On 2018-07-22 4:17 PM, Jules Richardson via cctalk wrote:
On 07/20/2018 10:57 PM, Tony Duell wrote:
It is not uncommon for the NiCd cells to act as a shunt regulator in
such
calculators. The charger is of relatively high voltage (say around
9V), it is
applied to the cells through a current limiter (often just a simple
resistor as
you say), and the fact that the on-charge voltage of the cells is
perhaps 2.5V
(for a pair of NiCds) limits the voltage applied to the rest of the
calculator.
Thanks, all - that does seem to be the case here, supply via the
external jack just runs through a 39 ohm resistor and then hooks
straight to the +ve battery terminal, with no other 'magic' involved.
From there, the entire calculator is switched via the on/off switch on
the keypad, i.e. there's no standby power.
The switch is good, and I've reseated the display and MOS IC, but
without any signs of life when feeding it 2.4V via the battery
terminals. The three electrolytics in it are all reading low on my
multimeter - I generally question the accuracy of that somewhat, but
as it's quick and easy (and they're over 40 years old) I'll probably
try replacing those.
One thing possibly of note - while I strongly suspect that the 4-pin
Astec module is generating some necessary voltages from the battery,
the MOS MCS2529 and the pair of ITT 510-5N display ICs that are in the
machine both receive battery voltage to various pins (pin 28 on the
MOS and pins 1 and 16 on the ITT's). That makes me wonder if the
battery pack voltage isn't supposed to be higher (perhaps around the
4.5V mark) - but obviously I'm reluctant to increase battery voltage
if there's actually some other fault that's preventing things from
working.
cheers
Jules
While I don't believe I have seen a calculator that uses that particular
chip, I have seen the inside of a few Commodore calculators that of
course use MOS chips and a pair of ITT510-5N display driver chips seem
to be the usual for them. My PR-100 have a small blue and white Astec
module hanging on wires (AA4080 in my SR4190) in them that does indeed
generate some other voltages used by the chips. These calculators all
have 3 NiCd cells in them if the batteries where already removed, you
may get a clue from the shell how many cells it was designed for.
Paul.
