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Re: How do you test a backlight ? IBM a31p

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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.

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.

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. 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.

> 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.

> 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. 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.

-- 
Bill
Gateway M465e ('06 era) - Thunderbird v14
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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