Possible current drop-off in two merged power adapters?

[oxymoron_02]

First and foremost, apologies; this is probably the only time I'll use this forum as I simply cannot find the information elsewhere. Secondly: Hi!

My synopsis:
I have a small television that takes 12v 4A and to combat the problem of, you know, not having a power supply for it, I spliced together two 12v 2A adapters to create a Frankenstein's monster 12v 4A.
I know this was of course stupid; it's a short-term solution just to make sure the set actually works before I splash out on an adapter that will actually do the job.

My problem: The merged supplies seem to do the job brilliantly, but after some time the screen still blanks while the audio continues to play. There is no sleep timer enabled and flicking between sources results in only a momentary flicker of life before the screen dies again.

My question:
Is it possible for there to be some kind of current loss after a prolonged period of use, as a result of my using two spliced adapters; or is the set simply knackered and only fit for the recycling centre?

Thanks in advance (seems rather presumptuous, doesn't it?),
- Oxy

 

[davenn]

Its possible that the 2 adaptors cannot handle the load. Specially if the load is a full 4 amps. It would mean that the 2 x 2 Amp plugpacks are working to max capacity... NOT a good thing.

Its also possible that because the outputs of the plugpacks may not be identical, then one could be sharing more of the load than the other. This will also cause its early failure as you are seeing.
If the plugpacks are both regulated then the regulators will have a temperature and overcurrent cutout.

Dave

 

[oxymoron_02]

Thanks for your input Dave. The two should be identical (in as far as they both give their ratings at 12v 2A) though they are not actually identical supplies.

I'm going to use my meter to measure the current and leave the set running until the screen goes again, then observe the current again. I'll keep you posted on my results.


But not if I blow up.

 

[oxymoron_02]

Well the set was drawing a pretty much steady 3.5A (+/- .03A) until the screen died at which point the meter of course dropped to 0.8A. It wasn't until actually seeing the figures that I realised of course it's going to draw less with the screen dead, but that doesn't tell me why the screen is dying.

Back to square one :|

 

[davenn]

Thanks for your input Dave. The two should be identical (in as far as they both give their ratings at 12v 2A) though they are not actually identical supplies.
I'm going to use my meter to measure the current and leave the set running until the screen goes again, then observe the current again. I'll keep you posted on my results.


But not if I blow up.

ratings written on the plugpack are pretty meaningless when referenced to two of them operating the same way when paralled. variations in manufacturing etc etc will almost guarantee that they wont evenly share the load. the 3.5A contineous is pretty tough for a 4 amp max capacity.
You really really need to have the monitor on a decent PSU capable doing the job, only then will be able to determine if its a PSU problem or a problem within the monitor.

This is one of those things you just cant skimp on ... if you want to do a good assesment test, then you need the gear to do it properly, else you are just stumbling in the dark.

Is this TV a LCD type screen or something else ? you havent mentioned
this is also important to determine next steps in testing

Dave

 

[oxymoron_02]

Thanks again Dave It's a small-ish LCD, widescreen and off the top of my head a 19". My friend was going to take it to the local recycle centre because he didn't have the PSU, so I said I'd grab it just for infrequent use for movies when I'm on my PC but don't want to use up the monitor.

I'm going to be grabbing the official PSU but in the mean time is there anything I can do in terms of poking and prodding which might ascertain if it's got a genuine problem?

 

[duke37]

You could try the TV on a car battery if you have a car!

 

[davenn]

You could try the TV on a car battery if you have a car!

Try Duke37's suggestion for a decent PSU

now knowing its an LCD screen, I'm wondering if its the inverter PSU for the LCD backlight fluroescent tubes.
a bit of background you may not be aware of .... ( sorry to repeat, if you are)....

LCD screens usually have two fluro tubes that run the length of the top and bottom of the screen. they are fed from an inverter PSU that will either be an intergral pary of the main PCB or on a separate PCB. The inverter takes the 12VDC and converts it to ~ 1000V or so for the tubes

They are a VERY common source of problems, its possible one or more of the electrolytic capacitors in that section have failed ... you will see usually them bulging. In extreme cases one or bother of the driver IC's will fail ... generally in a cloud of smoke.... this is also obvious as the IC chip(s) is physically destroyed.

Take a couple of CLEAR photos of the PCB(s) and let us guys have a look over them. I emphasise the word clear as some post very blurry pix haha

cheers
Dave

 

[oxymoron_02]

Apologies for taking so long, I wanted to get the working week over with before taking apart the TV

So after several takes I managed to... persuade my phone to take some decent, unblurred shots and I have to say on first inspection I can't see anything wrong with any of the caps. I forgot to mention previously that I work at Maplin Electronics and while we do deal with the retail of components and thus have to know about them, we recieve no formal training so I'm no expert. I know enough to recognise basic components, and can read and understand their values.

I'd really hoped I'd see an obviously blown cap and be able to replace it, once you suggested that, but I'm not sure I see anything See what you think.

Simple top-down view:




A bit more depth to properly see the top and sides:





I only uploaded this one because it looks kind of awesome

 

[davenn]

That cant be the only cct board, I cant see the hi voltage supply for the LCD screen

Dave

 

[oxymoron_02]

I removed another cover and found this :| Sorry.

Though to my eye they still seem fine. It's going to be easier just to buy the actual PSU, isn't it?

 

[(*steve*)]

That board is the one which provides power to the backlight.

Open another cover, you're not there yet

 

[oxymoron_02]

The remaining cover is that which mounts the main boards above to the LCD panel itself. In the last two pictures I posted, the upper one depicts (just in view on the right hand side) a four-pin connector which is mirrored on the other side of this board.
I'm having immense difficulty unplugging them but they are connected to both the upper and lower edges of the panel, echoing what Dave earlier said.