The earliest scanners were crystal-controlled. These scanners had one or more internal slots for a crystal which allowed the radio to receive a single frequency. Crystal-controlled scanners are not currently being sold through RadioShack; however, they still turn up in pawn shops and garage sales. For example, the PRO-25 (Cat. No. 200-0106), PRO-27 (Cat. No. 200-0108), and PRO-53 (Cat. No. 200-0122) scanners were of this type. Generally, if you are looking at a RadioShack scanner and it does not have a keypad or program button, it is probably crystal-controlled.
The disadvantage of crystal-controlled scanners is that they were slow to tune, expensive to set up for new frequencies or channels, and they could not follow the new trunked radio activity. Crystal-controlled scanners have been replaced by programmable scanners.