samsung tv s51a chassis - smps problem, advice ?

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extract from schematic:
http://www.fileden.com/files/2007/6/4/1143893/s51a%20extract.jpeg

This Tv came to me with the fuse blown and the KA3S1265 chip missing.

I replaced the ic, but have not yet plugged in. Basically , doing a
few double-checks I saw that I have a dead short on the output of the
bridge rectifier. I have removed the rectifier, c805 and c806 with no
difference. So I'm now suspecting the smps transformer, as it is the
only thing 'in-cct'.

Does this sound reasonable? I've occasionally had these transformers
cause problems before in other Tvs, but haven't worked on many
samsungs. Any thoughts welcome.
-B

Re: samsung tv s51a chassis - smps problem, advice ?



"b"

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** What about C807 ??

   That is one highly stressed poly cap.


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**  Insulation failure between layers is possible -  but you may have to
remove the tranny ( or cut some tracks) from the PCB to prove the fact.


.....  Phil







Re: samsung tv s51a chassis - smps problem, advice ?



"b"
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** You have used an insulator between the new chip and its heatsink ??

The KA3S1265 comes in metal tab TO3P-5 and fully coated versions.



....  Phil



Re: samsung tv s51a chassis - smps problem, advice ?


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You just hit the nail on the head Phil! Wish you'd posted that an hour
or so earlier!
After more head scratching, I remembered that when I went to buy the
IC (several months ago now) the TO3PF- 5L wasn't available at my local
dealer and I got the TO3P-5L.
After I checked that cap you said, I  took another look at that IC I'd
installed back then. I had fitted mica insulation but the tensioning
screw arrangement I had used to fit it to the heatsink was in slight
contact with the IC's top metal edge .After releasing it I tested
again with the multimeter and hey presto - short gone. I isolated the
whole heatsink from the chassis for good measure and the set came on
OK. D'oh...

 thanks again
-B.


Re: samsung tv s51a chassis - smps problem, advice ?




Quoted text here. Click to load it

You just hit the nail on the head Phil! Wish you'd posted that an hour
or so earlier!
After more head scratching, I remembered that when I went to buy the
IC (several months ago now) the TO3PF- 5L wasn't available at my local
dealer and I got the TO3P-5L.
After I checked that cap you said, I  took another look at that IC I'd
installed back then. I had fitted mica insulation but the tensioning
screw arrangement I had used to fit it to the heatsink was in slight
contact with the IC's top metal edge .After releasing it I tested
again with the multimeter and hey presto - short gone. I isolated the
whole heatsink from the chassis for good measure and the set came on
OK. D'oh...


** Often, the hardest faults of all for any service tech to find are the
ones  HE  puts there himself -  by accident.



....   Phil




Re: samsung tv s51a chassis - smps problem, advice ?


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Well, that didn't last long. I ran the Tv on soak for about 30 mins
and there was a fizz and it went off. The IC heatsink was very hot, so
I suspect it failed again.

incidentally, when I got this set there was a zener diode connected
track-side between ground and one of the IC pins (don't know which
one, it was hanging loose after someone had desoldered the original
IC). this was not on the schematic and looked to be a modification.
Looks like this should be checked and reconnected, I presume it is to
improve regulation. I'll take a closer look this evening. meanwhile,
any thoughts?
-b

Re: samsung tv s51a chassis - smps problem, advice ?

"b
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Well, that didn't last long. I ran the Tv on soak for about 30 mins
and there was a fizz and it went off. The IC heatsink was very hot, so
I suspect it failed again.

** Horizontal transistors often overheat and fail because of incorrect drive
waveforms  - but you have a IC with an included Mosfet.

Anyhow, check or replace C808  on the Vcc  - it might be high ESR.

The IC is almost certainly buggered.


.....  Phil








Re: samsung tv s51a chassis - smps problem, advice ?



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Update: Replaced the IC and a handful of zeners, I modified the
heatink by drilling a new hole so as to attach it well, and added
fresh heatsink compound. I think this was the reason for the first one
failing after 30 mins,  it was not the tightest fit on the heatsink.
c808 had already been changed.

Set ran all night without probs and heatink ran at a reasonable temp.
So - touch wood - it is fixed
cheers.
-B

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