If an SR-52 consistently misreads known good cards and you've checked all the obvious (head is clean, motor is spinning evenly, no wires are broken) it's time to check the head coils. The SR-52 card reader has a four-track head; in some models, this means eight wires as each coil has a separate return wire, whereas in other machines there's only a single return wire. In both models, the resistance to ground through each coil should be roughly 7.5 kOhms. I've seen a number of machines in which one or more head coils are broken. Unfortunately, these head assemblies are not repairable; the only known repair method is to replace the head assembly from another, "for parts" calculator.
SR-52 consistently misreads cards
