Repairing a weird HP12C


Hi, all;

based on what happened, I guess that this would be of interest for those who repair Voyagers and newer calcs. Having my bother in a rush (hollydays), he asked me to try finding what was wrong with his HP12C: it did not turn on. Fresh bateries, slight twist and voilà!

[ Pr  Error ]
'It's alive...' Then the problem: each time it was turned to off, it was necessary to twist it back again so it would turn to ON with a big Pr Error in the display. It was too fast to be a bad contact, maybe the internal capacitor itself was bad. After openning it up I saw one of the internal assemblies I was searching for: the one board only style, three batteries and two IC's, mainboard layout like the HP11C/15C. With this one, I have already seen at least three different HP12C internals with three batteries, missing only the one with flex circuit (earliest of them all).

Now the works: resolder all terminals in both IC's and components, because if comms fail between them two, I guess that it would cause a Pr Error as well. O.K., IC's terminals resoldered, turn the calculator ON and no way to start the self test. Awkward: keys in both right-side rows 2 and 3 generating similar codes. Press [1] and 4 appears in the display, press [2] and 5 appears in the display... either {+] or [-] generate code 20 in program mode. My conclusion: short circuit in row lines. Open it again and visual check: nothing. Clean all solder pads and... nothing. I decided to go for awkward tests and firmly pressed the processor against the m-board: calculator works perfectly fine! What to do? Surgery! I removed the IC. Look below:

Click to enlarge

I inspected the surface and it was not hard to see some green crusts over some golden-coated trails, suggesting some corrosive product or residual liquid was allowed to get inside the calculator. Unfortunately I tried to remove them prior to take the picture below, but some remaining dust can be seen:

Click to enlarge

I carefully cleaned the surface, removed all visible dust (I use a small lens) and soldered the IC back:

Click to enlarge

My brother is happy, and one other HP12C is alive again!


(to my three friends that have calculators with me: M.B. (41CX), R.T. (29C) and J.E.G.(25,41CV), please be sure that this experience enhanced my wish on having your calculators back to life... I'm regrouping and getting ready to attack!)

Edited: 27 Mar 2005, 10:54 p.m.


Hi Vieira,

Great post ! I love these elegantly explained practical informations.

How do you do to unsolder cleanly the small pins of the IC circuit ? What tools do you use ?


Pierre Brial


Hi, Pierre; thanks for your kind words.

If you take a look at the first picture, you'll see the tip of a sewing needle close to the Voyager's processor. In fact, I use dessoldering mash to remove the excess of solder and then I ground the needle, wrist and batteries contacts altogether. Then I use a small soldering iron with a thin tip and for each terminal I briefly heat the cooper trail wihle I pull the therminal with the needle. The needle tip is positioned between the IC's body and the terminal to be pulled. Sometimes you need to go gently inserting the needle and pulling as many terminals as possible. The needle itself helps part of the heat not to get inside the IC, protecting it. It takes some time, but if you are patient enough, I tell you it can be done. Most important of all: brief touching the thin cooper trails. They tend to get loose when excessive heated. After removed, the terminals must be realigned. I proposedly left them "twisted" in the picture so you can see the SMD IC after removed. Also, don't bend them more than enough to remove, or they may be broken when realigning.

I know that it can be done with special tools, mostly when you have them... I do what can be done with what I have in hands, otherwise I go for my own solutions. Sometimes works, others I must face a failure and start again. So far I successfuly removed and resoldered many SMD devices, and I recall one single time I was not able to remove and had to let the dammaged IC in place.


Luiz (Brazil)

Edited: 28 Mar 2005, 2:35 a.m.



I remember trying to remove an IC from a broken Sharp PC-1251, but I damaged the trails.

But I didn't use a needle !



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