Basically a programmable version of the famed SL-10, the SL-15 was however no gadget at all and is an excellent turntable.
Motor-wise, specs-wise, dimensions-wise, features-wise the exact same as the SL-10 - with a bonus, a preview of sorts of what was to come only two years after the SL-10 and SL-15 were launched: programming !

The latter feature is done, previsibly, by an infrared LED emitter scanning the surface of the LP to find the wider groove spacings that determine different tracks. However, the beam isn't located onto the headshell or on a dedicated cartridge (as Toshiba had done in the early 1970s) but within the upper half of the SL-15.
Apart from that, SL-15 = SL-10 so you can refer to the SL-10 post for the details of the construction and features - just add to the already long list the programming ability, one side repeat, one song repeat, program repeat, program overriding (etc) and retrieve the possibility to power the SL-15 with 12V DC which the SL-10 had.

Pity one cannot find MC cartridges in T4P mounts because the SL-15 and SL-10 deserved the very best and they still do.

You can find related info and service manuals on the trio (SL-15, SL-10 and SL-7) right here.