Panasonic TH-42PX70B Blowing mains fuse.

[Steve.B]

Would appreciate some views on this problem. My daughter-in-laws Panasonic TH-42PX70B HD Plasma is blowing the 5amp fuse in the mains plug. Replacing the fuse simple results in a brief flash of the power light along with one click of a relay before the fuse blows again.

I would normally associate this with a power board problem, but this tv also displayed one other problem for a couple of days prior to the fuse blowing failure. A vertical line (one pixel wide apparently - did not see it for myself) was being displayed, the line varied in colour and intensity dependant on what picture was being displayed. The line was towards the middle of the screen. Incidentally i have been told that the screen suffered an identical thin line a few months ago, but that one was towards one end of the screen, that line cleared itself after a few hours.

If it was not for that 'extra' bit of information, i would not think twice about lugging the tv the 100 miles back to my house in order to try and fix it for her, but i am weary about that thin line - could that mean a panel failure? And could a panel failure cause a mains plug fuse to constantly blow? Or is there a good chance that the power problem is a separate issue? (I remember reading Tom saying that some Panasonic 1080p panels were prone to some kind of internal short - i am hoping this is not one of those causing this problem).

If you guys here think that the power issue could be a separate issue, then i would not mind having a go at it. But if the general opinion is that the panel is the cause, then it's obviously not worth me even lugging it the 100+ miles back to my place. My daughter-in-law is not too bothered about the thin line, it's mainly my 4 year old granddaughter that watches it anyway.

I have never worked on a plasma before, but have had experience fixing lcd monitors, psu's and various other electrical items. Bad caps, shorted diodes - mosfets etc. So am not shy of using soldering irons and multi-meters.

I do not have the tv at the moment, so obviously no nice pcb photos, but would appreciate any views on this one. Could this be the panel causing the massive overload that is knocking the mains fuse out, or is there a good chance that the fuse problem is a separate issue that would be a more viable repair?

Thanks guys.

[RJARRRPCGP]

Re: Panasonic TH-42PX70B Blowing mains fuse.

A vertical line (one pixel wide apparently - did not see it for myself) was being displayed, the line varied in colour and intensity dependant on what picture was being displayed. The line was towards the middle of the screen.

With an LCD, it's likely just a bad connection to the T-con.
Sounds like what the LCD screen does on my Core 1 Duo 1.66 Ghz Acer laptop.

I hear that with plasma, it's more serious. At least one folk reports that with a plasma, it's in the panel and thus unrepairable!

[tw2005]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Would appreciate some views on this problem. My daughter-in-laws Panasonic TH-42PX70B HD Plasma is blowing the 5amp fuse in the mains plug. Replacing the fuse simple results in a brief flash of the power light along with one click of a relay before the fuse blows again.

I would normally associate this with a power board problem, but this tv also displayed one other problem for a couple of days prior to the fuse blowing failure. A vertical line (one pixel wide apparently - did not see it for myself) was being displayed, the line varied in colour and intensity dependant on what picture was being displayed. The line was towards the middle of the screen. Incidentally i have been told that the screen suffered an identical thin line a few months ago, but that one was towards one end of the screen, that line cleared itself after a few hours.

If it was not for that 'extra' bit of information, i would not think twice about lugging the tv the 100 miles back to my house in order to try and fix it for her, but i am weary about that thin line - could that mean a panel failure? And could a panel failure cause a mains plug fuse to constantly blow? Or is there a good chance that the power problem is a separate issue? (I remember reading Tom saying that some Panasonic 1080p panels were prone to some kind of internal short - i am hoping this is not one of those causing this problem).

If you guys here think that the power issue could be a separate issue, then i would not mind having a go at it. But if the general opinion is that the panel is the cause, then it's obviously not worth me even lugging it the 100+ miles back to my place. My daughter-in-law is not too bothered about the thin line, it's mainly my 4 year old granddaughter that watches it anyway.

I have never worked on a plasma before, but have had experience fixing lcd monitors, psu's and various other electrical items. Bad caps, shorted diodes - mosfets etc. So am not shy of using soldering irons and multi-meters.

I do not have the tv at the moment, so obviously no nice pcb photos, but would appreciate any views on this one. Could this be the panel causing the massive overload that is knocking the mains fuse out, or is there a good chance that the fuse problem is a separate issue that would be a more viable repair?

Thanks guys.

I'm going to say you have separate issues. I've had and worked on both the PX70 and PX7 which are essentially the same except the 7 was chinese ans the 70 was japanese with some extra HDMI.

The psu has a common fault, a FET shorts, takes out the fuse and sometimes the ceramic resistor blows open cct too and maybe the rectifier bridge. Mine had more damge because my mate had no idea and blowing it up several times probably took out extra items.

The other will most likely be the COF ( chip on film) on the panel ribbons at the bottom failing. I had that too, swapped out the C boards and digital boards with no change on mine so it's the panel. Not repairable but tolerable to watch.

The PSu should be a cheap repair if you can do it.

[tom66]

Re: Panasonic TH-42PX70B Blowing mains fuse.

One pixel line is a bad panel.
Blowing fuse see this video: https://www.youtube.com/watch?v=OuykZxLURis

[tw2005]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Just thought of someting else. 5amp fuse in mains plug? That's different. I think and I have not checked but the PSU mains fuses internal are 8amp.

[Steve.B]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Thank you so much for these replies, needless to say that i will indeed be going for the repair on this one. I of course realise that the thin line is almost certainly a panel problem, i am not even going to think about solving that one, but knowing that the power problem is probably not related to the panel issue is good news.

tw2005, thanks for the info, and a big thanks for the .pdf files on the likely components. I am sure they will be very useful. With regards the 5amp / 8amp plug fuse, i am fairly certain that it was a 5amp in there from the start, but i will get that double checked. I have never come across an 8amp plug fuse over here in the UK, but that's not to say that Panasonic are not using one of course.

tom66, thanks for the video link, certainly nothing in that would worry me, only thing i do not have in my kit is an esr meter, so i will just have to do a visual check on the main filter cap, though being a panny i am not expecting to find some cheapo brand cap in there anyway. So hopefully it will not be an issue.

I am seeing my daughter-in-law at the weekend, so will pick the tv at the same time. I will of course post back here when i know more. In the meantime, thanks again for all of your replies. You have been a great help already. Very much appreciated.

EDIT: tw2005 - just re-read your post, just realised that i think you were referring to the tv internal psu fuse, if that is 8amp then that explains why it's blowing out the plugs 5amp fuse first. I was wondering why it was taking out the mains plug fuse before taking out it's own internal fuse. That would explain it.

[tw2005]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Yeah that's why I mentioned it but I'm not familiar with the UK setup but I had a quick look at the parts list and internal 8amp I do remember now that it was 8amp fuses i had to replace but we have the standard panasonic lead with the EMI filter but it's a sealed lead no fuses anyway it should be a straight forward job, fingers crossed

[tom66]

Re: Panasonic TH-42PX70B Blowing mains fuse.

The plug fuse is only for protection of the socket, plug and wire, never for the device attached. For example it's OK to use a 13 amp fuse on an appliance rated 5 amps, provided that the cord is rated to 13 amps.

[Steve.B]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Update time. I feel i am in need of some further help.......10 blinks now.

With reference to the video that Tom posted above - https://www.youtube.com/watch?v=OuykZxLURis.

Before even looking at the tv i pre-ordered a replacement Mosfet, (RJK5020), along with the 3 x MUR460 diodes. I also ordered a replacement bridge rectifier, just in case i needed it. I was hoping this would be an easy fix i could do with the tv still at my daughter-in-laws house, instead of having to lug it back down to Devon with me.

I looked at the tv at the weekend, and sure enough, the mosfet was indeed shorted out. The shorted component was a RJK5010, which i replaced with the RJK5020 that i had - i presume that this would be a suitable replacement? Please correct me if i am wrong.

Like in the video, i checked the large diodes that are connected in parallel, i only had 2 on my board, and both checked out ok. But i decided to change them anyway - seeing that i had replacements. My board only had 2 of these diodes fitted, (SBK69) but the holes where there for a third one - like in the video, so i went ahead and fitted all 3 of the diodes (I replaced them with the MUR460), i figured that seeing that all of them are in parallel it would do no harm, again, correct me if i am wrong.

I also checked the other components that the guy checked in the video, i believe these are connected to the fet that i have just changed. All of them seemed to check out ok.

R406 - 5 ohm
R439 - 11.9 ohm
R493 - 12 ohm
D415 checked out ok, and i found no shorts on Q404.

From the information in the video, all of those readings seemed good, and hopefully meant that it was just the blown fet.

After changing the fet and the diodes, i reassembled the power board back into the tv and plugged all cables back in. I then plugged the set into the mains and turned it on. I heard relays clicking a couple of times, this was followed by that unmistakable sound of something shorting out. I did not see any magic smoke, but did smell a bit of it briefly. Unfortunately i was standing in front of the tv at the time i turned it on, so did not have chance to see any evidence of the fault location.

The 5 amp mains fuse however did not blow, neither did any of the fuses on the power board. But all i am getting now is the ten red blinks when i push the power button, these follow the green light flashing a couple of times, along with a couple of relay clicks.

I took the power board back out of the tv to give it a visual check - looking for any sign of a burnt component. I could find nothing obvious. I then made some checks with my meter. The mosfet i had replaced was not shorted, neither were the diodes. I also checked carefully with my magnifying glass to make sure i had not splattered solder and caused a short somewhere. all looked ok.

I then discovered that some of the components i had checked earlier were now damaged.

R439 is now reading 'open circuit', but no visible signs of burning.
R493 is now reading 22 ohm (was reading 12 ohm before), again, no sign of burning, but obviously it is now damaged.

Both of those are 12 ohm surface mount resistors.

This time around i also checked the two large white 5w 0.12 ohm resistors that sit beside the diodes i changed (R449 and R448), both of these are reading 'open circuit'. But because i did not check these out before i fitted the new parts, i cannot say if they were blown before or not.

So, by replacing the Mosfet i have gone from having a dead short that was blowing the mains fuse, to a 10 blink code with at least two components damaged when i powered up - R439 and R493. And of course i have discovered that the two large 5w resistors are open circuit.

How do you guys think i should proceed from here? Obviously i need to replace the parts i know are fried - R449, R448, R439 and R493.

Could it be that the two large 5w resistors were originally blown open when the original fet shorted out? And that powering the set back up with those blown 5w resistors still in place caused the damage to the previously good R439 and R493?

Also, could the fact that i replaced a shorted RJK5010 with a RJK5020 cause a problem? Or could the fact that i put three diodes back in place of the original two cause a problem?, bearing in mind they are all connected in parallel.

Once again, any advice on how i should proceed will be much appreciated. I have attached a couple of pics below.

I have marked the components concerned in one photo (Heat sink removed for better view). Also please ignore the tape on top of the mains filter cap, that was just to stop reflections from the camera flash.

EDIT: Yes, i know that one leg of R493 is not attached in some of the photos, i wanted to confirm that the resistor was in fact reading 22ohm, rather than the 12ohm that it should be reading.

[Steve.B]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Been doing some more checking. I have found another of the large cement resistors to be open circuit, this one is R411, a 5w 10ohm.

So that is all 3 of the cement resistors on the board reading open circuit. Is that something that is possible with the faults i have had? Or am i doing something wrong when trying to measure this type of resistor. I am just using my normal meter on it's 200ohm range, i am not getting any reading at all. I have checked that the probes are making good contact with the pads. But all three of these cement power resistors are giving me an open circuit reading.

this is a photo showing R411, which my meter is reading as being open circuit. Question, this type of resistor should be measurable with a normal meter set on a low ohms setting?

All help greatly appreciated guys.

[tw2005]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Ok, sorry you are correct, now that I see the Rev AE RJK5010 is correct, it was 5020 on our 42" but would not matter, same voltage and the 20 will be higher current.

Is the board currently assembled with all heatsinks? If so , test for short between the metal tab or centre pin of the RJK5020 to the heatsink. If it is then that is a problem. Are the 8 amp fuses still intact or open too?

[Steve.B]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Ok, sorry you are correct, now that I see the Rev AE RJK5010 is correct, it was 5020 on our 42" but would not matter, same voltage and the 20 will be higher current.

Is the board currently assembled with all heatsinks? If so , test for short between the metal tab or centre pin of the RJK5020 to the heatsink. If it is then that is a problem. Are the 8 amp fuses still intact or open too?

Thanks tw2005,

Board is now assembled again, no, i am not showing a short between the centre pin or metal tab and the heatsink. The 8 amp fuses are still good, along with the 5 amp plug fuse.


With regards to the RJK5010 and RJK5020, no worries, i was fairly certain that the 5020 would do the job with the difference being it's current handling capabilities. (I figured the same would apply when i replaced the two diodes with the three).

[Steve.B]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Anyone know of a good place to get the resistors i need in the UK? I am having trouble finding a source for the following....

R449
R448

Both are 5w 0.12r ceramic cement. (Flame proof).

I cannot find anything like them at either RS or Farnell, the closest i have found so far are these from Rapid Electronics. http://www.rapidonline.com/Electroni...sistor-62-8290

Those are 0r47, does that mean that they are 0.47ohm? If so, how critical is the value of these resistors? Could i use these instead of the 0.12ohm that are blown? Less than 1/2 ohm difference in value.


I am also looking to replace these, but i am unsure of what type they are?

R493
R439

Both are 12ohm through hole. To me they look like they could be 0.6w Metal Film, or even wire wound. They may also be the flame proof type.

If anyone knows the exact type, and where in the UK i could source them, i would be grateful.

The only one i can get an exact match for is R411, 5w 10ohm ceramic cement, those i can get from Rapid electronics.

But ideally i would like to source them all from one place, otherwise the postage costs just get silly.

Again, any help is appreciated, thanks.

[tw2005]

Re: Panasonic TH-42PX70B Blowing mains fuse.

I'm at a loss, when we had issues due to a very silly mistake of losing the insulator and we had Q406 arcing on the heatsink, The items that blew were the 2 fuses, R411. We did manage to kill the bridge rectifier. Apart from that it was straight forward.

So R406 - 5 ohm,R439 - 11.9 ohm, R493 - 12 ohm are bad
D415 , Q404 are Ok?

Sounds like something is shorted on the board possibly, we're missing something here. I don't know what to suggest except grab the schematic and check everything out .

[Steve.B]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Just to clarify.

Before i fitted the new main mosfet i checked R493 and R439, they checked out good. I also checked D415 and R406, they were good. And Q404 was not showing any shorts, so i assumed it was ok.

But after fitting the new mosfet and trying to power up the set, i found that R493 and R439 had gone bad, one completely open circuit, the other had nearly doubled it's resistance value.


So like you, i am also a bit confused, i know the insulator was in place because it's stuck tightly to the heatsink. I am kicking myself for not checking those large 5w ceramic resistors before i tried to power it back up, it might be useful to know if they were already blown, or they blew when i applied power.

I have done some checking on the board, but have not found anything else that is shorted or blown open, but when it comes to something like a chip, i really would not know how to check it. I notice that there is a small 8 pin smd chip very near to the Q404 circuit, i am wondering if that's been damaged in some way.

I am wondering if i would be better off getting a replacement board, i might end up spending £20 on parts and postage, only to find that it's still faulty. It is annoying, the parts cost pennies, it's the postage that stings.

I will keep looking tonight, if i can find the parts i know i need, then i will order them, if i can't find them i will probably grab a replacement board from ebay, there is an 'AE' version on there at the moment for £30 + £10 postage, i am very tempted.

I will let you know. Thanks again tw2005, i will keep you informed on how it goes.

[Steve.B]

Re: Panasonic TH-42PX70B Blowing mains fuse.

I don't know what to suggest except grab the schematic and check everything out .

Have started looking through the schematic for the P board, have come across something that's really confused me.

You will remember that earlier i had noticed that both R448 and R449 were both o/c, they are the 5w ceramic cement ones. this is what is written on the actual resistors...

5w 0.12ohm

According to the schematic, they should be either 10r for the AB / AD / AG / versions of the board, or 12r for my AE version of the board.

So am i interpreting the numbers wrong on my R448 and R449? Or have i actually got 0.12ohm resistors instead of the 12ohm that is stated in the schematic?

I was about to order up some 5w 0.47ohm ceramic cement replacements, hoping that they would be close enough to the 0.12ohm that are blown, now i am left wondering what to order?

Any ideas?

[tw2005]

Re: Panasonic TH-42PX70B Blowing mains fuse.

As far as those 0.12ohm resistors go, you've mixed up what the schematic states. In cct yes they simply have 12 so you could think 12ohms. The parts list definitely tells you 0.12 ohm 5w, the note in the schematic actually says R10 or R12 not 10R,12R.

Treat the R as a decimal place, so R12 = 0.12, 12R = 12, 1R2 would be 1.2

[Steve.B]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Thanks very much for explaining that, i knew i had to have something mixed up. Very glad you mentioned it because i could have made a bad mistake there.

I am guessing they use 'R' for clarity, less chance of an R being missed. (Unless your not sure to start with like me.

But i am still wondering how critical the value is, i am still struggling to find replacements at that value, closest so far is 5w 0.22ohm. Not understanding how these resistors are being used, i would not like to guess how critical that value is.

[tom66]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Likely, they will set the current ramp rate or limit of the converter, so increasing them might lead the TV to cut out on bright scenes, for example.

[tw2005]

Re: Panasonic TH-42PX70B Blowing mains fuse.

I'm refraining from guessing because I'm not sure if it's for current control and also part of the cct protection. at that resistance it's effectively a short, not exacty going to be a large volatge drop.

Instead of radial are they available in axial package? I would try to get the right value or very close to it.

Found any shorts, q404, D415 ?

This was to be an easy job. I am wondering whether Q404 has an issue but like I said we had no issues.

The other thing, maybe you have a different part of the cct failed which blew Q406.

Most of my litlle experience is with other boards, the PSU is usually very reliable in the Pannys.

BUDM is someone that comes to mind for PSU fault finding although I think most of his posts are on LCDs I think.