HP29C problems

[Barry]

hello to the group: picked up a 29C awhile ago, in the box w/ charger/manual/carrying case; problems w/ running it: there is some very lite corrosion on one battery contact
(calc side) and a little on one end of the battery pack;
i've cleaned them a little. when powering up w/ the charger
(no power w/out the charger), i first get flashing zeroes
across the board (w/ a brite zero second from right end),
then it seems to charge up after a little bit, but i then
get only a "8."; will not accept any input from any key,
etc.
any suggestions? this is a very clean unit, hope it can be
brought back to life (i do have to sell it tho, as my finan-
cial predicament does not allow me to keep much of what i
locate).
tnx in advance,
barry
carson city, nv

[David Smith]

You need a GOOD battery pack in the machine before it will work. Even then, be VERY careful. It is HIGHLY recommended that you charge the pack externally.

If the machine is plugged into the charger without a good pack (and making proper contact) then voltages high enough to burn out chips in the machine are produced. On the 25C and 29C this will happen even if the machine is turned off (because the RAM chip is powered up even when the switch is off. The 29C is the most electrically fragile machine HP produced.

[Tony Duell]

It's actually _worse_ when the machine is turned off (This applies to the 25C, 29C, 33C, 34C, 38C, BTW -- they all have much the same PSU circuitry).
The HP chargers deliberately ahve a high internal resistance, and when the calculator is turned on, it draws enough current to pull the charger voltage down considerably. THis is less likely (note, I didn't say 'won't'!) to damage ICs.
But with the machine off, the voltage can rise to 12-15V. On the -C models, the RAM chip receives power all the time (that's how it maintains the 'continuous memory') and will normally burn out if it gets 12V. Never plug the charger into a -C model without a known-good battery pack in place. And if you have any doubts at all, keep the machine turned on when you have the charger connected...

[Barry]

what is the fix for the bad battery pack? are replacements
available, or is this one of the units requiring "surgery"
to make workable?
sorry, wasn't paying attention when this was discussed before (probably a hundred times before!).
tnx in advance,
barry
carson city, nv

[David Smith]

It is very easy to fix the pack. You can either cut loose the end of the center strap (where it comes down bethween the battery contact holes) and slide the cells out or slice off the top of the pack, replace the cells, and glue it back together.

As a quick check you can just clip a couple of alkaline cells to the calculator and verify it is still OK.

[John Garza (3665)]

Bottom line for a woodstocks.....

Cut the center plastic bar in the battery case, chuck the old NiCd batts, install alkalines (they snap fit in nicely). Never worry about recarging again.

Before I did this, I had one 25C heat up so much the charging resistor slightly discolored the plastic case!

cheers,
-John