HP Forum Archive 21

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Re: Maintenance and Durability: From NIB to abused calculators Message #1 Posted by McAllan on 27 Feb 2012, 3:54 p.m. Interesting thread here. Usually while I clean old calculators etc. I change electrolytic capacitor for modern film types. Usually They're not much bigger and they'll never need replacing ever again or at least for the next many many decades. Sometimes they can't be as big as the original electrolytic ones but really even a smaller value filmtype will do the purpose of the original electrolyte better that it did in it's days. Changing them in the PSU/inverter on many VFD seems to have an instant impact. Display is visible brigther and more crisp even nothing else was changed. Changing tantalums usually makes no difference. Here I only do it if I by no means want them to fail. Quote: I have two counter-examples for this. I have two calculators: a Ti-68 and an Elorg-51. Both stored for years in a box without batteries, and with silica-gel to keep humidity low. The Ti-68 developed a fault at the flex cable connecting the display to the motherboard. Even though it doesn't move or actually flex in the Ti68, still it got degraded somehow, leading to display problems (in my case, no decimal point was displayed). That can usually be repaired. Also I believe it's from a manufacturing defect right from the start only that it didn't show right from start. Yes of course the materials changed a little or the tracks it was bonded to oxidized underneath or similar because of insufficient heat setting and that's why there's now loose connections. But as said that can be fixed in many cases and without any special tools. All you need is a soldering iron and some heat resistant silicone rubber or just a piece of bicycle tube although that'll melt slowly but it's useable (and it smells "great" too). Find the spot to press to have missing segments visible. That's where to "solder". Put the piece of (silicone)rubber over the place and heat until it seems to have seated properly. The rubber gently distributes the heat, making dosing easy and therefore does not stick to the outer side unless you've heated it too much. Quite difficult to explain when it has got enough (not too much) to get a few dollar store calculators to practice on - really it's not that hard. Also to not heat at the very edge of the place as heat and the edge could cause track to melt over. But hey you can get conductive paint if you're so unlucky. First tried this myself on a TI-30Xa instead of just throwing it out and it has worked great ever since :) And since then on a few other things with same fault. Happy practicing!

Re: Maintenance and Durability: From NIB to abused calculators Message #2 Posted by uhmgawa on 3 Mar 2012, 10:38 a.m., in response to message #1 by McAllan Quote: I change electrolytic capacitor for modern film types.. Changing them in the PSU/inverter on many VFD seems to have an instant impact. Display is visible brigther and more crisp even nothing else was changed. I suspect that's in part due to electrolytics of that vintage not being generally available for switching converter applications. Yet for battery powered VFDs converters of relatively low energy it just may be electrolyte loss due to age vs. internal heating. Quote: Quote: The Ti-68 developed a fault at the flex cable connecting the display.. leading to display problems (in my case, no decimal point was displayed). That can usually be repaired. Also I believe it's from a manufacturing defect right from the start only that it didn't show right from start. Yes of course the materials changed a little or the tracks it was bonded to oxidized underneath or similar because of insufficient heat setting and that's why there's now loose connections. But as said that can be fixed in many cases and without any special tools. I've had mixed results trying to reset heat seal connections. Part of the problem being the anisotropic adhesive is typically thermosetting vs. thermoplastic and you may not be able to soften it sufficiently to restore the contact and have joint tension reliably maintained after the repair attempt. Keep in mind if adhesive does flow in response to applied heat, pressure should be maintained on the full heated area so the tension in functional contacts isn't released resulting in a worse problem. The conductive tape used to establish a heat seal connection consists of an adhesive impregnated with metallic plated polymer balls. The joint is assembled and thermally set typically by a hot bar bonding tool which most importantly maintains full joint pressure until the adhesive has set (or cooled in the case of thermoplastic adhesive). So a repair attempt would need to do likewise to keep the plated particles in tension between the conductor tape and pcb. The most success I've had was using a bare (uncoated) metal thumb tack head-down on the joint held by the stud in tweezers. Move it around constantly while heating the back with a soldering iron until you estimate it is sufficiently hot and then remove the iron while maintaining pressure on the tack. Agreed some experimentation is needed to get the correct temperature for the adhesive (assuming it even will reflow) short of destroying the polyester backing. The recommended repair method for a heat seal connection is to disassemble it and clean off the old conductive isotropic tape. But with econo carbon ink printed heat seal conductors mechanical removal can easily damage the carbon traces.

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