Riddled TV Repair Forum

toolmaster · Joined: 21 Apr 2010 Posts: 7

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?

Google

kuhurdler · Joined: 28 Dec 2006 Posts: 2875 Location: Overland Park, KS

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

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: 2379 Location: Far, Far Away

toolmaster · Joined: 21 Apr 2010 Posts: 7

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

Google

JTS1957 · Joined: 21 Jan 2009 Posts: 2379 Location: Far, Far Away

What's the heatsink have to do with anything?

toolmaster · Joined: 21 Apr 2010 Posts: 7

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: 2379 Location: Far, Far Away

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

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: 2379 Location: Far, Far Away

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.