29C Charger



#2

Cleaning out a closet I came upon my old 29C. Alas, I can't find the charger and I want to see if this thing works. Anyone know where I can find one or borrow one?

Thanks in advance.

Michael


#3

WARNING!

If you plug a 29C into a charger, without having a GOOD battery pack installed, the 29C will likely be damaged. This is one of the few cases of poor design practice on HP's part, where they saved a few cents by using the battery to limit the voltage.

Even if you have a good battery pack in place, if the contacts are corroded or for any other reason don't make good contact, the 29C can be damaged.

The CMOS RAM chips are usually what is damaged, though sometimes other parts including the processor are damaged. These were all custom chips, and there is no source for replacement chips aside from other HP calculators.

Many HP calculator enthusiasts will only charge the battery packs outside the calculator for this reason. HP sold a separate "reserve power pack" that had a charger that connected to the AC adapter supplied with the calculator.

#4

As Eric said, forget about charging the batteries in your 29C!!!
The safest thing to do is to remove the NiCad batteries and replace them with fully charged NiCads (or NiMi) cells or just use alkaline batteries. It is easy to do and much safer than attaching the charger.

It is a shame HP did not put a bit more design effort into the charging system on the Woodstock series (HP-21, HP-25, etc). Relying on the presence of the NiCad batteries to avoid destroying the calculator is NOT good design practice. Worn out cells, poor connections or the user simply removing the battery pack during charging could result in a failed product. I wonder why HP engineering let this design compromise (defect) get into a finished product?


#5

Thanks for the info, I'll look for rechargables. I think it needs 1.2V is that correct? Also, the batteries are very tight in the pack, is there a recommended way to remove them without breaking the mold?

Michael


#6

HP claimed that the NiCd cells were nominally 1.25V, but most sources claim 1.2V. It's not critical; any normal NiCd or NiMH cells should work fine.

The cells can't be removed from the pack without cutting it.

#7

This is what I did to replace the batteries on my HP-25. Carefully use a sharp hobby knife (exacto type) blade to slice through one end of the thin plastic bar that runs between the two batteries. Cut the end closest to the open end of the battery pack. With a bit of effort, you can then gently pull it aside enough to slip the batteries out, one at a time. The plastic bar will set back in place after you are done. If you don't care about appearance, just cut the bar off completely.

WARNING! BE CAREFUL TO OBSERVE THE POLARITY WHEN REPLACING THE CELLS!

When looking at the back of the calculator with the silver Hewlett Packard label end down, the Negative pack battery end should contact the terminal above the H in Hewlett. The positive battery end should contact the terminal above the P in Packard (Picture of 20 Series battery polarity). The bump on the positive battery end does not always sit well on the PC board terminal since the original NiCads were flat on both ends. If possible find Nicad cells that are flat on both ends.

See this HP Museum link for more information on replacing batteries:
http://www.hpmuseum.org/batts/battery.htm


When I originally did this conversion (in 1986!) I used ordinary 1.5V alkaline batteries. I had some issues with the Low Battery warning (all decimal points lit except the true one) randomly coming on the display. If finally cut the battery holder spring and carefully soldered a diode in series to reduce the battery voltage. I also soldered a flat piece of copper to the PC board terminal to make better contact with the positive bump on the battery. Although it still works great, I don't recommend doing all this extra work! Just used 1.2V rechargeable cells.


#8

My sincere thanks to everyone that responded; my 31 year old calculator is back in business. I was able to pull the batteries without any damage to the holder and dropped in two fully charged 1.2v Ni Cads. Flipped the switch and voila!

Michael

Edited: 23 Aug 2008, 6:24 p.m.


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