Need help to diagnose laptop LCD problem

[pbl]

Hi - I have an older Dell Inspiron 8500 running Windoes XP and the LCD
doesn't work. I purchased a 2nd hand LCD backlight and inverter cheaply some
months ago and only tonight got around to trying to fix the problem. I was
planning to replace the inverter first to determine whether the inverter or
lamp was faulty but upon opening the package I discovered that the inverter
sent was the wrong one. Before I go ahead and order another, I hope I can
satisfactorily describe the problem so that someone can help me to decide
whether it is the inverter or backlight at fault.

The symptom started months ago with a reddish tinge to bottom half of the
display at start-up. This was annoying but it usually went away after the
display "warmed up." There was no apparent loss of brightness, although
compared with my new laptop display is was less bright. One day, the display
simply went dark to the point that I couldn't see anything. Not jet black
because if I was to turn the laptop off you could see the display go even
blacker.

Now I find this problem - I can turn on the computer and the display fires
up normally but with the reddish tinge. After 30 - 60 seconds (it varies) by
the time the red tinge has generally gone, the screen cuts out and goes dark
again. I can repeat this over and over and each time I restart, the display
works for a short period and then goes out.

Can anyone tell me if these symptoms suggest a faulty inverter, backlight
assembly, or (dare I say) both.

Many thanks - pbl

 

[Paul]

Hi - I have an older Dell Inspiron 8500 running Windoes XP and the LCD
doesn't work. I purchased a 2nd hand LCD backlight and inverter cheaply
some months ago and only tonight got around to trying to fix the
problem. I was planning to replace the inverter first to determine
whether the inverter or lamp was faulty but upon opening the package I
discovered that the inverter sent was the wrong one. Before I go ahead
and order another, I hope I can satisfactorily describe the problem so
that someone can help me to decide whether it is the inverter or
backlight at fault.

The symptom started months ago with a reddish tinge to bottom half of
the display at start-up. This was annoying but it usually went away
after the display "warmed up." There was no apparent loss of brightness,
although compared with my new laptop display is was less bright. One
day, the display simply went dark to the point that I couldn't see
anything. Not jet black because if I was to turn the laptop off you
could see the display go even blacker.

Now I find this problem - I can turn on the computer and the display
fires up normally but with the reddish tinge. After 30 - 60 seconds (it
varies) by the time the red tinge has generally gone, the screen cuts
out and goes dark again. I can repeat this over and over and each time I
restart, the display works for a short period and then goes out.

Can anyone tell me if these symptoms suggest a faulty inverter,
backlight assembly, or (dare I say) both.

Many thanks - pbl

The state of both components has an impact on circuit operation.

I found a datasheet, which describes some of the conditions an
inverter would have to deal with. This chip has an "ignition state"
and a "burn state", with the "ignition state" overcoming the high
resistance of the tube when it is cold. The voltage is reduced
once the tube starts to glow.

http://www.nxp.com/acrobat_download/datasheets/UBA2070_2.pdf

In terms of controlling the tube, a popular technique is pulse
width modulation (not mentioned in that data sheet). That means
the oscillator delivers a burst of pulses to the transformer,
operating the tube for a fraction of a second, then stops for
a fraction of a second. The alternation is done at a frequency
above human flicker perception. The current flow level is the
normal "burn" level, during the burst. That results in the
color of the tube being preserved (white). By varying the
duty cycle of the PWM, a large intensity range can be
achieved (weak white to strong white). This data sheet has
pictures of the waveforms used.

http://www.fairchildsemi.com/an/AB/ABR-42023.pdf

If, on the other hand, you were to try reducing the voltage,
there would likely be a color shift on the tube.

Your "dim and white" condition, implies a reduced PWM duty
cycle (like the control signal was wrong). Your "reddish tinge"
suggests to me, the tube is not operating in its normal
operating range. The duty cycle may be proper, but the
voltage developed across the tube might be too little.

For the tube to fail, you'd need a lot of service hours
on the tubes. Or, if the tube seals failed, that would be
a possible reason for the tube to shift characteristics.
Also, the connectors to the tube can corrode or misbehave,
and sometimes, simply removing and reinstalling the connector
is enough to restore operation for a while.

When working on that stuff, put it back exactly as you
found it. Every part in there is important. The tubes
are capacitively coupled, and if you find foil around
the tube area, the foil might actually have electrical
properties. The whole thing is very much a "flaky pastry"

If you've already taken it apart, checking the connectors
is a good first step.

Good luck,
Paul

 

[pbl]

The state of both components has an impact on circuit operation.

I found a datasheet, which describes some of the conditions an
inverter would have to deal with. This chip has an "ignition state"
and a "burn state", with the "ignition state" overcoming the high
resistance of the tube when it is cold. The voltage is reduced
once the tube starts to glow.

http://www.nxp.com/acrobat_download/datasheets/UBA2070_2.pdf

In terms of controlling the tube, a popular technique is pulse
width modulation (not mentioned in that data sheet). That means
the oscillator delivers a burst of pulses to the transformer,
operating the tube for a fraction of a second, then stops for
a fraction of a second. The alternation is done at a frequency
above human flicker perception. The current flow level is the
normal "burn" level, during the burst. That results in the
color of the tube being preserved (white). By varying the
duty cycle of the PWM, a large intensity range can be
achieved (weak white to strong white). This data sheet has
pictures of the waveforms used.

http://www.fairchildsemi.com/an/AB/ABR-42023.pdf

If, on the other hand, you were to try reducing the voltage,
there would likely be a color shift on the tube.

Your "dim and white" condition, implies a reduced PWM duty
cycle (like the control signal was wrong). Your "reddish tinge"
suggests to me, the tube is not operating in its normal
operating range. The duty cycle may be proper, but the
voltage developed across the tube might be too little.

For the tube to fail, you'd need a lot of service hours
on the tubes. Or, if the tube seals failed, that would be
a possible reason for the tube to shift characteristics.
Also, the connectors to the tube can corrode or misbehave,
and sometimes, simply removing and reinstalling the connector
is enough to restore operation for a while.

When working on that stuff, put it back exactly as you
found it. Every part in there is important. The tubes
are capacitively coupled, and if you find foil around
the tube area, the foil might actually have electrical
properties. The whole thing is very much a "flaky pastry"

If you've already taken it apart, checking the connectors
is a good first step.

Good luck,
Paul

Thanks Paul - that is very helpful.

 

[pbl]

Why ask here? This is a pure hardware problem that has nothing to do
with the OS.

Ask elsewhere

Why - because this newsgroup is entitled ...windowsxp.hardware. Please don't
bother saying anything if you have nothing constructive or helpful to say.

 

[M.I.5¾]

Why ask here? This is a pure hardware problem that has nothing to do
with the OS.

Ask elsewhere

Just ignore our resident ****wit.

This *is* a *hardware* newsgroup.

 

[M.I.5¾]

Why - because this newsgroup is entitled ...windowsxp.hardware. Please
don't bother saying anything if you have nothing constructive or helpful
to say.

Unfortunately, Encino Man (or any of his numerous previous incarnations) has
had nothing constructive of useful to say in this (and other) newsgroups for
years. Unfortunately he is a mentally retarded 13 year old child.

 

[M.I.5¾]

But this post had to nothing to do with WindowsXP. It was pure
hardware and as such is out of place in this group. Or did you not
notice the WindowsXP part of the title?

You are obviously confused. You are confusing him with someone who gives a
damn for your ****wit postings.

That his question was answered clearly (at least to everyone else) shows
that it was entirely the right place to ask.

 

[M.I.5¾]

The state of both components has an impact on circuit operation.

I found a datasheet, which describes some of the conditions an
inverter would have to deal with. This chip has an "ignition state"
and a "burn state", with the "ignition state" overcoming the high
resistance of the tube when it is cold. The voltage is reduced
once the tube starts to glow.

http://www.nxp.com/acrobat_download/datasheets/UBA2070_2.pdf

In terms of controlling the tube, a popular technique is pulse
width modulation (not mentioned in that data sheet). That means
the oscillator delivers a burst of pulses to the transformer,
operating the tube for a fraction of a second, then stops for
a fraction of a second. The alternation is done at a frequency
above human flicker perception. The current flow level is the
normal "burn" level, during the burst. That results in the
color of the tube being preserved (white). By varying the
duty cycle of the PWM, a large intensity range can be
achieved (weak white to strong white). This data sheet has
pictures of the waveforms used.

http://www.fairchildsemi.com/an/AB/ABR-42023.pdf

If, on the other hand, you were to try reducing the voltage,
there would likely be a color shift on the tube.

Why? The phosphor only glows one colour regardless of the amount of UV
illuminating it. Dimming standard fluorescent tubes by reducing the drive
voltage is a common method. The colour does not change, though the white is
perceived as a sort of dirty gray at the dimmer levels, but that is down to
the eye not the actual colour.

In actual practice it is not feasible to dim cold cathoe tubes by drive
voltage becasue they rely on the discharge current to heat the cathodes (the
term cold cathode means that the cathodes are not independently heated and
has nothing to do with their actual operating temperature - which is very
hot).

 

[Ka2H]

Unfortunately, Encino Man (or any of his numerous previous incarnations) has
had nothing constructive of useful to say in this (and other) newsgroups for
years. Unfortunately he is a mentally retarded 13 year old child.

Minor correction; the "thing" - Encino Man - gotta still be in Kindergaten
by noting its lack of ability to read. The OP forwarded relevant information
concerning his problem, running Windows XP and having a hardware problem, in
accordance with rules of posting in the Newsgroup - and the title of the
group: "Windows XP Hardware". Note the latest post of the "thing".

Regards,
Ka2H

 

[M.I.5¾]

However the problem has nothing to do with the OS and would be the
same regardless of the OS. It is therefore purely a hardware problem
and does NOT belong in this group.

Too bad you have trouble reading that group title and putting 2 and 2
together...or are you using Orwellian math and getting 5 from that
equation?

Too bad that you can't grasp the fundamental priciple that no body gives a
shit what you think.