Powering a HP 67 without battery pack?

[Lode]

I heard a brief motor sound when connecting a DC voltage supply with 3.7V to the + and - terminals (with the calculator switch *off*). It also draws as much current as given (I stopped at 0.5 amp). But the calculator does not power on.

With the HP power supply connected to the 3-wire input, the calculator does work (so phew, that's not broken!).

So is it possible that the broken card reader is affecting how the power to the calculator itself from the battery terminals works, but somehow using the external 3-wire input with the official power supply manages to get around that problem?

How does one disconnect the card reader board? Are the screws under the sticker on the back? Shame to have to remove it in that case...