samsung tv s51a chassis - smps problem, advice ?
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From: b on 19 Jan 2010 19:58 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
From: Phil Allison on 19 Jan 2010 20:22 "b" > 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'. ** What about C807 ?? That is one highly stressed poly cap. > 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. ** 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
From: Phil Allison on 19 Jan 2010 21:20 "b" > > 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. ** 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
From: b on 19 Jan 2010 22:18 On 20 ene, 03:20, "Phil Allison" <phi...(a)tpg.com.au> wrote: > "b" > > > > > 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. > > ** 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 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.
From: Phil Allison on 20 Jan 2010 05:00
"b" <reverend_rogers(a)yahoo.com> > > 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. > > ** You have used an insulator between the new chip and its heatsink ?? > > The KA3S1265 comes in metal tab TO3P-5 and fully coated versions. > 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 |