On 20 ene, 11:00, "Phil Allison" 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.

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