LCD repair tip: Missing segments on heat seated ribbon

[McAllan]

Hi

In my search for information about the ghosting on classics I've encountered a few repair tip threads.
One if the things I've read which I do not agree with is that general consensus is that missing segments on calculators with a heat seated ribbon cable to the LCD is not repairable.

Here I disagree. I've done it in the past and I can do it again.

At a flea market I bought this TI-30Xa for pennies and "of course" it has a defect (hence low price):

Missing segments in the exponent

Mostly you don't need new ribbons, specialized tools etc. All you need is the soldering iron and some heat resistant silicone rubber. You can get special heat resistant mains wire at most lamp stores. It's the highest temperature one with tinned cores you should buy as it has silicone rubber on the outside.

Put a small piece on your (cold) iron and heat it up. Now you can reheat the ribbon cable in a gently manner and it'll not stick to the soldering iron. Be careful you don't bend the cable on the PCB edge. Or just stop a millimeter before the edge to be sure.

As you can see - the missing segments are working again :)

If the fault is at the LCD end you probably need specialized tools. At least because the LCD isn't as heat tolerant as the PCB. On the other hand all the faults I've seen have been on the PCB end.

Yeah I know I'm a heretic using a TI as example but the technique is the same on all calculators with similar construction ;)

[Andres Capdevila]

Interesting tip, thank you for sharing it. I wonder how many calculators I threw away because of that type of failure... Oh well, too late to be sorry!

[Lincoln R.]

Another thing I've had success with was completely removing the ribbon cable and soldering in pieces of wire. This was on a TI-83 Plus that had the ribbon cable installed in a really stupid manner (the cable was too long and folded into a Z shape that had the conductive traces bent at 180 degree angles in two places causing the traces to crack and fail). I'd suggest testing these ribbon cables with an ohm meter, end-to-end resistance should usually be < 1 Ohm on a good cable.

[Bart (UK)]

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This was on a TI-83 Plus that had the ribbon cable installed in a really stupid manner (the cable was too long and folded into a Z shape that had the conductive traces bent at 180 degree angles in two places causing the traces to crack and fail).

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I found that on a Casio FX-850P.

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Another thing I've had success with was completely removing the ribbon cable and soldering in pieces of wire.

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How do you solder it onto the LCD? or even onto the bit of ribbon left on the LCD side?

[Lincoln R.]

Ah. This was an LCD that was attached to a PCB with more ribbon cable and black tape. The flex cable I had to fix ran between the two PCBs. See TI-83 Plus PCB

In that picture it's the cable directly above the "9923" date code on the bottom PCB.

[McAllan]

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Another thing I've had success with was completely removing the ribbon cable and soldering in pieces of wire.

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Yes but that requires you're able to solder both ends. And with normal LCD's that's not the case - like here.

For an inside picture take a look at Jörg's Datamath: http://www.datamath.org/Sci/Modern/JPEG_TI-30ecoRS.htm#PCB

Sure for those with such cable between keyboard and CPU board etc. that is both ends are PCB can easily be replaced with wires. At least if they're not using conductive printing to connect to the ribbon.

In that case I'd probably investigate conductive paint or ditto two component glue. That is if it's worth saving. This one hardly very valuable on the other hand repair is easy and doesn't take much effort. Bought it for daily use.

[Bart (UK)]

Thank you for the tip. I have recently experimented on a Sharp with a similar problem, but some of the segments dissapeared again after a few weeks, so I knew I needed to refine my technique. I will certainly try your method.

[Joerg Woerner]

Joerg,

Here's how you can fix some of the units with missing segments and or digits. Open the calculator and apply hot air to the heat seal film where it attaches to the PCB. Take care not to leave it on too long and destroy the film. A little experimentation will let you know how long to heat the film. Turn the unit over and look to see if the contacts come back. Do not touch the contacts or press something against them. If the segments/digits re-appear and stay there, place epoxy across the area where the film is sealed to the PCB. The epoxy should extend alittle past the contact area on the PCB. Allow to dry and reassemble. The units should then work for years.

If the segments/ digits will not come back until you press upon the film, the film has dried out and is not repairable (even though they may appear to work-they will fail later).

Regards, ----