A feature of older recorders is that when designed there were relatively few channels so less memory was required to store a full channel scan. For example, mine will start to malfunction if more than 40 or so are tuned in.
Disconnect the aerial and re-tune. You'll get a 'no aerial' error message, but will have cleared all data. Re-connect, and re-tune. Then delete all the channels you don't want. Don't just 'hide' them, delete altogether.
Fingers crossed.