Opening the HP-12C Platinum (original silver faceplate)

A while ago I got a HP-12C platinum (aka HP-12CP) off eBay. It had a depressed decimal point key. Today I decided to investigate, so opened it was. To open, first remove the battery and the rubber feet and then undo the 5 screws. With a plastic pry tool, lift off the back. the PCB is now visible. To go further, the tops of the heat stakes have to be removed. I cut them off carefully using the blade of a Stanley knife (Note - these will have to be replaced by screws. As the plastic stakes are hollow they take small screws quite easily but these may be difficult to find. I salvaged some from a 50G that fell into dis-repair due to screen problems. Look out for my next blog). Carefully taking the PCB off (to which the screen is attached) reveals a rubber mat under which is the keyboard.

Back removed showing PCB with heat stakes cut off and showing some of the replacement screws

All keys apart from the two shift keys are hinged plastic type. I have seen this (dis)proven technology before, in a cheap '70s four-banger (see the picture of the Satek 8M - suffering the same problem). This is not what I would expect in a top-of-the-range calculator that this is supposed to be! The shift keys are on separate rotating hinges - much better, but what do HP think? that the shift keys are used most often? is a calculator not used for numbers? wouldn't it seem  reasonable that the number keys would need to have the better design?

The loose 12CP key

Cheap '70s Satek 8M with broken hinged key

In this case the decimal key has become totally detached from its hinges and the 9-key has one leg detached and the 5-key is showing signs of weakness at the hinges - as I would expect the number keys are first to go as they are likely used more often.

To get the key more-or-less back to normal, it needs a spring to keep it lifted up and a "hinge" to rotate on. The hinge was fashioned from aluminium tape and a sponge added as I did not have a spring of the right size handy. (Other keys were taped as well in the hope that it will prevent breaking, note the white piece of sponge under the decimal key in the last picture).

The repaired unit put back together:

The decimal key feels a bit squidgy, so I guess an actual spring would be the better solution.

PLEASE NOTE: the newer black faceplate model apparently has a better keyboard (it certainly feels better).