Panasonic TH-42PX70B Blowing mains fuse.

[Steve.B]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Thank you both. So fitting the 0.22ohm ones i have found could cause problems. But i am wondering if they would be good enough just to determine whether this thing is going to go into meltdown again once i have replaced the known faulty parts? Once i know that is not going to happen, then i would not mind waiting a few weeks for the correct value to arrive from the far east. I have searched a lot of places in the UK, and 0.22 is the closest i have found. I have indeed even tried looking for them in an axial package, but no luck there either.

With regards to D415 and Q404, no shorts. Normal sort of diode drop reading from D415, and similar sort of readings from Q404, but nothing even close to a short. The only known problems in that part of the circuit are R439 and R493.

IC406 is something i am wondering about, seeing that R439 and R493 go down to pin 7 of that chip. I also notice that one of my blown ceramic resistors - R448, also connects to that chip via pin 6. (GND).

Talking of which, pin 6 of that chip (IC406) is show on the schematic to be 'GND', but i have looked for continuity between that pin and one of the pcb ground points (ground to chassis), but am not getting continuity. What ground would that pin be connected to?

[tom66]

Re: Panasonic TH-42PX70B Blowing mains fuse.

It should fire up I'd just expect pink tinge due to insufficient Vsus on bright scenes.

[tw2005]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Can you see the part number of the device for maybe a datasheet? Also a close up photo of that and surrounding areas.

I was just reading again previous post and I was wondering about that too. Is that where they also recommend that mod adding the cap to prevent Q406 failing again. i guess that's the driver IC?

Intersting you mention no continuity to gnd. One issu i had just before I got my PSU going again was I found that the PCB track had fused under a joint somewhere within the circuit near Q406.

Could not see it but when everything was replaced properly, It no longer would blow up or power on and i think it was ten blinks.

For whatever reason I decided to test for example a pin on the transistor to the other end of the cct traces and found no connection. Had to scrap the board back and bridge it.

I remember it well because i was stumped for a while as nothing else was making sense. It was our fault because of that shorted to heatsink transistor and we ran out of fuses and started using bigger ones which is dumb.

[Steve.B]

Re: Panasonic TH-42PX70B Blowing mains fuse.

It should fire up I'd just expect pink tinge due to insufficient Vsus on bright scenes.

Thanks, Tom, I will go with the 0.22ohm for now, at least until i know that we have life again.

[tw2005]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Talking of which, pin 6 of that chip (IC406) is show on the schematic to be 'GND', but i have looked for continuity between that pin and one of the pcb ground points (ground to chassis), but am not getting continuity. What ground would that pin be connected to?

Looking at the schematic, I'm thinking if R448,449 are both open it may not be grounded.

If the resistors are out, confirm pin 6 to the pad of where it connects to r448,449. then try from there to gnd, any mounting pad should be gnd, then try to other side of where those resistors are to gnd and see what you get.

Not sure if I'm right or not but I'm curious to read what happens.

[Steve.B]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Can you see the part number of the device for maybe a datasheet? Also a close up photo of that and surrounding areas.

I was just reading again previous post and I was wondering about that too. Is that where they also recommend that mod adding the cap to prevent Q406 failing again. i guess that's the driver IC?

Intersting you mention no continuity to gnd. One issu i had just before I got my PSU going again was I found that the PCB track had fused under a joint somewhere within the circuit near Q406.

Could not see it but when everything was replaced properly, It no longer would blow up or power on and i think it was ten blinks.

For whatever reason I decided to test for example a pin on the transistor to the other end of the cct traces and found no connection. Had to scrap the board back and bridge it.

I remember it well because i was stumped for a while as nothing else was making sense. It was our fault because of that shorted to heatsink transistor and we ran out of fuses and started using bigger ones which is dumb.

IC406 seems to be a standard L6561D, could not find it in the parts list, but the schematic gives that number, and that matches what i can see on the actual chip. Plenty on eBay, so no problem getting hold of one. £2.46 with free postage. - http://www.ebay.co.uk/itm/Stmicroele...item1c32de1bb7

Here is the data sheet for it. http://www.st.com/web/en/resource/te...CD00001174.pdf

I am also under the impression that this chip is driving the large Q406 Mosfet.
I have included a photo below that shows the chip, about the best i can get with my camera, but you should be able to zoom in. I have also marked R439 and R493, you will see i have managed to destroy one of the pads for R493, my own fault, i had the temperature on my iron set too high (had it turned up to do those large Q406 joints, forgot to turn it down before messing with the smaller stuff), but i can see where the trace runs down to pin 7 on the IC, luckily there is plenty of the trace left to scrape back and solder the new resistor to.

With regards to IC406 pin 6 to GND, i am not sure if that means the chassis gnd, if it does i will have to recheck, because i am sure i have no continuity to ground. Like i mention before, i am using one of the pcb's chassis ground points.

[tw2005]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Are those 2 0.12 resistors open cct and removed? I think it may go to gnd via them? Earthing pads on the PCB, or the - pin on the bridge rectifier.

[Steve.B]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Looking at the schematic, I'm thinking if R448,449 are both open it may not be grounded.

If the resistors are out, confirm pin 6 to the pad of where it connects to r448,449. then try from there to gnd, any mounting pad should be gnd, then try to other side of where those resistors are to gnd and see what you get.

Not sure if I'm right or not but I'm curious to read what happens.

Ok, i have continuity between pin 6 of the IC to the pad that connects to R448, 449. But i do not have any continuity between the resistor pads (either side of resistors) and chassis gnd points.

I seem to remember reading in other threads about different gnd points. Would there be a difference between the DC and AC ground points? If so, would the GND point for that IC be a DC GND point, rather than an AC ground point? (I am presuming that chassis ground would be on the AC side of things? Am i making any sense?

[Steve.B]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Are those 2 0.12 resistors open cct and removed? I think it may go to gnd via them? Earthing pads on the PCB, or the - pin on the bridge rectifier.

No, they are not removed from the board yet, but seeing that they are totally open circuit, i don't think removing them will change any readings, but i will pull them out now, just to make sure.

With regards to the bridge rectifier, yes, pin 6 of the IC does have continuity to one of the rectifier pins, the one that connects at ZH17. Will that be the GND reference i am looking for with regards to the IC? But again, that connection does not have continuity to any of the chassis GND points.

[tw2005]

Re: Panasonic TH-42PX70B Blowing mains fuse.

I know there's hot gnd and cold ground. never gone right into it. Cold ground I would expect not chassis. That point connects to - on the bridge so maybe you have the wrong ground point. The ic is connected to where it needs to be.

[tw2005]

Re: Panasonic TH-42PX70B Blowing mains fuse.

I would not bother , like you say it does not prove anything if they are confirmed open,

[Steve.B]

Re: Panasonic TH-42PX70B Blowing mains fuse.

I know there's hot gnd and cold ground. never gone right into it. Cold ground I would expect not chassis. That point connects to - on the bridge so maybe you have the wrong ground point. The ic is connected to where it needs to be.

Thanks, so pin 6 is getting what it needs, that is the main thing.

I am double checking everything at the moment, D415, i have taken it out of circuit to check it properly, using the diode check range on my meter, i am only getting a reading one way, like i would expect, but the reading is rather low to what i normally see with a diode, i am getting around the .205 mark. Would that be just down to the type and value of the diode? Or is it something to look at do you think?

[Steve.B]

Re: Panasonic TH-42PX70B Blowing mains fuse.

I am double checking everything at the moment, D415, i have taken it out of circuit to check it properly, using the diode check range on my meter, i am only getting a reading one way, like i would expect, but the reading is rather low to what i normally see with a diode, i am getting around the .205 mark. Would that be just down to the type and value of the diode? Or is it something to look at do you think?

I can answer that one for myself, D415 is a Schottky Diode - D1NS4. Just looked at it's data sheet and the low voltage drop i am getting is normal for this type of diode.

[Steve.B]

Re: Panasonic TH-42PX70B Blowing mains fuse.

A further bit of information. It looks like i have found another blown fet on the board. Q422 is showing a constant resistance of 57ohm between the gate and source pins. I have received advice elsewhere on the forum that suggests that the fet is damaged. Again i am lucky that it is a commonly available part. I have ordered a couple of them, just in case i mess up fitting it the first time around. It a tiny SOT-323 device, could be fun fitting that!

So this is my current list of known bad components:

R448 & R449
R439 & R493
R411
Q422

So i am making progress. Finding that dodgy Q422 may explain why the driver circuit for the main power fet had a meltdown (R439 and R493).

[Steve.B]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Q422, looks like the drain from that Mosfet supplies the Vcc for IC406? I am guessing that the Vcc is very important for IC406, so a faulty Q422 would cause a major issue.

I think i might replace IC406, not sure how to test it, but seeing that i now have 5 different faulty components that connect to it, i think changing it would be a wise move.

Yes, it would have been easier to just swap the whole board, but this is so much more interesting, and rather fun. Shame my living room now looks like a workshop though. That did not take long to happen!

[tw2005]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Q422, looks like the drain from that Mosfet supplies the Vcc for IC406? I am guessing that the Vcc is very important for IC406, so a faulty Q422 would cause a major issue.

I think i might replace IC406, not sure how to test it, but seeing that i now have 5 different faulty components that connect to it, i think changing it would be a wise move.

Yes, it would have been easier to just swap the whole board, but this is so much more interesting, and rather fun. Shame my living room now looks like a workshop though. That did not take long to happen!

Well, good you found it because all I could think of was there had to be something else shorted out but I had no idea really where to point you, you have a different fault to what mine wa and the common fix.

probably would be just as easy to get a board but yes once you get your teeth into it it's hard to nit enjoy the challenge of fault finding it.

Hope 2nd time round it works for you.

[Steve.B]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Well, i will wait for all the parts to arrive, then i will know for sure if i have found everything. If it's still not working after all of this, then a new board it will be.

I am thinking about using budm's trick of wiring it up to the mains via a couple of light bulbs when i first put power to it, hopefully that will stop it having another meltdown if there is still a fault.

Seeing that when i first checked the mosfet driver circuit it seemed fine, no blown resistors or shorts, i can only think that when the original mosfet blew, it took with it the large ceramic resistors, those i did not check. Perhaps powering it up with those open circuit caused the extra damage i now have. That is what i am hoping anyway.

I will of course post back once the board is back together - hopefully with some good news. Fingers crossed.

Thank you again for your input, it is very much appreciated this end.

[tw2005]

Re: Panasonic TH-42PX70B Blowing mains fuse.

The bulb trick works, I never mentioned it but it was actually the only time I've ever tried it was on this very PSU. Certainly helped stop destroying more fuses and board damage until I worked out the FET was shorted still.

[Steve.B]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Success!!!

The last of the parts arrived today - the little SO-8 mosfet driver chip, and two of the tiny SOT-323 2sk3018 mosfets. I fitted them, along with the 5 replacement resistors, plus i also replaced the main power mosfet (Q406), this time i put a RJK5010 back in, rather than the uprated RJK5020 i put in last time.

I felt great relief and satisfaction when i pushed the power button and was welcomed with a flashing green light, then she booted up like normal. I even had an added bonus, the thin vertical line that had been reported was nowhere to be seen!

In all i guess i used about £15 worth of parts. Plus an hour or two looking carefully at the schematic for the power board, something i am glad i did, for it led me to those two tiny mosfets that were found to be blown - Q402 and Q422, both 2sk3018's. The schematic also showed me that the common link between all of the blown components was IC406, the chip that drives the main power mosfet i believe.

So, list of parts replaced.

R448 & R449 (5w 0.12ohm)
R439 & R493 (0.6w 12ohm)
R411 (5w 10ohm)
Q402 & Q422 (2SK3018)
IC406 (ST L6561)
Q406 (RJK5010)

So thank you again to tw2005 and tom66 for your input, very greatly appreciated by me, and i am sure it will be appreciated even more by my 4 year old granddaughter.

Strange how that one pixel wide vertical line seems to have fixed itself, my daughter-in-law said it done the same thing a year or so ago, only with a line in a different place, that also disappeared on it's own. That one never came back, so lets hope this one doesn't either.

[tw2005]

Re: Panasonic TH-42PX70B Blowing mains fuse.

Nice , So all those were bad or were the driver and smd FETs just precautionary?