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Re: How do you test a backlight ? IBM a31p

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On 8/29/2012 9:15 PM, mike wrote:
> On 8/29/2012 6:12 PM, BillW50 wrote:
>> On 8/29/2012 7:01 PM, mike wrote:
>>> I've got a Thinkpad a31p with no backlight.
>>>
>>> I put another backlight on the inverter and got no light.
>>> I tried the Thinkpad backlight on another inverter and got no light.
>>> Initial guess is both bad.
>>> Is there any reason the Thinkpad light should be different from any
>>> other?
>>> I put a 1M resistor in series with a DC power supply.
>>> A Toshiba backlight fires at about 1500V and stays lit
>>> back down to 1000V or so.
>>
>> You used a DC power supply and cranked it up to 1500V? And you used a
>> one megohm resistor in series with the power supply? I am not sure
>> that would work even if everything works fine.
>
> Well, the paragraph above says that it does work: fires 1500V drops
> out at 1kv or so...that's volts into the resistor.  I hadn't built
> the high voltage probe to check the lamp voltage.  That's next.

Sounds good to me. What I hear a lot is that inverters generally put out
like 1800v unloaded and about 800v when the lamp is lit. So a value
similar is what I would expect.

>> As once a florescent tube fires and ionizes, the internal resistance
>> drops really fast. And the trick of the inverter is to get the tube
>> working partly shorted and open by pulsating the voltage. Straight DC
>> voltage won't work. How they regulate this pulsing, I do not know
>> exactly how they work the timing.
>
> There may be service life issues with DC, but it definitely works. see
> above.

That is very good to know. ;-)

>> At first, I understood that a tube and an inverter were matched.
>> Although later I learned from others who didn't follow that belief
>> who said this. Inverters are matched to a given length and diameter
>> of a tube. And they say you can cheat a bit. Like taking an inverter
>> 15.4 inch tube and using it on a 14 inch or something. But that is
>> pushing it a bit.
>>
>>> The Thinkpad light starts drawing current at about 1500V
>>> but doesn't light. I'm trying to think of a failure mode
>>> that would cause this behavior. If the bulb is cracked,
>>> wouldn't is just be an open circuit at any rational voltage?
>>
>> Well a tube not firing, ionizing is going to act totally open. And if
>> you monitor the current flow, you will see none.
>
> Like I said in the above paragraph.  It's drawing current, just not
> lighting.
> And it displays the negative resistance effect, just not lighting.

It could be like some switching supplies draw current unloaded, while
some truly draw nothing at all.

>> The tube doesn't have to be cracked or anything. Tubes generally have
>> a lifespan of 25,000 to 30,000 hours. Once they get this old, they
>> get dimmer and dimmer and then at some point may start sometimes or
>> not. Then one day, it will never fire.
>
> My understanding of the theory is that the firing voltage goes up
> with age and the inverter can't put out enough to get it started.
> The plasma is either ionized or it isn't.  If it's ionized, it oughta
> be putting out some light?  Maybe not much, maybe it's pink, but
> oughta see something?

Yes that is my understanding too. Although I suspect at some point no
matter how much voltage you feed it, it just won't ionize.

>>> Is there a source for schematics or reference inverter designs?

Sadly, manufactures nowadays keep these under lock and key. They seem to
be really worried that their competitors stealing their designs.

>>> IBM put LED indicators on the inverter board, so you can't just
>>> cobble in another. They're rare and more expensive than buying
>>> a whole used A31p.
>>> It's got the 1600x1200 display, so it's worth some effort to try
>>> to fix it.

eBay is usually a good place to find used working parts. For some things
anyway. Like these Gateway M465, zillions have come off lease and I can
buy perfectly working ones all day long for about $50 a piece off of
eBay.

>>> According to the service manual. There are several ways to turn
>>> off the display, but I'm assuming that if there's data on the
>>> display, the backlight should always be on???
>>
>> Speaking about most laptops in general. They can turn off through the
>> lid switch (usually magnetic reed switch) or if the external monitor
>> port is in use. Some can be disabled through the BIOS settings. Of
>> course, Windows can turn it off too.
>
> As I stated in the above paragraph, if there's data on the display,
> shouldn't the backlilght be on?  Anything that turns off the backlight
> oughta also turn off the data to the display???  I'd hate to chase my
> tail only to find out I hadn't pushed some key.

I don't think that is always true. For example most laptops if you close
the lid, the lamp turns off. But I believe the display itself is still
functioning. Although I haven't experimented to test this or anything.
Another test would be to have Windows turn off the display. Then with a
flashlight, spotlight, or even sunlight you should be able to make out
part of the image if it is still functioning. Anything white is very
easier to see. Like having notepad or something on the screen should
make this much easier to check.

My old Toshiba 2595XDVD laptops from 1999 have a hotkey to turn off the
backlighting on the display. So at least some laptops it is possible
that it could be a key press away. I don't think it sticks if you reboot
though. As I believe it defaults to being on when powered up.

>>> The service manual mentions that fuses aren't covered under
>>> warranty, but there's no mention of a fuse anywhere.
>>> I once had a thinkpad with a bad display fuse. Haven't taken it
>>> that far apart yet.
>>>
>>> Suggestions?
>>> Thanks, mike
>>
>> I don't think you will find replaceable fuses in a laptop. They
>> should be all those surface mounted fuses.
>
> Said nothing about the type of fuse, just asked if they often exist.
> The only display fuse I've ever seen was on a thinkpad, but I've never
> really looked for fuses.

Yes they do exist on motherboards. I can't say they always exist on
every design though. But they are indeed common enough.

>> If you have replaced components on a motherboard before, you should
>> be good to go. If you haven't, well a laptop would not be my first
>> choice to practice on.
>
> Been designing/fixing this stuff for decades.

Ah good! ;-)

> I'd rather not have to reverse engineer the thing if info is
> available.

I know the feeling.

> The chip is marked HL397 3456.  Google has failed me finding a
> datasheet or reference design related to it.

This is the chip on the inverter, right? I haven't found anything here
either yet. But I'll continue checking.

-- 
Bill
Gateway M465e ('06 era) - OE-QuoteFix v1.19.2
Centrino Core2 Duo T5600 1.83GHz - 4GB - Windows XP SP2

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Thread

How do you test a backlight ? IBM a31p mike <spamme9@gmail.com> - 2012-08-29 17:01 -0700
  Re: How do you test a backlight ? IBM a31p BillW50 <BillW50@aol.kom> - 2012-08-29 20:12 -0500
    Re: How do you test a backlight ? IBM a31p mike <spamme9@gmail.com> - 2012-08-29 19:15 -0700
      Re: How do you test a backlight ? IBM a31p "BillW50" <BillW50@aol.kom> - 2012-08-30 05:42 -0500
        Re: How do you test a backlight ? IBM a31p mike <spamme9@gmail.com> - 2012-08-30 05:55 -0700
          Re: How do you test a backlight ? IBM a31p BillW50 <BillW50@aol.kom> - 2012-08-30 10:25 -0500
            Re: How do you test a backlight ? IBM a31p mike <spamme9@gmail.com> - 2012-08-30 08:33 -0700
              Re: How do you test a backlight ? IBM a31p BillW50 <BillW50@aol.kom> - 2012-08-30 11:08 -0500
                Re: How do you test a backlight ? IBM a31p BillW50 <BillW50@aol.kom> - 2012-08-30 11:33 -0500
                Re: How do you test a backlight ? IBM a31p mike <spamme9@gmail.com> - 2012-08-30 10:43 -0700
                Re: How do you test a backlight ? IBM a31p BillW50 <BillW50@aol.kom> - 2012-08-30 17:19 -0500
                Re: How do you test a backlight ? IBM a31p mike <spamme9@gmail.com> - 2012-08-30 10:37 -0700
                Re: How do you test a backlight ? IBM a31p BillW50 <BillW50@aol.kom> - 2012-08-30 17:06 -0500
                Re: How do you test a backlight ? IBM a31p wrat@panix.com (the wharf rat) - 2012-09-01 06:33 +0000
                Re: How do you test a backlight ? IBM a31p BillW50 <BillW50@aol.kom> - 2012-09-01 12:36 -0500

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