Bear in mind with used cassette decks that there may be more aging issues than just the belt. Rubber idler wheels, pinch rollers, take-up reel clutches, and plastic parts can crack or get hard over time and need attention. A well used deck may have have wear in the motor, capstan and other bearings, as well as head wear, especially if metal type tapes were used in them. A good cleaning and lubrication is also advisable, including front panel and internal switches.
Most cassette decks have what's called the "play-record slide switch" which is a long narrow centipede type that reconfigures the circuitry when switching between the play and record modes. In my experience of repairing many hundreds of decks over the years, this switch is responsible for about 95% of cases of electronic failures. To properly clean one of these it must be desoldered from the pcb, dissassembled, and manually cleaned, then all put back together. I would automatically do this on any deck that was more than ten years old.
All of this can make for a reasonable DIY project if a person is interested in working with cassettes. Building a "better" cassette deck from scratch, however, would require top notch machining skills and equipment, as well as some hard to find parts such as heads and rollers. If you have the resources and ability, then go ahead, and please post your progress here as I would enjoy seeing that. However, I agree with others here that it would be much easier to find a used deck in good general condition and experiment with upgrading it after addressing any needed maintenance issues.
Take care,
Doug
. All jokes aside I am serious in this project .
