Picture for 3 secs

[sinclairtv]

Sry no mod no at present. Got a 22" Swisstec portable where the backlight only lasts for 3 secs..changed inverter board to no avail. In general what should I be looking for. No bulged caps in PSU and when the pic is on, its fine. Tx.

[E Parton]

Hi Sinclair..
I'm assuming sound remains when the picture vanishes and that the backlight shuts off ?

Idealy you need to test the ccfls in the lcd panel individually to make sure you don't have a faulty substandard one ...or indeed one that doesn't light at all ..
Most invertors can sense the current drawn when all ccfls are performing correctly..
If this isn't the case ..the discrepency in current _can_ shut down the invertor control chip ..resulting in exactly the fault you are seeing ..
Do you have access to some sort of test device like the Zippy or similar ..?
If so test all ccfls ..
If all prove ok ...and you know the invertor board is good ..then the backlight is probably being shut down under cpu control from the main board ..
Check for changes in the Backlight On voltage before and after the shutdown ...
If it changes then the cpu is indeed shutting down the backlight.
As to why ...anyones guess ..
If its ccfls bellow par or faulty ..then its either a new screen ...or you can try sourcing the ccfls and replacing it/them ....

A tricky task most engineers avoid as its often uneconomical..and really needs to be done in a clean dust free environment..
Not impossible ...just damned awkward.
rgds
Evan

[ProDave]

Another trick some of us do if it's slightly dodgy ccfl's, is disable the inverter shutdown circuit.  There are several well known modifications.

Tell us the chip used on the inverter and we'll see if a mod is available.

Even if you don't do this as a "permanent" fix, it will allow the inverter to run for longer so you can see the condition of the tubes etc.

[Turnip]

Hi P/D,

Chap has replaced inverter, Guess CCFLs are the biz with 22/24 Tv and monitor screen stuff probs becoming popular - wonder why, large screen sets usually groan on for ever though their DVB panels won't update  - Chris

[sinclairtv]

Update..I could hear a hissing sound which stopped when pic went off. Changed screen and problem solved though I couldn't get into screen to see if a tube was arcing. 

[Turnip]

Hi Sinclairtv,

Congratulations - would suggest on future posts more model info would be helpful to other folks - Chris