Service a " HP-28c"


I recently got a HP28c through ebay as-is condition. Put the batteries in and it will not turn on. Any body know how to open it up ? There is no screws under the rubber feet. I like to check the power connection to the cpu. Help is much appreciated.



did you test this reset-"trick" ?

Best regards



Hi Andreas,

Is the reset trick in the HP28c manual or is it a off the record trick ? I never owned a HP28c before, I am willing to give it a try first before I open it up. Thank You.



Yes, with great difficulty :-(. Take out the batteries, and peel off the overlay on the right-hand heyboard (the side with the display), and the 'HP28C' label at the top on that side. Then using a 3mm (or 1/8") twist drill bit held in your fingers (or a large pin chuck, but not a hand or electric drill),
cut away the tops of the heat-staked posts that hold the case together. Take off the front panel on that side (and don't lose any of the keys!), and you
can get the PCB out. Don't separate the PCB from the LCD glass -- it's almost impossible to realign them.
On the back (accessible) side of the PCB are 2 ROM chips. On the LCB side there are 3 chips directly mounted on the PCB (and covered with epoxy). IIRC the outside 2 are display drivers/RAM and the middle one is the CPU.


Hello Hugh,

Before you are going to open-up the calculator, try the following.

Take two metal strips, and place one on each battery-sprong. Then short-cut the to metal strips for several or more seconds. In otherwords, short-cut the battery-sprongs.
I have 21 HP calculators, and used this trick several time's now. It allway's brought back live into the calculator, after i thought it had died on me. :~) Especcially the 28 serie like's to pretend it's dead.

Greatings, Erwin

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