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toolmaster
Joined: 21 Apr 2010 Posts: 7
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Posted: Wed Apr 21, 2010 8:10 pm Post subject: Sylvania LC155SL8 Dead |
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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: 2879 Location: Overland Park, KS
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Posted: Wed Apr 21, 2010 9:18 pm Post subject: |
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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. |
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toolmaster
Joined: 21 Apr 2010 Posts: 7
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JTS1957
Joined: 21 Jan 2009 Posts: 2395 Location: Far, Far Away
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Posted: Thu Apr 22, 2010 8:46 pm Post subject: |
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toolmaster wrote: | and it appeared to be shorted. All 3 legs tested against the heat sink were wide open. |
Seems contradictory to me... 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 |
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toolmaster
Joined: 21 Apr 2010 Posts: 7
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Posted: Thu Apr 22, 2010 9:13 pm Post subject: |
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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: 2395 Location: Far, Far Away
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Posted: Thu Apr 22, 2010 9:25 pm Post subject: |
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What's the heatsink have to do with anything? |
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toolmaster
Joined: 21 Apr 2010 Posts: 7
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Posted: Thu Apr 22, 2010 9:28 pm Post subject: |
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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. |
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JTS1957
Joined: 21 Jan 2009 Posts: 2395 Location: Far, Far Away
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Posted: Thu Apr 22, 2010 9:34 pm Post subject: |
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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). |
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toolmaster
Joined: 21 Apr 2010 Posts: 7
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Posted: Thu Apr 22, 2010 10:33 pm Post subject: |
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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. |
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JTS1957
Joined: 21 Jan 2009 Posts: 2395 Location: Far, Far Away
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Posted: Fri Apr 23, 2010 1:40 am Post subject: |
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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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