22 Jul
2018
22 Jul
'18
12:17 p.m.
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