Bad batch of cards, or reader/writer issue?

[Dave Britten]

I've got a wallet of blank cards that I've been using with my 97 since I bought it early last year, and in going through them this morning, I'm finding that at least half of the recorded programs can't be read - so far 18 cards that are unreadable on at least one side, and usually both. These all were readable immediately after I wrote them, so it seems like the recordings have deteriorated. And I've confirmed with my 41 that I can't read them on that machine either. I haven't encountered this issue with my 65, and the batch of used TI-59 cards I bought a few months ago were all still perfectly readable on arrival (after fixing the gummed up reader and adjusting the speed).

So the question is what's more likely? Is this a bad batch of cards (maybe heat damaged at some point), or is there an issue with the writer in my 97 that's causing it to lay down weak signals that fade quickly?