There's a double-edged sword, so to speak, with the Tandberg rtr's: on one hand, even their single motor/non logic controlled models had far better performance than most of what Sony and Akai (for example) offered in the day...however: Tandberg HEADS are notoriously "soft" (I believe, the reason attributed to why they -while they last- can sound so good...the softer metal of their surface induces the best coercivity from the tape's magnetism; opposite, say, of Akai's "GX"/glass design...which, by contrast, tend to reproduce treble response with almost a muted and veiled feeling). Anyway, I'd make sure the pressure pad leading from the erase head remains in good shape or I'd replace it if not --- so, that, the tape wrap doesn't start veering off and (unevenly) wear the heads out to the point of being unable to ever relapp them (without, ultimately, resorting to having to sand off any more metal than necessary).
Mechanically, there's a large "figure 8" drive belt which wraps around the motor and both clutches of the supply and take-up reels (you might want to check to see if the surface of it has any cracks). Next, the old grease on the gear to change the speed is best cleaned and relubed (the gear is plastic and can crack if it requires undue force). Third, the take-up spindle has a felt disc sandwiched between the outer and inner halves which provides torque for the reel to catch tape. This often becomes worn down; causing the take-up reel motion to slip and fast forward to not work. On the back of the frame: the spindles are held in their bearing with a plastic set screw fastened to a spring clip retainer. If the take-up reel seems to wobble or pack too loose, tightening the screw on the back will put more tension on the winding of it (if, of course, the felt disc part is still serviceable enough to use).
