Abstract: A reader (20) for a magnetic card for an automated teller machine (40) or other self-service financial terminal comprises a magnetic reading head (22), a wave shaper (26), and a data memory (28) to supply card-related data to a central authorization position. A wave shape monitor (30) is preset with a threshold amplitude, and detects the amplitude of analogue signals from the wave shaper; if the signals are readable but below the threshold level, the same magnetic reading head (22) or a different magnetic head (34) is instructed to rewrite the magnetic data on the card.