The question of gear-driven turntables and the derivation of standard rotational speeds from feasible gear ratios came up in this earlier thread:
Question on Recording speeds,
https://www.vintage-radio.net/forum/...d.php?t=159711.
The RCA 70-series transcription turntable was quoted as an example with synchronous motor and gear drive, and more comment on that turntable had been provided in this thread:
Why both 33 and 45 ? https://www.vintage-radio.net/forum/...d.php?t=151267 (towards the end)
Cheers,