Sylvania lc320ss2 LCD TV keeps blowing fuse

[Mark369]

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

but couldn't he use two 100w lights in series and just keep adding more.

or is it less watts?

[tom66]

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

Lightbulbs are resistors, two lights in series is more resistance, two in parallel may be enough to start PSU up.

[Beowulf]

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

I have a 150w lightbulb I could try.
The thing is that, if for example, I remove the FET and have the lightbulb in place, I do get the voltage on the filter cap and the lightbulb stays off.

[tom66]

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

Yes, the circuit oscillates around the transformer and transistor.
Without the transistor in place = No oscillation.
No oscillation = no power draw.

This circuit will run reliably down to just 3VDC on the caps! And it will readily continue running on very little energy. So I have half a suspicion that it is tricking the lightbulb test.

But I'm not sure how to rule it out.

[Beowulf]

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

Hi,
Yesterday I continued removing and testing components. Besides the two large inductors everything else from the primary side has been verified not to be shorted. And the two inductors were checked with the blue ring tester and they seem very OK.

Those components I'm nor sure how to test I removed and tried turning the system on to see if it'd make any difference and it did not.

Even removed those ceramic capacitors that are soldered along the line between hot/cold.

I'm waiting now for a Sencore PR57 I just bought last night, unless someone has any pointers I'll wait til I get the isolated variac to resume work on this set.

[Beowulf]

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

"Would love to learn when and when not a component could be tested in-circuit."

Logging off now - I dont know if you have seen this guide written by retiredcaps, it contains lots of tests most are carried out without power to the boards.
https://www.badcaps.net/forum/showthread.php?t=10419

There are few updates the latest starts at post19.

Selldoor, I'll check it out, thank you.

[Beowulf]

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

Hi guys,

I got the Sencor today and what a relieve when you have its benefits.

Anyway here is the update.

First thing let me tell you that I blew up the fuse and the mosfet again and I was concerned I had killed the other components I replaced before too, but after checking they're alright.
I blew them up when trying to pull a fast one and gave it 20ACV

Well luckily I had a replacement FET and a fuse too, so I installed them and started experimenting again.

Here is what I've been doing. I've been giving it around 13acv at which the TV is pulling 2amps, then I leave it for a few minutes so I can unplug the TV and go touch to see what's getting the hottest. Well various components are . The two cement resistors, the four diodes, the green resistor etc, and of course the FET. They all get pretty warm, but the filter cap stays ice cold.
The transformer starts chirping too, which I believe is normal. Only the hotside gets warm.

So, could it be that this replacement FET I got is not up to the task?
I tried to get it from digikey but it was not available so I went and got three off ebay (2SK3569), this one here:

http://www.ebay.com/itm/350648230374...84.m1497.l2649


The original one was a K10A50D.

Regards,

[budm]

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

It can be that instead of the Power MOSFET being switching on and off at high frequency like it should, it is just being Bias fully on, other word, it is not oscillating. You have to check the component around the Gate connection to see what you have at low AC setting like 10VAC.

[Beowulf]

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

That sounds interesting and makes seance.
I have a scope, still learning how to use it. Seems it'd be the best way to find out if the FET is switching?
I assume if your theory is true I'd get a constant signal rather than a square oscillating one from the gate pin?
And since I'm powering the TV from an isolated source I should place my scope's ground lead on the dot's ground (hot ground in this case)?

[budm]

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

Yes, you will get constant DC voltage at the Gate. You can use DC meter for now for the verification.

[Beowulf]

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

Cool.
Alright I did the test at about 10 VAC and the reading from the transistor's gate is a constant 5.37 VDC.

What exactly I'm I looking for on the components attached to the gate?

[tom66]

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

The zero current clamp resistor & capacitor (runs off a transformer tap to the gate directly), and the current limit transistor (connected through a 1k~10k resistor to the source of the MOSFET; which goes through a low ohm resistor to ground.)

[budm]

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

Your power supply design will be really close to what Sylvania/Emerson/Philps use. Most of them based on this design for 2-transistor power supply.

[Beowulf]

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

Thanks.
I was checking earlier and what I have in that line are a cap, a resistor, a cap (all surface mount) then a transistor, which gets hot too. I think it's defective, I should have spotted this before since I had already checked this transistor a few weeks ago but I'm still not too confident when testing transistors.
One of you experience guy could confirm? Here is the test I'm finding suspicious about it.

With the positive on B and negative on C it reads .719 (good, I believe), but with the positive on B and negative on E it reads 1.411 (isn't this too high? )
All other test pass ok.

[Beowulf]

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

Cool Bud M.

I've found some service manuals for other models but all of them have the inverter circuit separate from the main power supply board.

[budm]

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

Is it the small transistor (2sc2120 NPN) on that primay side. E to B shuould also show like diode, around 0.6~0.7. When that big MOSFET shorted ou, it could have taken out anything connected to the Gate.

[Beowulf]

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

Yeah Q601 (it's a 2SC5344 actually) behind the power FET.
It's starting to make seance now, I'll order a replacement.

[budm]

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

As you can see in the example diagram, there are Zener Diode, Diode around the Gate drive, you should check them out also.

[tom66]

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

And that transistor is responsible for the circuit oscillating correctly.

BTW, these TVs are all made by Funai. Philips just licensed their name out. So lots of customers thought they were getting a decent Philips TV, but they were getting a re-badged model instead.

[Beowulf]

Re: Sylvania lc320ss2 LCD TV keeps blowing fuse

Yeah guys I'm ordering that transistor. I think we found our problem. Everything else had been checked before on the primary side. It was this transistor that I failed to see it was defective due to inexperience testing transistor, plus a misread of the datasheet where the pin assignment wasn't too clear.