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Any standard failure modes in old TI calculators?

Discussion in 'Electronic Design' started by Joerg, Dec 4, 2012.

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  1. Joerg

    Joerg Guest


    My trusty old 1974-vintage Texas Instruments SR-50 is beginning to
    sputter <sniffle>. It turns on and at first only reacts to very few keys
    like "Clear" or "Equals". When pressing those it does erratic things
    such as incrementing by some large number until overflow occurs.
    Gradually its marbles come back, more and more keys respond seemingly
    correctly and it does math correctly. But of course it can't be trusted
    much in this sort of state.

    Inside it looks clean, signals are healthy, solder joints look ok. Not
    sure what's underneath the membrane keys, I haven't dared to tear that
    apart because then it's probably all toast.

    Does anyone remember if there's a standard failure mode on these I could
    look for? Or maybe it doesn't like very long periods of non-use?

    What amazes me is how much power they use. With all LEDs on it slurps
    120mA out of a 3.8V supply. Cleared down to just one zero it's still 80mA.
  2. How are the batteries? On some of those old calculators they worked to
    filter the power supply. Have you tried powering it with a lab supply?
    That one has 3 NiCd cells, right?

    I've still got an SR-52 (the one with the magnetic card reader) around
    somewhere. But I never liked TI's keyboards.. felt cheap and were
    never 100% sure one and only one keystroke got through. Went HP and
    never looked back.

    Best regards,
    Spehro Pefhany
  3. Joerg

    Joerg Guest

    Yup, three cells and they had leaked so they are on the way to E-waste.
    Luckily these leak into the bottom shell, not the electronics. Right now
    it's on a bench supply and a 100uF cap where the battery was. If I get
    it going again I'll give it three new NiMH cells and put them on a C/10
    or a little less trickle. I know that's not too healthy for them but
    they seem to last a long time in cordless phones that way.

    Same here, the HP-11C is my office workhorse. But I keep the SR-50 at
    the lab. Mainly because my parents gave it to me in the mid 70's when it
    was unbelievably expensive. I am forever thankful to them for that
    because it opened the door to many intricated projects. And because of
    the rather good quality LED it is much easier to read in bright light.
    No RPN though which throws me a curve every time.

    Right now it's totally on the fritz again, totally erratic :-(
  4. Joerg

    Joerg Guest

    I've got the original, way on the left.

    Possible. What puzzles me is that the thing worked fine 15 minutes ago
    and now is all dead again on the keys.

    Yikes, that looks like a bear of a job.

    The charger plug was the first thing that went but that was easily fixed.

    Thanks. Not looking forward to this one though.
  5. If one of the keys is 'stuck' on due to (say) spilt wine or perhaps a
    fine single-malt scotch (not that I'd have any direct experience with
    that sort of disaster), then the scanning stops and the calculator may
    appear dead, or may do something else (such as count up) if it
    triggers auto-repeat.

    If something like that happened, you might be able clean it out or
    not. Once it 'tracks' the conductivity seems to stay. If you can get
    it open without wrecking it, it should be possible to clean. IIRC it's
    metal clicky domes in a polyester overlay on the TI calc's but it's
    been a LONG time, my memory may have faded.

    I wonder if putting the keyboard through the dishwasher and then
    subjecting it to a vacuum to dry it inside would be less risky than
    trying to open it up.

    I've also seen the keyboard PCB crack on remotes, but that's when they
    use the crappiest paper-based phenolic you can buy. Your TI probably
    has nice FR4.

    Some remote controls just seem to spontaneously fail:

    Best regards,
    Spehro Pefhany
  6. Right, the polyester is glued right to the PCB, right? Should seal it
    against ingress of fluids, but also prevents cleaning.
    Bitch chewed it. Female Golden Retriever, to be specific. I'm sure
    that there are people who would try to return something like that
    under warranty. After all, Panasonic didn't have to make it so tasty
    and bone-shaped.

    Best regards,
    Spehro Pefhany
  7. Uwe Hercksen

    Uwe Hercksen Guest


    if you are really used to RPN, you have problems using another
    calculator without RPN.

  8. Joerg

    Joerg Guest

    True, and I do. But it does train the brain so I like the challenge,
    just like driving a car with right-hand steering. I also am sometimes
    forced to use non-RPN because I often don't take my HP on the road. The
    usual, some innocent EMC job and then they need me to also fix internal
    noise and calculate a filter, on whatever calculators they have. Lately
    on guy handed me his smart phone turned calculator. No RPN either and
    then you have no time to load a new app.

    No joke: This can help in at least pushing out diseaese like Alzheimer's.
  9. Joerg

    Joerg Guest

    Unfortunately that's the case. Jeff has described how to fix it but it
    seems to be a rather onerous procedure.

    That looks like the "Golden Retriever effect" :)
  10. Joerg

    Joerg Guest

    At least there's a pinout for the scan chip:

    That would be another option. I don't think these SR-50 have much of a
    resale value and I am not concerned about that anyhow. I could just buy
    a new calculator for $30-$40 but it has some sentimental value to me.
    That sentimenatl value has its limits though :)
  11. John Larkin schrieb:



  12. mike

    mike Guest

    I'd expect some evaporated electrolyte condensed somewhere it doesn't
    I expect you'll have to clean all the boards very thoroughly.
  13. Jeroen

    Jeroen Guest

    In the case of an HP35, that would be '1+', I suppose.

    Jeroen Belleman
  14. Guest

    I carry my HP11C (emulator) everywhere I go. Well, I don't carry my
    cell phone around the house. ;-) Loading a new app on my cell phone
    takes all of 30 seconds.

    I can't stand "algebraic" calculators. No two are the same. There are
    always minor differences in the way operations are chained.
    Yeah, suicide is faster.
  15. Joerg

    Joerg Guest

    Can't do that. My cell is a no frills phone-phone. No apps possible,
    whatsoever. Kids think it's a steampunk model.

    Slowly I get the feel this old SR-50 is toast. Even the pins of the SCOM
    chip appear to have started some sort of dry-rot, all the way up to and
    possibly into the chip.

  16. Guest

    We had those until we took the plunge and bought Android phones in
    January. We make very good use of them and haven't been sorry at all.
    Working as designed. ;-) I hated SR-50s. I would have stuck with my
    slipstick if they were the only alternative.
  17. Martin Brown

    Martin Brown Guest

    They wear out after a certain amount of use. My SR-51 eventually expired
    but largely because it fell off my belt clip and got run over.

    I am surprised one lasted as long as this.

    We once tormented one by turning the clock speed right down so you could
    see the refresh. ISTR the TI patents are available (not sure if online)
    so you should be able to get full circuit diagrams for the
    implementation. Actually I think it was a 56 we did that too.
    I suspect wear and tear - nearly 50 years operation isn't bad. Most of
    my calculators have died within a decade. Although I do still have a
    working SR-59 and print cradle in the loft somewhere as I could not
    bring myself to part with it.
    I think it is the even longer periods of use inbetween that does for them.
    That is 6 segments on so 12mA a segment average. A "1" might well be
    lower by a factor of 3 depending on how much juice the core takes.

    Martin Brown

    (also a fan of TI calculators)
  18. Joerg

    Joerg Guest

    Yeah, one of the reasons I decided to hand it over to E-waste now is
    that some of the domes appear to have a hole rubbed in, and not just
    through the plastic. This calculator got some extreme workouts because I
    did not have a computer for my early filter designs. My dad had an
    IBM-5100 but it ran APL which I never really warmed up to.

    I could have swapped the CLEAR and EQUALS domes with ARCTAN or other
    lesser used ones. But there comes a limit. Walmart sells a nice HP
    scientific calculator for around $50 and it has RPN.

    They were never very bright but nice small segments and very contrasty.

    Me, not so much, I have to say that afetr I married I began to prefer
    HP. Feels like better quality and I started to really like RPN. My wife
    brought an HP-11C into our marriage and that's the one I use right now.
    We could never divorce because then I'd have to give up the 11C, and
    they don't make those anymore :)
  19. Joerg

    Joerg Guest

    We are on $5/mo pay-as you-go plans. Plenty for us. Virgin does have
    smart phones but then AFAIK it pops up to $40/mo or so and I can't see
    us using that. I hate it when people fidget around with their handheld
    gizmos all the time. Some are literally obsessed. Middle of a nice
    dinner ... *BING* ... "Oh, that could be so-and-so sending a text" ...
    and out comes the schmart fone. Yuk.

    It's been good to me. I could not possibly have calculated all those
    filters with a slide rule. Plus back then HP's were truly unaffordable
    for the masses, you had to be on a professor's pay grade for those.
  20. Martin Brown

    Martin Brown Guest

    The first calculator I ever encountered belonged to a school friend. It
    came back from the US as his dad was a drugs rep. HP20 I think.

    I RPN Obscure Very Find - Yoda.

    I was just lazy and liked entering formulae in normal order without
    having to think. I got away with mental arithmetic and writing SR for
    "slide rule" in the margin until first year uni. Answers to 3 sig fig
    and SR did not go down at all well with our crystallography supervisor
    so I got a calculator. SR51 - I have fond memories of it too.

    Martin Brown
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