Samsung 305T Plus black screen

[budm]

Re: Samsung 305T Plus black screen

See the specheet of the MOSFET 4614:
Check Source and Drain pins.
https://cdn.badcaps-static.com/pdfs/...df367ff0cc.pdf

[andrew1]

Re: Samsung 305T Plus black screen

Let's see. So I have to measure pins:
1-5 / 1-6
3-7 / 3-8

(2 MOSFET from LEFT to RIGHT) short S2 - D2 pins / S1 - D1
This is definatelly bad.

(6,7, 9, 10 MOSFETS) "1 aka OL" S2 -D2 at first trial but will charge after a few attempts to measure them.

Last two MOSFETs (9, 10) - first time I got short only between S1 - D1.
After a few trials I found out that last two give me a few readings.
0.72 / 0.450/ 0.470 / 0.270 / 0

Picture attached.

[budm]

Re: Samsung 305T Plus black screen

Put meter in 200 oHms scale and see if it show real low Ohm. When semiconductor shorted out, it will be just like reading low Ohm resistor, no need to use Diode mode for testing them.
So many MOSFETs are on that board?

[andrew1]

Re: Samsung 305T Plus black screen

Put meter in 200 oHms scale and see if it show real low Ohm. When semiconductor shorted out, it will be just like reading low Ohm resistor, no need to use Diode mode for testing them.
So many MOSFETs are on that board?

budm. I think so, alot of them, all in the first row have the same code.

Tried your method with 200 Ohms scale.
Now it show "1" everywhere with one exception - 2nd MOSFET. The one I was sure it was bad.

Can you verify if these are the right pins?
1-5 / 1-6
3-7 / 3-8

Thanks.

[budm]

Re: Samsung 305T Plus black screen

If just one is shorted it, I would not think it will blow both fuses since they are separated from each other.
I would order a bunch of the MOSFETS for now and replace the shorted oout one.

[andrew1]

Re: Samsung 305T Plus black screen

If just one is shorted it, I would not think it will blow both fuses since they are separated from each other.
I would order a bunch of the MOSFETS for now and replace the shorted oout one.

So we have two fuses / one MOSFET but the main problem is not yet found. I will replace that mosfet later...for now let's move on to the next step.

I want to ask you a question. How we found out that we have a short circuit in the inverter? Is because I got a short between ground and one end of the fuses? or is just because these bad fuses show an increase in current after...not before them. I wonder what would happen if I connect the mainboard to the inverter and measure the ohms between mainboard ground and the other end of the fuses. Make sense?

[Lumberjack777]

Re: Samsung 305T Plus black screen

So we have two fuses / one MOSFET but the main problem is not yet found. I will replace that mosfet later...for now let's move on to the next step.

I want to ask you a question. How we found out that we have a short circuit in the inverter? Is because I got a short between ground and one end of the fuses? or is just because these bad fuses show an increase in current after...not before them. I wonder what would happen if I connect the mainboard to the inverter and measure the ohms between mainboard ground and the other end of the fuses. Make sense?

OK Andrew. You don't have to do that. The reason we suspected the inverter was out was because the backlights were not lighting up. A short on the mosfet will cause the fuse to blow. I'd just do what budm suggested and buy a couple fuses. And buy a few mosfets. That should be all you'll need to get fixed up. It's just a matter of if it was a single mosfet that went out, or another one somewhere. This is a common repair in inverters. Mosfets and fuses.

[budm]

Re: Samsung 305T Plus black screen

Just make sure that you have really verify all the MOSFETs real well since it still does not make sense why the other fuse that feed the other set of MOSFETs which shown as tested OK had blown the fuse.
Make sure there is not hidden MOSFET under the white glue.
Those yellow caps should also be replaced at the same time.
By the way:
"Can you verify if these are the right pins?
1-5 / 1-6
3-7 / 3-8"

This is how the pins are setup:
Pin 1 (dot) =S2
Pin 7~8 = D2

Pin3 = S1
Pin 5~6 = D1

[budm]

Re: Samsung 305T Plus black screen

OK, I look at the inverter transformer drive circuits setup.
There are 5 sets of transformer drivers.
The set at both ends of the board uses one set of MOSFETS to drive one transformer, the other 3 sets have the transformer's primary windings connected in parallel and driven by one set of MOSFETS per two transformers.
I would inspect the two MOSFETs on the right side of the board since it is setup the same as the set on the left side of the board that has shorted MOSFET.

[andrew1]

Re: Samsung 305T Plus black screen

Just make sure that you have really verify all the MOSFETs real well since it still does not make sense why the other fuse that feed the other set of MOSFETs which shown as tested OK had blown the fuse.
Make sure there is not hidden MOSFET under the white glue.
Those yellow caps should also be replaced at the same time.
By the way:
"Can you verify if these are the right pins?
1-5 / 1-6
3-7 / 3-8"

This is how the pins are setup:
Pin 1 (dot) =S2
Pin 7~8 = D2

Pin3 = S1
Pin 5~6 = D1

Hi budm. Thank you for corections. I forgot how to read pin order but multimeter was used the right way.

OK, I look at the inverter transformer drive circuits setup.
There are 5 sets of transformer drivers.
The set at both ends of the board uses one set of MOSFETS to drive one transformer, the other 3 sets have the transformer's primary windings connected in parallel and driven by one set of MOSFETS per two transformers.
I would inspect the two MOSFETs on the right side of the board since it is setup the same as the set on the left side of the board that has shorted MOSFET.

Now that you're mention there is something strange with those two MOSFETs on the right side (S1/D1 only). If you see the last picture I got a few random readings . In 200ohm mode everything is fine "1" but if I switch to diode mode and I measure them a few times at some point I get "0". and if I go back to 200 ohm mode fast there is "0" also for a few seconds.

I just now measured S1/D1 in diode mode:
0.67
0.147
and last 0.532 which is the quite usual for other S1/D1 MOSFETS.
Both give me identical readings when the value change.

Another strange thing is that a couple of MOSFETs show "1" for S2/D2 in diode mode. "1" is to low I think.

Should I use the gate to charge/discharge the transistor first to get a good reading?

[budm]

Re: Samsung 305T Plus black screen

"1" on the meter means Over range.

[andrew1]

Re: Samsung 305T Plus black screen

"1" on the meter means Over range.

Thank you all for your support.
Last question. How many MOSFETs should I buy. 2, 3 ,5?

[budm]

Re: Samsung 305T Plus black screen

Since it uses 5 pair (10 pieces to make it worth shipping charge), I would oder 10. You should replace them in a pair, since when one shorted out it may have weaken the other one. Make sure to replace those 6 yellow caps at the same time, get PANASONIC FM/FR/FC series.

[andrew1]

Re: Samsung 305T Plus black screen

Since it uses 5 pair (10 pieces to make it worth shipping charge), I would oder 10. You should replace them in a pair, since when one shorted out it may have weaken the other one. Make sure to replace those 6 yellow caps at the same time, get PANASONIC FM/FR/FC series.

I bought everything I need but I have to wait two weeks. I'll keep you posted

[Lumberjack777]

Re: Samsung 305T Plus black screen

I bought everything I need but I have to wait two weeks. I'll keep you posted

Thanks Andrew. Please do! It gets frustrating when some of the techs spend so much time volunteering to help a person. The person get the parts. Fixes it. And we never hear from them again, or even know if it was fixed. And another person comes along with a similar problem; reads the thread, and has no idea what fixed the problem.

Make sure to get those caps like budm mentioned. What probably happened was the caps started going high esr. That caused the mosfets to short out. Which then caused the fuses to blow. By replacing all 3 types of parts the repair will most likely last.

[andrew1]

Re: Samsung 305T Plus black screen

Thanks Andrew. Please do! It gets frustrating when some of the techs spend so much time volunteering to help a person. The person get the parts. Fixes it. And we never hear from them again, or even know if it was fixed. And another person comes along with a similar problem; reads the thread, and has no idea what fixed the problem.

Make sure to get those caps like budm mentioned. What probably happened was the caps started going high esr. That caused the mosfets to short out. Which then caused the fuses to blow. By replacing all 3 types of parts the repair will most likely last.

Still doesn't work. Was tested a few seconds. Fuses are ok but I don't want to try again. Maybe I should find a service.

[selldoor]

Re: Samsung 305T Plus black screen

Are you sure you have it all connected up? Check all the cables.

Are the fuses still ok on the inverter and with a source connected can you
see anything on screen. Did it make any pops or ticking is the power light on.

Check you put the caps in the right way round - it does happen

[andrew1]

Re: Samsung 305T Plus black screen

Are you sure you have it all connected up? Check all the cables.

Are the fuses still ok on the inverter and with a source connected can you
see anything on screen. Did it make any pops or ticking is the power light on.

Check you put the caps in the right way round - it does happen

I realised that I can't do it by myself...mosfets have 8 pins and I need a heat gun to change them. Everything was done in a service. I told the guy from there to change caps and mosfets but he said one mosfet is bad...and the rest are good. When I went home I saw the same problem...so I decided to let him take a look at SMPS, MAIN & INVERTER. Maybe the problem is not so simple...or he did a mistake by not changing all four mosfets and caps...not sure. Anyway I don't have any experience and is to complicated for me. Maybe there are some bad rezistors or caps on the mainboard to.

[selldoor]

Re: Samsung 305T Plus black screen

Mosfets nearly always should be changed in pairs or groups and as it was suspected the caps caused the problem with the mosfets ( at least two of us
said they should be changed) then they should have been changed. Did the service not have the whole set to test the board after their work?

[andrew1]

Re: Samsung 305T Plus black screen

Mosfets nearly always should be changed in pairs or groups and as it was suspected the caps caused the problem with the mosfets ( at least two of us
said they should be changed) then they should have been changed. Did the service not have the whole set to test the board after their work?

Yesterday I took with me only the inverter. But after I saw the same problem I went back with the other two boards (SMPS and MAIN). They can't do a full test...I called an hour ago and they have no clue about the problem. BTW is not a certified service or anything. Maybe you still can help me find the problem. I'm thinking if both fuses are blown...the problem should be with the inverter or maybe CCFL lamp? Thanks.