Powering a HP 67 without battery pack?

[rprosperi]

Q1 - I will let more knowledgeable folks reply on this one.

Q2 - [Corrected] As Tony pointed out below, and actually seen in the photo, that machine in the linked article is a 35. And though on 35/45 the tabs come up from the right, they are still in the same relative position, so the battery sits in the same orientation, with (-) terminal on top and (+) on bottom, so even though tabs are different, the polarity is the same. Sorry for earlier comment and thx to Tony for correction.

Q3 - No idea why that would not work; the battery pack used 3 NiCD cells, nominally 1.2V ea. (but often a bit higher when fully charged of course), so 3.6V certainly should work.

Q4 - Yes USB can be used, there is a special cable available which does just that, but note that this cable connects to the CHARGING pin, not the operation pin (see the connector pinout in that same link). I can't say if direct connection to the battery tabs is safe, but I would not attempt it until someone here absolutely and clearly confirms so, the risk of damage is not worth it.