The 927 was introduced in 1951 and was joined by the smaller 930 in 1956, aimed at customers who didn't require the 16" disc capability of the 927. The two machines are similar in design, construction and performance; one of the manuals I have covers both models. The 930 motor is smaller (but still wouldn't look out of place on a small lathe!)
The 928 is a later belt-drive turntable introduced in 1968. It's a heavily re-engineered Thorens TD-125 (few common parts, and it sold for rather more than the TD-125.) So I'm assuming EMT carried over at least the oscillator/amplifier motor drive of the TD-125, if not the motor itself.
I've gone through some documentation looking for clues; I may have found where the 'synchronous motor' idea originated. An
article in 'Sound Practices' issue 16 (.pdf) (late nineties?) states:
"The motor is massive. It looks like an industrial motor, 13.5cm in diameter and 20cm long! It is a 3 phase synchronous type with phase shifter."
This may be where some have got the idea that it's a synchronous motor from; an article which isn't entirely error-free. This statement is obviously incorrect:
"A precision manufactured 3-step pulley is mounted on the motor axis [...]"
As can be seen from my photo of the rotor, the rotor shaft itself is stepped. (I was wondering how it came off before I dismantled the motor, I couldn't find any grub screws...)
So, I guess it
must be a synchronous motor because it says so in the 'Sound Practices' article...
