Repair Panasonic PT-56LCX66 TV Power Supply Board LSJB 1206-2

[SgtRock]

Greetings EEVBees:

--Please see attached picture(s) of Power Supply Board LSJB 1206-2 from the Panasonic PT-52LCX66 I am trying to fix for a friend (I.E. no charge). I have checked everything visually and can find no sign of anything, the board appears to be pristine. Checking the resistors in situ with a DMM revealed only one definite problem. The Grey Red Green Gold Green resistor in the pictures which is 3/8" or 9.5mm long, should be 82.5 Ohms ± 0.5% is reading 8 Megohms. It connects directly to the 120 V power chord, white wire, wider plug blade.

--So it looks to me like I will need a 1 Watt (or better) 82.5 Ohm ± 0.5% resistor. I have tried DigiKey, no luck. Mouser has them in 1/4 watt, so I could series parallel 4 of them easily enough, if that is acceptable. Any fault finding or advice, would be sincerely appreciated. Thanks again.

"Measure twice. Cut once"
Norm Abram 1950 -

Best Regards
Clear Ether

[steve_w]

try futurlec http://www.futurlec.com/Res1W.shtml
 part no R082R1W

futurlec are slow but reliable and its only 40 cents plus postage; plus a week or two for delivery.

:-)

regards

Steve w

[T4P]

try futurlec http://www.futurlec.com/Res1W.shtml
 part no R082R1W

futurlec are slow but reliable and its only 40 cents plus postage; plus a week or two for delivery.

:-)

regards

Steve w

0.5% NOT 5% .

[Psi]

Yeah, it's quite common for those large resistors between the high/low sides of a psu to die.
I think they're used to bootstrap the flyback and they take quite a big wack everytime you turn the power on.
The designers often spec a smaller wattage resistor than it technically should be because the resistor has plenty of time to cool down afterwards.

We had quite a few psus die repeatedly (every 6-12 months) and they just needed a bigger wattage resistor.

[nukie]

Are those caps a mix of Nichicons and Chemicons?

[SgtRock]

Greetings EEVBees:

--I want to thank everyone for holding my hand. I feel much better.

--To Nukie: The largest caps are Nichicons, the medium sizes are Chemicons and Rubycons, the very small ones are KY series Chemicons. Panasonic uses good caps. All test good for ESR.

--The aluminum SMD caps are marked UA, and have not been checked. They are to difficult to get to, and hard to decipher, being on crowded right angle daughter boards. This board is so delicate, I hate to handle it let alone do a bunch of desoldering to check anything else. I have decided to replace the resistor with four 82.5 Ohm ± 0.1% in series parallel to get the correct resistance and wattage. I will leave a little room in between the to help with heat, although as Psi says, it is the turn on surge, rather than heat that probably destroyed it. This is probably not a great solution, but I do not know what else to try. I have a calibrated Fluke 87 which in this range is only rated to ± (0.2% + 2 counts) which works out to about 0.4% in this case, so I will have to trust Mouser. The odd thing is that I will now have a standard to use to hand pick 1% resistors which cost 22 cents instead of $1.90 each. Arrg. If anyone has a better way to go, I am all ears.

--Thanks again for all the help.

"I wear suspenders and a belt. I am a security man all the way"
Justin Wilson (The Cajun Cook) 1914 - 2001
 
Best Regards
Clear Ether

[damo]

I could be wrong here, but that looks kind of like a fusible resistor. If so you may have another fault in the set somewhere that you need to find. If you want to make sure, you can get the service manual (around 8 US bucks) from here http://www.tradebit.com/filedetail.php/9028513-panasonic-pt52lcx66-service-manual. I tried very hard to find a free one so i could check for you myself, but could not find one. Anyway, good luck with the repair.

[SgtRock]

Dear Damo:

--Thanks for the tip. I already downloaded a free copy of the Official Service Manual at the below site.
For any that happen to be interested in adding a copy to the library, the download link appears about a minute of so after you get to the site.

http://elektrotanya.com/panasonic_pt.../download.html

--Damo. Do you happen to know where I could view a fusible resistor like you are talking about. Also, I thought about checking the Transient Surge Absorber marked ZNR V10681U, which is connected to the other end of the resistor in the picture, and is the fat little bugger right next to the resistor, but I do not know how to check one. I thought about getting one, but I cannot find one. All I know is that it is a Panasonic ZNR. It shows no damage. If someone could give me an idea of something to use in its place, if necessary, I would appreciate it. By the way, non of the fuses are blown. Thanks Again.

"People must understand that science is inherently neither a potential for good nor for evil. It is a potential to be harnessed by man to do his bidding."
Glenn Theodore Seaborg 1912 - 1999
 
Best Regards
Clear Ether

[davec]

The Grey Red Green Gold Green resistor in the pictures which is 3/8" or 9.5mm long, should be 82.5 Ohms ± 0.5% is reading 8 Megohms. It connects directly to the 120 V power chord, white wire, wider plug blade.

Is it just a coincidence that on the old 3 band system Grey Red Green is 8M2, which is about what you're measuring?

Before the modern 5 band (4 values + tolerance) system was adopted, weren't there some 5 band systems where the value was the first three, next would be tolerance (gold=5%) and finally power rating or temperature coefficient or something?

I take it that it is marked on the schematic as 82.5ohm?

[SgtRock]

Dear Davec:

--If you are trying to frighten me you are doing a very good job. The Service manual I have is fairly complete, but it does not have a schematic for that board. This TV was made in Sept. of 2006. The other color coded resistor on the board is a 4 band. Indeed, I have heard of 4 band resistors with a fifth proprietary band. Panasonic's published material with regard to 5 band resistor code, indicates that 82.5 Ohms ± 0.5 would be correct. On the other hand, the resistor looks mighty healthy. So I can prove your contention neither wrong nor right, and thus I sit upon the horns of a dilemma So far I am unable to find even a comparable schematic with resistor values. I guess I will try to develop a hand drawn partial schematic of the first few nodes after the power chord, and see what I can reason out. I still have the option of allowing my friend to purchase and entire new board for about $80. Panasonic quoted her $300. In any case I appreciate the warning. If it is a 8.2 Meg and I replace with 82.5 the smoke could possibly roll.

"He didn't know where he was going. But he knew where he was wasn't it."
Lord Buckley 1906 1960
 
Best Regards
Clear Ether

[davec]

My apologies, it was certainly not my intention to cause fright. I was just raising a query based on the fact the resistor appeared intact and the measured value was within 5% of 8M2. As you say, if you were to replace it with 82.5R it wouldn't react well.

It may be that the resistor is fine, in which case, time to check the silicon. I've found with TV power supplies that if it isn't dried up electrolytics (carefully placed by the designers right next to the heatsinks), it can be shorted mosfets or bridge rectifiers - although they usually cause a fuse to blow.

As you suggest, reverse engineering that part of the circuit should give an idea what it is doing, or perhaps someone else can confirm the correct value.

Thanks,

Dave

[quantumfall]

I think power resistors tend to be low values, 8 meg resistor is going to need a very high voltage to need a high wattage value.

if you see what I'm getting at.