samsung tv s51a chassis - smps problem, advice ?

"b"

** What about C807 ??

That is one highly stressed poly cap.

** 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

"b"

** 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.

"b"

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

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

"b

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

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