Powering a HP 67 without battery pack?

[teenix]

(05-27-2019 06:17 PM)Lode Wrote:  For powering a HP 67 without battery pack (and without willingness to use any batteries):

Question 1) I read in multiple threads that powering it with the HP DC adapter/charger can break the card reader. However, this one is already known to have a broken card reader. Is it safe to use in that case?
NOTE: I tried it *very briefly* and it worked, it turned on and could calculate.

Question 2) What is the +/- side of the battery terminals? The terminals in the photo in this thread (https://www.hpmuseum.org/cgi-sys/cgiwrap...ead=219662) are on the opposite side as in my case, in my case they are on the left (with the top of the calculator at the top of course). So are + and - reversed (+ up) in my case?

Questions 3) Is it safe to power it with an adjustable DC power supply to the battery contacts (once knowing what the + and - are)? should it be set to 3.75 V or to 4.5V? How much milliamperes is it expected to draw?
NOTE: a very quick experiment did not work, unlike the HP adapter afterwards.

Question 4) Is it safe to power it with USB (by modifying a USB cable first), given that USB is 5V?

Thanks!

The card reader chip is fed direct from the battery to drive the motor and from the internal power supply to power the rest of the chip. I haven't seen any specifications for this chip so I don't know the safe voltage levels. The logic power source is 6 volt, If it is damaged, I would unplug the reader circuit board from the main board in case it causes trouble. The calculator will still work.

Higher battery voltages may work but could stress the power supply circuit.

Isn't that a HP-35 in the picture? The battery is where the card motor is.
The HP-67 -ve terminal is closest to the calculator top.

Yes it is safe to use an external power supply. It would be best to set it to 3.7 volts where the calculator was designed to operate. If voltage gets below (around) 3.5 volts, the low battery LED will begin to light.

Just be careful with voltage and polarity as there is no internal power protection. An error could cause total calculator failure.

The calculator can draw up to 250mA from the card motor depending on the cam adjustment. In your case the digits on the display will determine the max current. The current will be in the range 150mA to 185mA.

USB should supply up to 500mA (or more) but the power depends on the source, ie. PC, Wall wart charger etc. It could supply a fixed voltage regulator circuit for the calculator.

cheers

Tony