LCD monitor - backlight tubes or invertor?

[[email protected]]

I have a Compaq 8020 18.1" LCD display (guess 4/5 years old) that
suddenly shows no picture - was running windows & display blanked for
a couple of seconds then came back briefly on before going again -
screen is now black as if power is not on, though LEDs & control panel
buttons seem to operate as normal.

Flicking around with with on-off control eventually resulted in the
COMPAQ splash logo coming up (this happened once & only briefly)
before going back to darkness

I have taken the back off the unit - it seems to have 4 backlight
tube positions, though I can't see any light leaking from the metal
panel covering the actual LCD unit (I'm loathe at this point to
dismantle any further as I'm not familiar what might spring out into
my lap/into the carpet...

Do backlight tubes fail in this manner (ie suddenly) or do they tend
to slowly degrade?

As they seemingly have all gone at the same time, would this rather
point to the invertor board - do they fail in this way? What about
that tantilising flash of the Compaq splash logo?

Thanks for any info

 

[Art]

Scope probe set near the ballast wil produce a high freq display if the
ballast is running.

 

[[email protected]]

Scope probe set near the ballast wil produce a high freq display if the
ballast is running.

Thanks Art - Googling around tonight brings up a similar tip - holding
a small discharge tube near the inverter board apparently causes it to
light up if board is working

Also have gleaned that these inverters tend to have an internal trip
such that if an HT feed circuit to *one* backlight goes down, it trips
a cutout & all the others are disabled on the board.

Guess it all points towards the inverter board - this particular one
is a "Hitachi invc551" - comes up on Google $99 - cheaper options?
(UK)?

There seems to be mention of (SMC?) fuses in inverter boards that can
blow - I would be glad of a pointer to a quick fix - tho I can't
immediatly see any of these on this board..

-----

Supplimentary question: there seems to be a small web industry
building up around the unreliability of these HT boards - what are the
issues that seemingly make them sooo fragile??


Thx for info !!

 

[James Sweet]

Thanks Art - Googling around tonight brings up a similar tip - holding
a small discharge tube near the inverter board apparently causes it to
light up if board is working

Also have gleaned that these inverters tend to have an internal trip
such that if an HT feed circuit to *one* backlight goes down, it trips
a cutout & all the others are disabled on the board.

Guess it all points towards the inverter board - this particular one
is a "Hitachi invc551" - comes up on Google $99 - cheaper options?
(UK)?

There seems to be mention of (SMC?) fuses in inverter boards that can
blow - I would be glad of a pointer to a quick fix - tho I can't
immediatly see any of these on this board..

-----

Supplimentary question: there seems to be a small web industry
building up around the unreliability of these HT boards - what are the
issues that seemingly make them sooo fragile??

They're just cheap, that's the main reason they fail.

Lots of surplus boards out there though not all will support dimming if you
need that.

 

[[email protected]]

They're just cheap, that's the main reason they fail.

Lots of surplus boards out there though not all will support dimming if you
need that.

Yeah yeah - also 1600V+ floatin arond - spookey things might happen -
PCB track capacance etc etc - beyond my knowledge (& capacity!)...

---

my dear old Dad used to say to me -

"Never trust an electrician with no eyebrows".

;-)

 

[[email protected]]

How many tubes?
Typically each bulb has its own inverter transformer and transistor
drivers, but they all have a common regulator ic. If any of the
inverters detect a problem, the driver ic goes into shutdown, after the
initial start attempt. Each inverter output will have its own surface
mount fuse. If one opens, the unit will just flash during lamp start
up.

The fuses are ultra super small surface mount jobs. Many times they
simply open up.
The backlight bulbs can also cause the shutdown circuit to trip, if one
bulb does not start, same symptom.