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Re: How do you test a backlight ? IBM a31p

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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.
>
> 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.
>
> 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.


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?
>
>> Is there a source for schematics or reference inverter 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.
>>
>> 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.
>
>> 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.
  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.
I'd rather not have to reverse engineer the thing if info is available.

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

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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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