Hello,
I have an HP-21 that was recently working just fine. I removed the battery pack and put it back in, and now the display shows all zeroes. It's possible that some small particles may have gotten inside the case.

Any suggestions as to what might be wrong?

Thanks!

Barry,

Katie has a very good article on this subject which may help. You can find her article in forum Articles area.

Seems like the solution may be to keep the unit on for up to 1/2 hour and turn off, turn it back on again. Quote from Katie's article "A seemingly common problem on some Woodstock machines is that they don't work until they are left on to "warm up" for some period of time. After leaving the on for several minutes (maybe up to 1/2 hour) then turning them off and back on again they start working normally."

Hope it helps,

Regards,Kerem

Edited: 23 Jan 2012, 7:47 p.m.

The article does not apply to the HP21 as it has no external ram.

Based on what happened, the problem is either a bad battery or bad contact with said battery. If the battery has been rebuilt, make sure the cells are connected with a welded strap, do not rely on the spring to do the job - as even a small amount of corrosion will cause problems. When you have a bad battery or bad contact, connecting the calculator to the HP charger can and usually results in catastrophic failure of the ACT (arithmetic, control, timing) IC. In other models of the 20 series, other components can be damaged as well.

Some questions.

Is this a NiCd battery pack ?

If so, was it charged inside the calculator ?

Is there any evidence of corrosion on the battery contacts inside the calculator ?

Do you have a voltmeter, and if so what is the battery pack voltage outside the calculator ?

I replaced the original batteries with Everready NiMH 1.2V rechargables, then charged them up in the calculator using the original charger.

All was working well until I attempted some additional cleaning of the battery contacts using an eraser:
https://picasaweb.google.com/rabbybuns/HP21#

I'll have to ask my brother for help with a voltmeter.

Thanks, everyone, for your advice!

Randy said,
"When you have a bad battery or bad contact, connecting the calculator to the HP charger can and usually results in catastrophic failure of the ACT (arithmetic, control, timing) IC. In other models of the 20 series, other components can be damaged as well. "

I am afraid what may have occurred is that when you cleaned the battery with an eraser, eraser residue (or another battery connection problem) may have caused the battery to not make a good connection with the calculator. If this happens AND then you plug the charger into the calculator, circuitry damage often occurs. The lesson from this is: NEVER EVER charge the batteries in the calculator. Always use an external charger. Using the batteries as a voltage regulator and over-voltage protection was a very poor decision on HP's part.