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50" plasma smps questions

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CalebG

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Hello everyone, I was given a nexus 50" plasma a few days ago. Model is Nexus nx-502, they said it made a loud bang and quit working. I expected to find a blown cap, and I did. It was one of two 330uf 450vdc caps. I got my replacements in from digikey today after installing them I powered up the tv only to have a small blue cap right after the chopper go nuclear and blow the fuse. My question is does the first failure cause the secondary or is it more likely that the chopper or transformer failed? or was it likely just a bypass cap that was shorted and burned out?
I did a quick search for a schematic and didnt find one> anybody got an idea where to find a schematic for this model?
Thanks Caleb
 
chances are there is several damaged components. they all need to be identified and replaced together or there will be more blasts
 
No lightning damage, unless it was awhile back and it has limped along before dying. I was thinking it would be an easy fix. So much for that : ) It is a brand Ive never heard of before, so who knows how good they are with smps designs.
How critical are the bypass cap values? I can get one about the same pysical size but with out a schematic I wont know the exact value. It is a dipped blue cap (well now it is a black mess)
On a better note I finally got a replacement transistor in the mail yesterday and was able to bring an old heath oscilloscope back to life. Still got a few bug in the vertical to work out but it will be so nice to have a scope.
 
Well it appears the power supply is made by advent (782.PHST18-200B) One of the chopper ics has 3 shorted leads it is a Stk 6468 which I cant seem to find on digikey or a data sheet on the web. Does anybody have a equivalent part number?
 
I am almost sure that what you have is actually a STRF6468 which is made by sanken and used on many of the power supplies. There is no direct sub or datasheet that I am aware of but it should still be available. There were a slew of those series used on many television sets and I used to have a collection of data but have no idea where it is now:)
The trouble with majority of the plasma &/or LCD television service manuals is the lack of the schematic for the power supply. Almost always it is made by someone else and you may be lucky to get a block diagram. Although I am not familiar with the specific nexus set, I do remember seeing the same supply used on many different sets during my time with consumer electronics.

Here is an example of a 42/50 plasma service manual that used the same power supply with just a block (page 38)
**broken link removed**
 
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I got out the magnifying glass and yes indeed you are correct it is indeed a STR-F6468. Thanks for that info btw. I found a couple on ebay, but none on mouser or digikey. May have to dig through a couple smps and see if I can rescue one.
 
I found a pinout on youtube as follows
STRF6468
pin1 feedback
pin2 emitter
pin3 collector
pin4 voltage in
pin5 ground

mine is testing short on board between 2,3 and 5

Edit, just got my scope up and running :)
**broken link removed**
 
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Well I pulled that ic today and it isn't shorted. My short appears to be elsewhere on the board. :(

Is there an online resource for finding the inductance of transformers? I have two transformers marked srw4ec-x58
tdk x 0638
one is measuring 3.11 inductance on the primary while the other is 2.94, Im guessing this means the lower one has shorted turns it is also the transformer next to the blown bypass cap.

Is there a way to measure it without a flyback tester? I downloaded a schematic for the dick smith flyback tester Im going to go dig in my junk box and see if I can find the bc328, lm393 and 4015 to make it.

**broken link removed**
 
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I tossed together a basic frequency generator, output of generator
**broken link removed**
tied to primary of one transformer, scope on secondary
**broken link removed**
and tied to primary of suspect transformer, scope on secondary
**broken link removed**
they seem to have the same basic waveform but the second one has some glitches to my eye, maybe because of the missing bypass capacitor?
 
Is there a way to measure it without a flyback tester? I downloaded a schematic for the dick smith flyback tester Im going to go dig in my junk box and see if I can find the bc328, lm393 and 4015 to make it.

I doubt you could measure it WITH a flyback tester - it's not a flyback transformer, and doesn't test as one (and I've tried numerous transformers on them).

Those little blue capacitors are really unreliable, and often blow for no reason - but my concerns are still that the large electrolytics blew as well.
 
From the pic in post #9 it looks like soot marks on the left tranny. Is there a burnt component underneath?
 
Yes one of the large electrolytics did blow, I replaced both with new panasonic caps. the little blue one blew next to the transformer when I put it back together.

Good to know about the flyback tester, now that I think about it a flyback only has one coil so that makes perfect sense. I looked through my junk pile and couldnt find any of the IC's to build the flyback tester anyhow.

I am down to my last fuse and am a bit nervous about trying it again, and I still havent found a suitable replacement for the ceramic that blew. all the other E caps are testing ok with the esr meter. So the slightly different waveforms are nothing to worry about? How do you guys test a regular transformer to verify its ok or am I worrying about a component that doesnt normally fail?
 
The bypass to the other transformer is a 102k 1kv, can I replace it with a non dipped ceramic of the same value or a metalized film or does it need to be the dipped version? I cant seem to find any in my collection of that value, guess Ill have to order some replacements.
 
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The bypass to the other transformer is a 102k 1kv, can I replace it with a non dipped ceramic of the same value or a metalized film or does it need to be the dipped version? I cant seem to find any in my collection of that value, guess Ill have to order some replacements.

Generally you're not going to find a capacitor of the required specification without it being a similar type.
 
Well I went ahead and ordered them from mouser, got the ics to build the flyback tester as well while I was at it. Will let you know how it goes when they get here.
 
Well I went ahead and ordered them from mouser, got the ics to build the flyback tester as well while I was at it. Will let you know how it goes when they get here.

As long as you're still repairing large numbers of CRT TV sets it will be useful :D - I can't remember the last time I used mine, or even where it is now? (it should still be where it always was, but it's been a long time since I looked).
 
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