Re: HP29C problems Message #3 Posted by Tony Duell (UK) on 8 Oct 2002, 7:43 p.m., in response to message #2 by David Smith
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...
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