Bush ltf22m4 mosfet replacement

[Studley]

Can anyone help me with a replacement mosfet. The attached photo shows the mosfet I'm trying to get hold of. The tv is a Bush LTF22m4 the board number is:- msdv 3206-zc01-01 (b)

Thanks in advance.

[selldoor]

Re: Bush ltf22m4 mosfet replacement

Are you sure its a mosfet and not a 1084 votage regulator?

test is here
https://www.badcaps.net/forum/showthread.php?t=22231

[Studley]

Re: Bush ltf22m4 mosfet replacement

Thanks selldoor, that does seem to be the case. Is this a case of searching the 1084 number and replacing with the same or are there different variants of voltages?

[selldoor]

Re: Bush ltf22m4 mosfet replacement

Do you need to replace it? if you were testing as a mosfet you need to test as a voltage regulator? or has it burnt up and this is not your board?
I cant tell from the picture as you have cut it off but as it doesnt indicate a voltage eg 1084-33 1084-5 it is probably an adjustable - it will have small resistors between the adjust leg and vout leg.
The 1084 is a hefty LDO and replacement needs to be 5A there are a number of
makes of 1084 but we have looked before and cant find who makes the one with the ( כ|| ) logo. They should all be a similar size but if you can find a datasheet for the one you decide on best just to check it going to fit.
Oh and you also need the same package with 3 full legs or 2.5 legs again I cant quite tell from the picture.

[Alastair E]

Re: Bush ltf22m4 mosfet replacement

Maybe we need to go back to basics....

That reg could well be just fine, O/P mistakes it for MOSFET.....

What is wrong with the set? Symptoms, and a picture of the results on the screen--If any!

[Studley]

Re: Bush ltf22m4 mosfet replacement

More information; The tv powers up and works fine on hdmi and av inputs. The problem is with the DTV. When selected, the channel comes on with good sound and picture but after about 30 seconds picture freezes and the sound ceases and doesn't come back on unless you turn off and back on again. On investigation of the board I managed to burn my finger on that particular voltage regulator. Its also situated in the area of the tuner which has led me to conclude its the culprit at fault.

[selldoor]

Re: Bush ltf22m4 mosfet replacement

I said it was a beast - they are supposed to work hot 5 amps is hot some of these work at 300 ma.
You really need to test it - I gave you the link..
I suppose it could be extra hot if the caps around it are bad making it work harder. I say test it first then consider replacing the caps

[Studley]

Re: Bush ltf22m4 mosfet replacement

Will get and test it later today and give the voltages. Just for your info, I have attached a larger view of the pcb. Thanks for all your help so far.

[selldoor]

Re: Bush ltf22m4 mosfet replacement

Ok - test the other one as well. Did it not n=seem as hot? what make series of caps are they? The one above looks like a changii but the other two cant tell.

[Studley]

Re: Bush ltf22m4 mosfet replacement

Have now measured the voltages of both the regulators. The suspect one is;- pin1, 6.60v pin2, 7.85v pin3, 20.01v
The one adjacent is;- pin1, 3.75v pin2, 4.99v pin3, 7.82v (this one is the coolest)
The caps adj are made by Aishi which I have changed out as some of the 470uf were reading just over 500uf. The others I have also changed. This has made no difference to the tv cutting out when on DTV.

[tom66]

Re: Bush ltf22m4 mosfet replacement

What happens to suspect's pin voltages when fault occurs?

[Studley]

Re: Bush ltf22m4 mosfet replacement

The voltages don't seem to change.

[tom66]

Re: Bush ltf22m4 mosfet replacement

Do you have any freeze spray? I would try cooling it with that, see if you can get picture/sound to come back. If you don't have any, PoundLand have cans 200ml for £1 which is cheaper than ebay.

I think that's an 8V regulator, for the audio circuit or such, the other regulator then uses the 8V, dropping it down to 5V for some other logic.

Very odd to see a 20V rail in a 22" TV, are you sure about that?

[Studley]

Re: Bush ltf22m4 mosfet replacement

Will have to buy some spray in the week. Glad you said 200ml cans are a pound because the most common question asked in poundland is "how much"? lol. The reason for the 20v I think is down to the mains adapter is rated at 20v output DC.

[tom66]

Re: Bush ltf22m4 mosfet replacement

Ah that makes some sense, 12V is most common. If so the heat in that chip is not at all unexpected, does TV have a USB port? If you attached 200mA load to USB port (typical when flash drive playing movie file) then the 8V regulator (the 5V ties into it) would dissipate 2.4 watts -- which would put it around 100°C. Just an example. Other loads would heat it up further.

[selldoor]

Re: Bush ltf22m4 mosfet replacement

Looks like we are looking elsewhere for the problem. Can you post full pictures of your boards

[Studley]

Re: Bush ltf22m4 mosfet replacement

Tom, I think i've just spotted the problem. I bought this tv from a guy who said the tv kept freezing and couldn't put up with it any more. You saying to me about it being strange having 20v at the regulator, I just double checked and yes the power adapter is 20v but noticed on the back of the tv, it says 12v dc input 3amp!! Conclusion, this guy has accidentally pluged the wrong power adapter by mistake thus creating this problem. The adapter he gave me is a Lenovo, (which I guess is for a computer?). Going to see if i can find something rated at 12v 3amp and give it a try. Can't believe this!!!

[selldoor]

Re: Bush ltf22m4 mosfet replacement

Sounds like that would do it- good catch.

[Studley]

Re: Bush ltf22m4 mosfet replacement

Hi selldoor and Tom, have just found a 13v adapter and tv is fine.
So lesson for today.....get right back to basics, even down to other people giving out the wrong information. I can't tell him he plugged the wrong adapter in.... only gave him £20 for the tv.

So whats he using the Bush adapter for?

[tom66]

Re: Bush ltf22m4 mosfet replacement

A friend had this problem before. Received a new 24" Bush LED TV for signing up to one of those life insurance things -- Argos direct -- inside the box was a 9V DC adapter for a cordless phone! The power light just flashed. Found a 12V adapter and it worked fine.

TBH, I'm surprised the 20V didn't blow anything up!