HP Forum Archive 13

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HP 12C Keys don't work Message #1 Posted by Stan (Sg) on 26 May 2003, 7:26 a.m. I have a Malaysian built HP 12C (1999 model if I am decoding the serial number correctly). The R/S and SST keys (and sometimes the R down key) don’t work intermittently (these are all in a row – keys 31, 32 and 33 per the program numbering). If I press the key many times, I can usually get it to work. I have tried removing the batteries for about 15 minutes and it didn’t help. The self test comes out fine. The keyboard self test works until I get to those keys, then gives “Error 9” when I get to X<>Y, which indicates that the keys in question are not “talking” to the CPU. My understanding is that the step of shorting a wire across the batteries is meant for the case where the unit will not respond to any key input. The unit is still under warranty for another couple of months. Should I try anything else before returning the unit? Any thoughts are appreciated.

Re: HP 12C Keys don't work Message #2 Posted by Randy Sloyer on 26 May 2003, 9:25 a.m., in response to message #1 by Stan (Sg) The battery gymnastics will not solve your problem. Nor will taking the batteries out for 24 hours as HP support recommends to "bleed off static charge". Nonsense stuff. It's a bad keyboard, plain and simple. The Chinese units would be more likely to have this defect, but it certainly is possible in a 1999 unit. The row of five keys R/S thru CLX should have the same problem. The keyboard is made up of eight strips of five keys each. If the strip looses contact with the keyboard scanner through the one connection point, the whole row of five will be dead. You might see intermittent behavior when pressing the R/S key, as the common connection is near that key and the force of the key actuation could help make the common connection. Here is Dave's picture of the strips in a 15C keyboard, the same as your 12C: Get a new one while you can.

First signs... Message #3 Posted by Vieira, Luiz C. (Brazil) on 26 May 2003, 10:43 a.m., in response to message #2 by Randy Sloyer ... of a short-life, disposable calculator. Sob! Luiz C. Vieira - brazil

i know how to fix that 12C Message #4 Posted by Norm on 27 May 2003, 2:09 a.m., in response to message #3 by Vieira, Luiz C. (Brazil) i know how to fix that 12C first you go down to main street where the trains rumble thru town. Then you ......

Re: i know how to fix that 12C - I tried Message #5 Posted by Renato on 27 May 2003, 1:06 p.m., in response to message #4 by Norm Norm, I tried that on my brand new 34c. It did not fix it. Maybe it was because it was in the box with all manuals, papers, case, charger, new batteries, etc. Would you care for the remains ? Maybe US trains work better. Renato

That method of repair Message #6 Posted by NH on 27 May 2003, 2:43 p.m., in response to message #5 by Renato That method of repair is only meant to be used on certain flavors of LCD calculators. If you used that method of repair on a 34C,it would be a grave error, also, it would not fix it.

Re: HP 12C Keys don't work Message #7 Posted by Renato on 26 May 2003, 5:30 p.m., in response to message #1 by Stan (Sg) Stan, I once got this type of problem on a Malaysian 12C. It was on the entire row that contains 7,8,9 (and others) keys. The cause was a bad solder on the single-chip - the pin was connected to all keys. I just checked continuity on the circuit board and checked keys - all ok. Then i pressed the corresponding chip pin, and presto - it started working reliably. I resoldered the pin, and the keys are working ok, although self testing them fails sometimes. I have also seem a 12c keyboard with corrosion on the metal domes, but have not tried fixing such a problem. Hope this helps, Renato

Re: HP 12C Keys don't work Message #8 Posted by Ellis Easley on 26 May 2003, 7:52 p.m., in response to message #7 by Renato It would be great if a fairly easy repair will fix it. It's tempting to say get it replaced by HP while you can, but the replacement will probably be a 3V Chinese unit. Maybe you should open it up, being extra careful with the feet, and see if there is a problem like Renato saw. Then if you can't fix it, close it back up and get HP to replace it

Re: HP 12C Keys don't work Message #9 Posted by Stan (Sg) on 27 May 2003, 8:19 a.m., in response to message #8 by Ellis Easley Thanks for all of the comments. I am surprised that the Malaysian and Chinese models would be different enough to justify opening this one up and possibly voiding the warranty, but I have never used a Chinese model. The keys on this model do feel pretty good. I will consider your suggestion. This development is especially disappointing when you consider how much abuse my HP15C (circa 1986) took over many years and still works fine. In fact, I have been using it some since my 12C developed a problem. When I got out my 15c I was surprised how many scratches / etc... that it has - but it still works great.

HP 12C Keys don't work - Help please! Message #10 Posted by james (UK) on 28 May 2003, 8:18 a.m., in response to message #1 by Stan (Sg) The plague seems to be spreading - I have got a Malaysian 12C (serial MY93908021) in which the top 5 right hand buttons (CHS, 7,8,9 and division) do not work. From Renato's reply and the picture of the 15C keyboard a broken connection somewhere would seem to be the cause. As the calculator was cheap I am considering trying to see if I can fix it as training for the day to come when I might need to poke around in my fullnut 41CV. From what Ellis said I'm guessing that like the 41 the casings are held together by screws which are hidden by the rubber feet. Is this correct? Can anyone point me to pictures of what the innards will look like and the best way of tracking down the duff connection? Any suggestions gratefully received. As it's rather a long trip to the railroad tracks I would like to try some other methods first.

Help is on the way Message #11 Posted by Randy Sloyer on 28 May 2003, 9:26 a.m., in response to message #10 by james (UK) Peel the feet, back out the screws and it's open. Pop the back off and before turning it over, locate the two very small springs that protrude from the front inside small tubes and remove them. They are part of the ESD protection and electrically connect the front and rear metal plates. Once open, you should be able to figure out where the 5 key strip makes its common connection and trace it. Sometimes on older units, the heat fused tops of the posts that hold the keystrips in contact break off. If you can find the post in question, you can clamp it and glue with gap filling cyanoacrylate. Sometimes the solder connection fails on the keyboard encoder chip if one sits on their calculator too often. That should cause the loss of 2 strips of 4 keys vertically as they are commoned at the encoder.

Re: Help is on the way Message #12 Posted by james (UK) on 28 May 2003, 9:42 a.m., in response to message #11 by Randy Sloyer Many thanks for the quick response. Lunch-hour has finished so I will need to postpone the operation until tonight! Will let you know how I get on.

Re: HP 12C Keys don't work - Help please! Message #13 Posted by Ellis Easley on 28 May 2003, 9:39 a.m., in response to message #10 by james (UK) Yes, under the feet there are four screws that hold the case together. I assume it will be the single PCB version inside, which means you can see the IC's (maybe just one in a 12C?) above the keyboard area (behind the LCD). From what Renato said, you might expect to find a pin on one of the IC's not properly soldered to the PBC. A tool like a dental probe is good for looking for such a loose pin. A large sewing needle might be a good substitute. Gently press the tip of the probe against the edge of each pin, one at a time, just above the pad where it is soldered to the PCB - in other words, push the pin toward an adjacent pin. If you do this to a loose pin it will give a little. A properly soldered pin will feel quite solid (it will bend if you push it too hard, but don't). Since you are touching each IC pin with a metal object, it is very important that there is no electrostatic charge on your body. Don't try to do this at all on a cold, dry day. Wait for a humid day. Run some hot water in the shower to raise the humidity (stop working if your glasses fog up!) Breathing on the work will raise the local humidity. Don't slide around on a chair seat. If you hold the calculator in one hand and probe it with the other hand, rather than lay the calculator on a table, there will be less chance of an electrostatic discharge since both your hands are at the same potential. Holding the calculator down on the table with one hand is good because the calculator will be at the potential of your body. If you are working and want to go and do something else, first touch the table top with the hand that is not holding the calculator, then lay the calculator on the table top, then take both hands away from the calculator, then stand up. When you come back to work, first sit down, then touch the table top with both hands, then pick up the calculator. These little habits will make it possible for you to stop worrying about static. Faith is essential! If you look up an article in the Articles Forum about upgrading a 42S to 32K RAM, I think you will find a good explanation of how to solder pins on a surface mount IC without shorting them together. I haven't heard of Malaysian HP calculators before - just US, Singapore, Indonesia, Brazil and China. Were other models made there? I guess since the 12C has been in production for so long, it has probably been made in the most factories.

Re: HP 12C Keys don't work - Help please! Message #14 Posted by james (UK) on 28 May 2003, 9:59 a.m., in response to message #13 by Ellis Easley Hi Ellis Many thanks (I've stopped shivering from the mention of a dental probe!) I'm in Scotland so no problem with the humidity!

Re: HP 12C Keys don't work - Help please! Message #15 Posted by Trent Moseley on 28 May 2003, 2:27 p.m., in response to message #13 by Ellis Easley I bought my 12C MY72002125 new at Fry's in Palo Alto in September 1997. tm

Photo of pin in question Message #16 Posted by Randy Sloyer on 28 May 2003, 5:28 p.m., in response to message #10 by james (UK) I cracked open a dead Malaysian unit SN MY90500345. It's a bit older than yours, but the guts should be the same. Here is the pin in question for CHS/7/8/9/DIV keys. Hope it helps.

Surgery postponed Message #17 Posted by james (UK) on 28 May 2003, 6:53 p.m., in response to message #10 by james (UK) Firstly, many thanks to Randy and Ellis for their advice, much appreciated. Got held up at work tonight, we're doing a presentation tomorrow and had to do a rehersal tonight so surgery has been postponed until the weekend. I'll report back on the patient once the op is done. Once again, many thanks.

Surgery successful - patient recovered Message #18 Posted by james (UK) on 29 May 2003, 3:55 p.m., in response to message #10 by james (UK) Hi guys Just finished open heart surgery on the 12C - many thanks for your assistance. When I opened her up all the joints looked fine, even under high magnification, but after setting up a bench in a steam filled bathroom and prodding the appropriate pin connection with a probe it was obvious the solder had not taken. However, a few tense moments with the iron and connection was re-established. The main self test and the keyboard test were fine and the patient is now happily sitting up in bed crunching negative numbers with 7s, 8s and 9s in them. As far as I can tell all functions seem to be working - difficult to tell with the financial stuff though but a NPV problem gave the same result as that from the Real Estate pac in my 41CV so must be right! One thing that surprised me was what seems to be a solder bridge between pins 3 and 4 on the bottom left hand side of the chip - I did wonder if this was the fault but figured it must be intentional as if it was a fault presumably a lot more than just the top 5 right hand keys would have been affected. It also looked like the bridge was there on the photo Randy posted so I left it alone. Once again, many thanks Randy and Ellis - couldn't have done it without you.

Re: Surgery successful - patient recovered Message #19 Posted by Ellis Easley on 29 May 2003, 7:15 p.m., in response to message #18 by james (UK) Great!

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