[ltp] Thinkpad 770 hardware (screen) question.

[Richard]

On Wed, 22 Jan 2003, Charles E Taylor IV wrote:

> I have a 770X (14.1" display) with a dead backlight.  Whether it's the
> inverter or the backlight that's toasted, I don't know (how can I tell?).
> It's not a loose cable, though. :)


You can try poking it with a voltmeter! I suspect[1] that the inverter
will be some kind of oscillator/step up transformer, coupled to a CCFL (col=
d
cathod fluorescent) tube. The outputs from these things are ususally
a few hundred volts and a few tens of kHz. If the inverter is running, you
may well pick up the radiated frequency signal with a digital meter even
without making a connection! That will allow you to tell which part is dead=
=2E
The CCFL should be an easy spare to obtain; the inverter may also be. If
its the inverter, my betting is on dead transistors - if you're lucky
these will be discrete parts and amenable to some "end user modification"
(voiding warranty, of course).

Note:

The high voltage can give you anything from a nasty jolt to a heart
attack, depending on how high it is, and how sensitive you are to electric
shock. Take CARE! It is also quite capable of arcing a few mm and this
could destroy part of your thinkpad, or start a fire.

If you are testing with a voltmeter, make sure that you use properly
insulated probes!

Hope that's of some use...

Best wishes

Richard


[1]I've not actually taken a thinkpad to bits. But I have experimented
with a CCFL/inverter of the type used for computer case-mod  lighting -
which is, I think, a similar circuit, although being only =A38, I was
willing to buy (and disassemble) it out of curiosity! This one had a very
simple 2 transistor oscillator, and step up transformer; it ran on
12V and drove a 30cm, 4mm diameter tube at 800V and 50 kHz; with the
tube disconnected, it could produce a 3mm long arc - and yes, I did manage
to set the pcb on fire!