HP98035 Real Time clock and AC5954N clock chip

[Tony Duell]

On Fri, Jun 7, 2019 at 4:44 AM Curious Marc <curiousmarc3 at gmail.com> wrote:
>
> Thanks, I’ll see if I can find replacements. You can easily see how they get zapped: they are 2.5V chips, the NiCd battery *is* the voltage regulator. Charging circuit is a simple diode connected to 5V via a resistor. Battery dies, goes high impedance, somebody plugs it in to try it out and poof! Clock chip gets zapped by 5V.

HP were fond of using NiCds as shunt regulators at that time. The did
it in many of
their handheld calculators (HP20 series 'Woodstock', HP30 series
'Spice', etc). In
those it wasn't normally a problem (the calculator electronics drew
enough current to
pull the voltage down) except in machines with 'continuous memory'
(battery backed
RAM). There, if the machine it turned _off_ the RAM is the only thing
drawing current
and it doesn't draw enough to pull the voltage down below the zapping level.

-tony