replacing backlight inverter - brightness control

[Alex Bird]

I've got an lcd monitor with a dead inverter here, screen and lamps are
good.

I can get two JKL BXA-12579 inverters from a local (uk) stockist,
though the bastards are just selling off their last stock so I don't
know where I'll get CCFL goodies after that...

What concerns me is the lack of brightness control I would then have.
I could turn off one inverter to halve the brightness, but that's quite
coarse adjustment ;o)

Any comments ?

I've got a dozen high brightness white leds here, I'm tempted to see
how well they light it up...

The original part is Lien Chang LI-2021, but I suspect the JKL parts
are tougher and easier to get.

Cheers,
Alex

 

[James Sweet]

I've got an lcd monitor with a dead inverter here, screen and lamps are
good.

I can get two JKL BXA-12579 inverters from a local (uk) stockist,
though the bastards are just selling off their last stock so I don't
know where I'll get CCFL goodies after that...

What concerns me is the lack of brightness control I would then have.
I could turn off one inverter to halve the brightness, but that's quite
coarse adjustment ;o)

Any comments ?

I've got a dozen high brightness white leds here, I'm tempted to see
how well they light it up...

The original part is Lien Chang LI-2021, but I suspect the JKL parts
are tougher and easier to get.

Cheers,
Alex

First of all are you certain the inverter is bad? I just fixed a laptop last
night that wouldn't light the screen, turns out the enable line to the
inverter was stuck low, funny thing is I jumpered it to +5v for a few
seconds and then removed the jumper and it's been working fine ever since,
guess there was something small shorting it that I managed to burn clear,
dunno.

 

[Alex Bird]

First of all are you certain the inverter is bad?

In fact, the lamps light briefly, then the monitor switches off (led
goes amber) then it tries again, indefinately...

If the inverter board is disconnected the monitor functions normally -
minus a backlight.

The supply to the inverter seems ok, the fuses on the board are okay,
nothing looks burned, and I don't have any SMD equipment, so I'm
inclined to replace unless anyone can give any specific advice.

Alex

 

[[email protected]]

Some of the inverter boards have a ceramic capaciotr across the primary
of the output transformer. (about 0.1Uf, I think). I have had a couple
of these go bad. The back light would come on then quickly shut down.
You might want to check these.
I use a NE2 neon bulb , taped to popsicle stick, to test if the
inverter is working. The gas in the bulb will glow, if there is high
voltage nearby. Hold it next to the output transformers.
Hope this helps...
John

 

[Alex Bird]

Some of the inverter boards have a ceramic capaciotr across the primary
of the output transformer. (about 0.1Uf, I think). I have had a couple
of these go bad. The back light would come on then quickly shut down.
You might want to check these.

I don't think it has these. There's a big polyester cap in each
channel, and those at the secondary.

See picture:
http://www.zen54410.zen.co.uk/linkfiles/inverter.jpg

I disconnected the primary on the right, along with the capacitor and
inductor there:

The lamps on the lefthand channel lit up more fully (before the mod) so
I thought the other channel might be at fault. I figured the
transformer could be at fault, though the transistors could too I can't
unsolder them!

The result is the other lamps now stay on for half a second or so, and
the monitor doesn't restart.
Still not working though.

I think I might have made a logical error:
Why would the monitor shut down and restart when the inverter
protection tripped? I'm not sure there's a signal there for that
purpose.
Should I be looking for a power supply problem ?

Many thanks for all your help,

Alex

 

[Alex Bird]

It's working beautifully now.

The inverter modules, BXA-12579 from JKL*, fitted quite neatly in the
space for the old quad module.

They have a more substantial transformer and no surface mount
components, so they'll probably outlast the rest of the monitor.

The brightness control seems fine, enough control via the lcd, possibly
it didn't vary the backlight level at all. I was thinking of laptop
brightness controls, which do.

http://www.zen54410.zen.co.uk/linkfiles/jkls_in_place.jpg

....Shows the fit. I had to cut a new piece of plastic to separate the
HV boards from the screen, I used LDPE from a commercial ice-cream tub.

I imagine there will be a lot more of these dying in the next year or
two, could be good business...

Alex


* - actually made by elevam

 

[James Sweet]

It's working beautifully now.

The inverter modules, BXA-12579 from JKL*, fitted quite neatly in the
space for the old quad module.

They have a more substantial transformer and no surface mount
components, so they'll probably outlast the rest of the monitor.

The brightness control seems fine, enough control via the lcd, possibly
it didn't vary the backlight level at all. I was thinking of laptop
brightness controls, which do.

http://www.zen54410.zen.co.uk/linkfiles/jkls_in_place.jpg

...Shows the fit. I had to cut a new piece of plastic to separate the
HV boards from the screen, I used LDPE from a commercial ice-cream tub.

I imagine there will be a lot more of these dying in the next year or
two, could be good business...

Alex

Nice, there's a lot of space in that thing, I would have guessed it'd be a
much tighter fit.

Yeah I'm hoping more of them start dying, greatly increases my chances of
picking up a nice dead one for free.