HP 28C Calculator not power on

[Francis]

All,

I tried to power on a HP 28 Calculator. When I pressed on button, it does not have any display on the screen. How can I know if this calculator is still functional? New battery has been placed.

Thanks,

Francis

[Maximilian Hohmann]

Hello!

The first thing I try with LCD calculators that show nothing is to adjust the display contrast. On the HP-28C that is done by holding down the "On" and pressing either the "+" or "-" key to increase or decrease. Note that this must be done with the calculator turned on. So you press "On" shortly first, release it, and then press "On" again and hold it.

Next thing to check would be the battery contacts. The ones inside the calculator are not easily accessible for cleaning. I found a glass fiber eraser to be a good tool for that. These things were widely used when technical drawings were still made with ink on transparent paper.

Using a multimeter that can measure in the mA (DC) range you can also check if there is any current flow when the "On" key is pressed. Just remove the battery cover and bridge the exposed battery contacts with the leads of your multimeter. That can be a bit difficult with two hands only because someone or something has to press "On" as well...

Open the calculator as a last resort only. Look for threads about how it can be done on this forum. The procedure is the same for all "clamshell" models (18, 19, 28) so there are multiple search criteria. The chances of killing the calculator in the process are about the same as to bring it back to life.

Good luck
Max

[mfleming]

This might sound obvious, but make sure you are using the 1.5V n cell batteries, not the 12V ones that are of similar size. I picked up a 28c once, very cheap, whose seller said the display only blinked briefly when turned on. I opened the battery door and out tumbled three 12V batteries! Calculator still worked with proper cells afterwards.

[rprosperi]

(11-27-2019 04:06 PM)mfleming Wrote:  I picked up a 28c once, very cheap, whose seller said the display only blinked briefly when turned on. I opened the battery door and out tumbled three 12V batteries! Calculator still worked with proper cells afterwards.

Surprising (that it still worked)! Definite progress from the power supply situation in the Woodstocks and Spice (though of course that damage was from the the AC in the transformer, not just batteries, but still!).