Issues with magnetic card reader on HP-41CV

[AeeeO]

I have found, that my two card readers work best on the CV and the CX that I operate them with, when no other module is inserted in them and the batteries are fresh. I also have very consistent results with the card readers when I power the 41 from a USB-rechargeable battery pack that I bought from a Dutch seller on eBay.
Still sometimes I have issues reading or writing magnetic cards which I attribute to a slightly flawed module slot due to the age of the machine.