Samsung LE40R87BD

[ronhayes]

Hi All
Now have this set up and running with the fantastic help I have received from Alastair what a star he is.
But now I am presented with another fault on the same set. After I replaced the main board and got the picture up it turns out that the whole screen does not light up only the right hand side lights up, The side that the inverter fits!!.
It looks like the tubes are only lighting half way , That is assuming they run horizontal not vertical. they stay on not fading out as the 2 second dark fault.
So now I am seeking more help could this be faulty tubes or faulty inverter board or maybe the transformer on the power board t1801.
Has any one had this kind of fault on one of these sets or is this a common fault, have tried searching for this fault but all I can see is either they don't come on at all or the 2 second to dark screen nothing on only lighting half the screen.
Any help much appreciated on how to pin point which parts are faulty, I will include some pictures of inverter and power board and of screen view.

Ron.[ATTACH][ATTACH][ATTACH][/ATTACH][/ATTACH][/ATTACH]

[Alastair E]

Re: Samsung LE40R87BD

Hmm--That is odd....

Normally the tubes--if they light at all--light their whole length--may get an odd one doing odd things sometimes--but not all of 'em at the same time....

Check/confirm PSU rails etc, make sure the caps are good/replaced,--dunno, not had that issue myself....

[Alastair E]

Re: Samsung LE40R87BD

Ah--Yes, I have, just thought--But you aint gonna like it!

Twas the panel tabs making poor contact, effected a cure with the foam-tape trick....

[ronhayes]

Re: Samsung LE40R87BD

Intrigued what is foam-tape-trick ?

[tom66]

Re: Samsung LE40R87BD

Doesn't look like a tab bond fault but Ron can confirm by setting TV to dark image and checking in a dark room - the backlight should fade if it is a backlight fault, or it will be solid if not.

My guess - the main HV transformer on SMPS is not being driven right,for one reason or another, and the output is not sufficient to strike the tubes and make them light fully. Once the tubes strike, their operating voltage requirement reduces considerably.

[ronhayes]

Re: Samsung LE40R87BD

With room light off at night and set powered up looking at the back of set you can see bright light through holes in metal casing near inverter board but further along by the main board you can't see any light through the holes, This makes me curious as it is quite clear their is no light coming through toward the end of the tubes so they are clearly not lighting up at the ends. And as you say I could imagine maybe one or possibly two tubes being faulty but not all tubes.
I did remove inverter board and cleaned all contacts that slot into connection with fibre glass pen, on the off chance they were poor connections . This made no difference what so ever.

[ronhayes]

Re: Samsung LE40R87BD

I think you are right Tom I think the problem is in the power supply as when the tubes are on they are steady not fading just not lighting all the way.
Even though I have upgraded all the 1000uF caps to 25v I think I will change them again as the caps I replaced them with although they were rated 1000uF 25v they were smaller type caps in case size as apposed to the ones removed.
I will send for some more and see what results from this. (who knows that may cure it)

[budm]

Re: Samsung LE40R87BD

Did you all the balancer board mounting screws back in place? It looks like the lamp current return path is open.

[ronhayes]

Re: Samsung LE40R87BD

Yes budm
I had put its cover on and all mounting screws were fully tightened .
Thank you all help. (think I will go with caps and see if this helps)

[tom66]

Re: Samsung LE40R87BD

The secondary side capacitors probably have nothing to do with this.

[ronhayes]

Re: Samsung LE40R87BD

Hi Tom
I am attaching a couple of pictures of the power board hoping you may point me in the right direction for the hv transformer on the smps not being driven correctly to strike the tubes to light up fully.
Any help would be much appreciated.

[tom66]

Re: Samsung LE40R87BD

I'd check that red cap near the transformer (check on cap meter, no need to do ESR test.) Also, make sure you're getting the full PFC voltage across the large 450V capacitor - around 390V when all running.

Drive waveforms are generated on secondary near TI802.

[ronhayes]

Re: Samsung LE40R87BD

Hi Tom
Checked the red cap it is 1uf 400v I was reading 1.2uf on my cap reader and the voltage across the 450v cap is 382v.

[ronhayes]

Re: Samsung LE40R87BD

Hi Tom
Any more info on this for me ?

[ronhayes]

Re: Samsung LE40R87BD

Ok thank you Tom

[tom66]

Re: Samsung LE40R87BD

I suppose I have one last thought: possibly the blue poly caps are bad. Run the set for about 10min. Turn it off (!!) and then check the capacitors near the output. And any on the inverter/balance board. Are any getting very hot? Do they show visible signs of discolouring or burning?

[ronhayes]

Re: Samsung LE40R87BD

Ok thanks Tom
I will give that a try tomorrow.

[ronhayes]

Re: Samsung LE40R87BD

Well I have carried out that test, with leaving set on for ten min then turned off and checked blue poly caps non on inverter/balance board but felt around all components but nothing getting hot !!
Oh well never mind worth a try.
Is their any way I can test tubes with a direct power source say with a variac transformer to see if they fully light up , And also could I check the inverter board using the same method to rule them out ? My variac transformer ranges from 0-260 volts.
See picture.

[tom66]

Re: Samsung LE40R87BD

Unfortunately the tubes require about 5000V to strike (ignite) then about 2500~3000V to operate. Also they operate on about 30kHz~60kHz so a 50Hz variac would not do it unfortunately.

You would need a high voltage probe (100X) for an oscilloscope or multimeter to test the output safely.

You can also try a common trick for old flybacks and that is to take a neon bulb (little indicator thing, like you'd find in an extension plug) and place it near the HV cables and the bulb connectors. It should glow near the HV. I have done this before. Where it does not glow requires investigation.

[ronhayes]

Re: Samsung LE40R87BD

Thank you for that Tom.
But unfortunately don't have any thing like that, I have an electrical tester screw driver handy would that work going near HV but not touching of course ?