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HP12C battery contact question
05-13-2019, 03:52 PM
Post: #1
HP12C battery contact question
My first post here & hoping someone can help me before I damage my 12C !
My 3-cell 12C often failed to turn on unless I re-seated the cells. Now that no longer works and I have looked inside for the reason. I see that the -ve terminal seems to be a tab on the PCB which makes NO electrical connection to the battery. But there is a small copper strip almost at the edge of the PCB tab which MAY have had a wire or spring pressing against the battery's -ve terminal.
Can anyone confirm that this IS the 12C's -ve connection to the battery pack; the +ve terminal is well-connected to the +ve terminal of the battery pack via compression spring ? I am hoping that a secure repair/restoration between this copper pad and the battery will reinstate perfect operation.
Many thanks,
MikeE


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05-13-2019, 08:56 PM
Post: #2
RE: HP12C battery contact question
There is a 12C in my collection with that type of coil spring at one end of the battery compartment. The other end looks like a gold block. It has more contact surface, but appears to be no taller or wider than the edge of the printed circuit card.

Alan
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05-13-2019, 09:00 PM
Post: #3
RE: HP12C battery contact question
On mine, the end of the tab (edge of the PCB) seems to be covered in metal foil. I haven't taken it apart though.

— Ian Abbott
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05-13-2019, 09:02 PM
Post: #4
Image of battery contact in 12C from 1996
See the attached image (sorry for the imperfect focus). It shows the block of contact that I mentioned.

Alan


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05-13-2019, 09:08 PM (This post was last modified: 05-13-2019 09:10 PM by burkhard.)
Post: #5
RE: HP12C battery contact question
(05-13-2019 08:56 PM)striegel Wrote:  There is a 12C in my collection with that type of coil spring at one end of the battery compartment. The other end looks like a gold block. It has more contact surface, but appears to be no taller or wider than the edge of the printed circuit card.

Alan

Yeah, I don't have a 3-cell 12C at work where i am now, but I have two 16C and a 15C and all three look like Alan describes [later edit: and has now pictured]. That fixed post that sticks into the battery compartment on yours seems to be missing its metal cap. If you were to fix that (and ensure a connection to the circuit is reëstablished, I would say you will be back in business.
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05-13-2019, 10:10 PM (This post was last modified: 05-14-2019 06:38 AM by ijabbott.)
Post: #6
RE: HP12C battery contact question
Here's a picture of mine from a different angle to striegel's. I took it by holding a magnifier in front of my phone's camera lens!


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05-13-2019, 10:40 PM
Post: #7
RE: HP12C battery contact question
It's not really a added cap covering the board stub, it's just the same material (copper/gold?) that they had used for the traces on the rest of the board. Is it just worn away? Corrosion?

The NEGATIVE connection stub is carried underneath the board (the part you can't see) and is actually connected to the top via hole just to the left of the top portion of the through-hole cap in your first post's picture. Next to the small spring cavity?

You could make a small wire connection to the battery compartment to complete the circuit.

I'll see if I can post a photo of the bottom of the board for clarification.
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05-13-2019, 10:49 PM
Post: #8
RE: HP12C battery contact question
Here's a photo of the area on the backside. This is from a 12C serial date-code 2651A. My board dosn't use the through-hole cap as yours does. Mine's a surface-mount cap on the far other side of the board, but that won't matter, the board was dual populated for either part.

Hope this helps.


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05-14-2019, 04:01 AM (This post was last modified: 05-14-2019 04:04 AM by SammysHP.)
Post: #9
RE: HP12C battery contact question
It's a corrosion issue, probably due to leaked battery. You can see it on several traces and at the remaining part of the battery contact. Even a tiny spot flaked off on the plastic. I'd give it a good clean with IPA and then solder a contact replacement to the remaining plating.

I'm not sure about the construction, but if possible I would also try to remove the board from the case and clean the other side.
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05-15-2019, 05:23 PM
Post: #10
RE: HP12C battery contact question
Well, thanks very much for the replies to my post. They were pretty much 'spot-on' and gave me the confidence to safely repair the calculator. After a good IPA clean around the PCB tracks, I managed to solder a short piece of 24 SWG tinned copper wire onto the track and then bend it to form a new contact for the battery's -ve terminal. All is now working perfectly. Particular thanks to jonese who identified an alternative connection route if the original track couldn't be repaired - so far, not required!

Many thanks to all for their help, Mike
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