A current issue with batteries for my HP-35 and HP-67 calculators

[teenix]

(05-30-2020 08:45 AM)aurelio Wrote:  Michael de Estrada explained it well in this old thread.

A good read.

It seems to me that the VCC pin of the sense chip seems the sensitive one.
The HP-67 has about 3.7V connected, unless the charger is connected without the battery in which case it will have the charger supply and is maybe why the warning.

The HP-65 has the same setup as the 67 but as mentioned in the text has a different sense chip and maybe would serve as caution to those swapping this chip to fix a 67 and expect it to run happily on a charger only. The 65 manual has the same warning about charger operation without the battery.

The HP-97 appears to have a power on delay attached to this pin, but connected to battery through Q1. In which case if the battery is out when the charger is in would be fed with rectified AC from the charger. Not sure of effect, but maybe why the extra circuitry on this pin.

The HP-41 card reader has 6V for battery power (4xNN I think) and this appears on the sense chip VCC pin via 3 diode drops bringing it back to about 4V.

So battery power from these 4 setups ranges from 3.7 - 6V and normally the VCC pin max is about 5V, but would be higher and maybe damaging without a battery. The 5V max may suggest why the card reader is happy running on 3 alkaline cells.

For me, I would just heed the manufacturer warning :-)

cheers

Tony