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Nixdorf LK-3000 Computer - Programmable Module (LK-3500) Battery Replacement?

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I recently acquired a Nixdorf LK-3000 "computer" in relatively good shape.  The "computer" itself is designed to not operate without a cartridge inserted.  With the machine, I received three cartridges which was two language cartridges and one Electronic Notepad Programmable Module (LK-3500).  The two language cartridges function well with no issues, however the Electronic Notepad Programmable Module (LK-3500) does not.  After opening the cartridge, it appears there is two batteries soldered to the board that need to be replaced.  My issue is that this cartridge is from around 1979 or 1980 so these batteries are quite old, and I am looking for a modern equivalent to replace them with.  The only marking on the batteries is the number 3079.  I have tried to research this battery using the number 3079 with no success.  I also have a copy of the documentation for the module, but it does not identify the battery type or voltage.   I would like to see if anyone has any suggestions for a replacement battery that I could attempt to replace these with hoping to get the Electronic Notepad Programmable Module (LK-3500) functioning.  Any suggestions are appreciated.  Thank You.

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I see that this thing has a 7400 series integrated circuit package.  These were widely used for things like NAND, NOR, XOR, OR, and co, package ICs in the 70s and 80s.  From what I am reading, they are 5v chips.

(The 7400PC is equivalent to NTE7400, which is a TTL Quad 2-Input NAND Gate IC. Its spec sheet says it is 5v nominal voltage (tolerant to 5.5v)) 


This suggests that the batteries are 2.5v nominal.


This suggests those are some variant of a lead acid cell, or similar chemistry.


Consider replacing them with a 6v lithium battery, with a buck voltage converter.

Edited by wierd_w
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