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toolmaster



Joined: 21 Apr 2010
Posts: 7

PostPosted: Wed Apr 21, 2010 8:10 pm    Post subject: Sylvania LC155SL8 Dead Reply with quote

Howdy - I'm new to the forum but I've used posts from here to repair a few tvs previously.

I was asked by my father in law to take a look at this tv and see what I could do. I don't know what happened to it - but when I got it, it was blowing the 4a fuses. I usually assume that a part with a heat sink is likely to fail first...so I tested a little fet transistor (to the best of my limited ability) and it appeared to be shorted. All 3 legs tested against the heat sink were wide open. I replaced that (thanks digi-key) and now it doesn't blow fuses. But it doesn't do anything else either.

Any ideas?







Last edited by toolmaster on Thu Apr 22, 2010 1:02 pm; edited 1 time in total
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kuhurdler



Joined: 28 Dec 2006
Posts: 2858
Location: Overland Park, KS

PostPosted: Wed Apr 21, 2010 9:18 pm    Post subject: Reply with quote

So, someone gave you a busted TV? And they didnt say what was wrong with it?

start checking diodes, and that transistor next to the one you replaced.
toolmaster



Joined: 21 Apr 2010
Posts: 7

PostPosted: Wed Apr 21, 2010 9:58 pm    Post subject: Reply with quote

Sure he gave me a busted tv. And he wants it back working.

I've tested all the diodes as per this instruction

http://www.electronics-radio.com/articles/test-methods/meters/multimeter-diode-transistor-test.php

They seem fine. I set the ohm meter to Rx10.

I've tested the transistor next to the one I replaced as per this instruction

http://www.stickycarpet.com/pinx/test.html

and it seems fine.
JTS1957



Joined: 21 Jan 2009
Posts: 2328
Location: Far, Far Away

PostPosted: Thu Apr 22, 2010 8:46 pm    Post subject: Reply with quote

toolmaster wrote:
and it appeared to be shorted. All 3 legs tested against the heat sink were wide open.


Seems contradictory to me...Confused sort of like...
It's better to be safe than sorry.
Nothing ventured, nothing gained.


Last edited by JTS1957 on Thu Apr 22, 2010 9:25 pm; edited 1 time in total
toolmaster



Joined: 21 Apr 2010
Posts: 7

PostPosted: Thu Apr 22, 2010 9:13 pm    Post subject: Reply with quote

You saying all three leads should conduct with the heatsink? After I took it off the board, it didn't seem to pass as per this test

http://www.noahtec.com/testing-fet.htm
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JTS1957



Joined: 21 Jan 2009
Posts: 2328
Location: Far, Far Away

PostPosted: Thu Apr 22, 2010 9:25 pm    Post subject: Reply with quote

What's the heatsink have to do with anything?
toolmaster



Joined: 21 Apr 2010
Posts: 7

PostPosted: Thu Apr 22, 2010 9:28 pm    Post subject: Reply with quote

Every other transistor I've checked with that same test didn't conduct on every lead. Seems to me that means a short. Quick and dirty test perhaps? Regardless...it must have had a short because when it was replaced, it stopped blowing fuses. The question is where to go from here.
JTS1957



Joined: 21 Jan 2009
Posts: 2328
Location: Far, Far Away

PostPosted: Thu Apr 22, 2010 9:34 pm    Post subject: Reply with quote

If one you pulled out is shorted it WILL read like a "jumper wire" between one or more pair of pins.

You may want to switch over from your analog to your digital meter and check for b-plus voltage between hot ground and the center pin of that FET. If not present check surge resistors in the area (analog/digital meter - your choice).
toolmaster



Joined: 21 Apr 2010
Posts: 7

PostPosted: Thu Apr 22, 2010 10:33 pm    Post subject: Reply with quote

I'm not sure what b-plus voltage is. I read 120v between the drain and hot ground. I did find two diodes that seem to be faulty.



D909 and D910 are faulty according to my trusty cen-tech meter.
JTS1957



Joined: 21 Jan 2009
Posts: 2328
Location: Far, Far Away

PostPosted: Fri Apr 23, 2010 1:40 am    Post subject: Reply with quote

Unsolder either end of both diodes and test "out-of-circuit."
FET's sometimes use a drain resistor, that if the FET was indeed shorted may have opened or changed value.
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