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HP Forum Archive 17

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Fixing Classic LED segments?
Message #1 Posted by Dan W on 2 May 2007, 9:11 p.m.

Hi all,

I have an HP-45 and a couple of the LEDs have some segments that don't light. What's the likely cause of this? Would I most likely need to replace the whole LED chip or could it be another part that is the problem?

TIA

-- Dan W.

      
Re: Fixing Classic LED segments?
Message #2 Posted by Thomas Radtke on 3 May 2007, 3:57 a.m.,
in response to message #1 by Dan W

Fixing might just be a matter of cleaning contacts. Have you had a battery leakage?

      
Re: Fixing Classic LED segments?
Message #3 Posted by Eric Smith on 3 May 2007, 4:34 a.m.,
in response to message #1 by Dan W

If a whole digit is out, or if the same segment is out on all digits, it could be a contact problem.

If there's a segment out, but it appears in other digits, and other segments appear in the same digit, that generally indicates that the segment has actually failed.

            
Re: Fixing Classic LED segments?
Message #4 Posted by Dan W on 3 May 2007, 10:16 a.m.,
in response to message #3 by Eric Smith

Just a few segments on 2 digits. One 8 looks like a 9. The other has only two horizontal digits working. The 2 digits are on 2 chips, the left and center.

Also, the digit that looks like a 9 has 2 horizontal segments that are always faintly lit.

There's no leakage visible but I haven't opened it up yet. Could have had a cup of coffee spilled in it. ;) But I won't know until I open it.

Owner said it worked fine when put in storage 20 years ago, but I doubt this problem would develop over time when not used.

I'd like to be able to repair this type of problem, isolating it to the LED chips, driver chips, discrete component or some mechanical PCB trace or something.

-- Dan

                  
Re: Fixing Classic LED segments?
Message #5 Posted by Paul Brogger on 3 May 2007, 11:36 a.m.,
in response to message #4 by Dan W

As alluded to in one of my MoHPC "HP Memories" entries (#1), I used to give dramatic demonstrations of my HP-21's durability by tossing it to the floor. Eventually, the display started acting up.

As told in the article, I took the calculator apart, and removed the clear plastic lens strip from the display board by melting the heat stakes. Under a dissecting microscope (binocular, ~20 power or so?) I could see that the inertia of repeated impacts had collapsed some of the little jumper wires that connect the chip pads to the PCB traces. With a sharpened and de-burred plastic soda straw, I was able to pick up each of the collapsed wires and fix the display.

I imagine that, if there was any moisture in the display, your connecting wires (or chips?) might have gotten corroded over time in storage. I don't know how you would solder anything so tiny, but there may be a way. Take a close look at the chip/PCB connections and you may find your cause.

[Edited: Ooops! I assumed the 45 and 21 had similar displays. Sorry!]

Edited: 3 May 2007, 5:42 p.m.

                  
Re: Fixing Classic LED segments?
Message #6 Posted by David Smith on 3 May 2007, 4:36 p.m.,
in response to message #4 by Dan W

The problem is a faulty LED module (DIP chip). There are three matched 5 character LED modules in a classic calculator. Only source is another machine. Modules were brightness matched at the factory and a letter code is stamped on the bottom of each one. The closer the letter code, the better the match. Also be aware that modules with gold and sliver leads have slighly different digit heights. Before soldering down a replacement module, inspect its alignment with a magnifying glass, solder two oposite corner pins, and recheck the alignment.

      
Re: Fixing Classic LED segments?
Message #7 Posted by Dan W on 4 May 2007, 1:02 p.m.,
in response to message #1 by Dan W

Thanks guys. You've told me pretty much what I expected but didn't want to hear.

Let me ask the opposite question: I have occasionally seen LED segments that are faintly lit all the time, typically opposite vertical segments on one digit. This I'm guessing is not in the LED chip itself, but what would cause it?

-- Dan


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