable and must be replaced.

Having check all the SMD diodes at the back and some selected resistors, I'm completely stumped of the problem. Anything else I should check before I just buy a new board, though I'm worried that if the AV board has a problem it may damage the new board too? I must have spent over 50 hours working on this board by now and on the edge of chucking the damn TV out of the window! Who would have thought a power supply would be be so difficult...

[tom66]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

I'd try without a main board and all connections isolated from the power board. I also have the A board for this set... but again, untested...
If you do get to the point of chucking it, I'm always on the look out for new projects . Though apparently, picture quality from these budget Panny's leaves a lot to be desired.

[Kairi]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

I just read an interesting topic about leaky capacitors and how it can draw too much current and I am wondering if all this could be down to bad capacitors after all... I measured the caps using one of those cheapo ones from eBay from China and according to my research, fault's might not show unless the caps are charged up to the stated voltage. As my meter is only powered by a 9V battery, this is nowhere near enough to the 450V main cap and so I may have been deceived to think it's OK whereas it fails under load.

As all the components tests fine, even down to SMD level zener diodes, the last thing I plan to do before swapping out the board is to replace the capacitors. My power mosfets were heating up real quick when connected to the "flashing" light bulb and was clearly blown due to something causing it to operate beyond its design paramaters more likely over current and the high current must have been pulled from a component further down the line..

What are your thoughts on the theory of leaky capacitors on the board?

The article I've read is this one... http://forum.allaboutcircuits.com/sh...ad.php?t=60189

[JRade]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

Is power panels repaired and which parts were replaced?
Please reply.
Best Regards

[Clydeuk]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

We serviced a lot of these under warranty from Panasonic from new, the combined power supply/SS design certainly seemed to be an issue from new which during my time at the company never seemed to be resolved by the manufacturer, Obviously as you stated Panasonic regard this as a replaceable assembly and therefore do not offer a schematic, which makes attempts at fault tracing extremely difficult.

In the years I have worked on SMPS I very much doubt your problem has anything to do with anything connected to the secondary side of the power supply, I would start by getting the datasheet for the gate driver i.c and seeing if you can check the DC conditions etc, if you have a scope you may be able to check it's output, I have no idea about the circuit used but would suggest removing the FET's to do this so you can apply power without them blowing, whether the gate driver will work without the complete circuit I don't know. I'm thinking something like a current sensing resistor gone high in value, not necessarily located near the gate driver i.c.

These are just thoughts, I think I have the manual for the X50 series as well but since you have this and there is no power supply schematic it's probably not worth me even looking at that.

I don't think your large 450v capacitor will be faulty, these seldom fail and I've never known one to cause this kind of problem.

If I still have access to the Panasonic service forums I could have a browse there at the weekend to see if there have been any reports of PSU repairs, It's over a year since I tried that. My ex employer's (now defunct) account may no longer be valid so I can only try and see.

[JRade]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

I ask because I have a Panasonic TX-P50X50E
with the same Error: Invalid MOSFETs Q207 and Q208 ?
Please reply
Best Regards!

[JRade]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

It is shema
Best Regards!

[JRade]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

It is.
Sorry!

[tw2005]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

It is.
Sorry!

Nice, thanks for sharing.

Where did you find that?

[Neelix]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

Thank you for the diagram. Darfon circuit diagrams are difficult to come by

[JRade]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

See link:http://www.radeel.co/index.php/en/various-parts

[Cornboats]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

Hi Kairi
I read your thread with interest. Did you get to the bottom of blowing fet's? I'm researching for solving a similar problem with a LG 50" plasma psu.

[Kairi]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

Hi all.

I don't come here regularly so I had missed this thread when new posts were added. I thought I had given this thread a closure by posting my final fix but obviously for some reason, I never.

Despite all the time devoted to fix the board, I never got to the bottom of the problem so in the end I replaced the entire board pulled from a 60 inch version which used a newer version of the power board. The TV is used in the living room has been working fine ever since.

I still have the old board so if someone wants to do a bit of detective work, I can post it to them... just pay postage.

[tri-comp]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

It is.
Sorry!

Darfon SMPSU schematic for Panasonic TX-P42X50E

Thanks for the schematic diagram.
I have a couple of questions:
This TV doesn't have stand-by 5V power as the switching-chip FAN6755U does nothing.
It's most likely defective as the surrounding components tests OK including the series string of resistors to facilitate initial start-up.
Now, here's the problem:
The schematic diagram shows pin-7 to be NC (Not Connected).
In fact it's missing on the original chip which has only 7 pin's in total.
The original chip is labelled 6755U so there can't be any question to what type is mounted.
Now go look at a FAN6755 datasheet (attached) and you'll find that pin-7 is very much in use.
It's actually the HV start-up pin of the chip which btw has all 8 pins supplied.
Now, if you instead take a look at the NCP-1271 datasheet (also attached) you'll see that this chip looks like a drop-in replacement for the FAN6755U in the Darfon schematic.
Only 7 pins and pinout to match the schematic.
What gives ...???