These unusual movements are characterised by:
- Tuning Fork
- Quartz Control
Possibly the only example of this type of watch is the Bulova Accuquartz (Calibre 224x). They are superficially very similar to the standard 218 Accutrons: they still have a tuning fork, the two coils, similar looking indexing system but the basics electronics (transistor, resistors, etc) have been replaced with a quartz crystal and associated circuitry.
In previous calibres, the tuning fork was responsible for the accuracy of the watch and tuning regulators were supplied on the coil cups to make small adjustments to the tuning fork frequency. In the 224x calibres, the tuning forks have become “passive” and the quartz crystal controls the tuning fork frequency — the regulation of the movement is now undertaken by “switching in” or “switching out” a series of internal capacitors by using small brass screws. You can see these 6 screw holes on the far right of the 2242 movement below.
Unlike the 218s whose tuning fork vibrates at 360 Hz, the 224 vibrates at 341 1/3 Hz. For a full explanation for this strange figure, Rob Berkavicius gives a very detailed explanation on his site here. His page also highlights why some of the wheels on the 224s are not interchangeable with 218s.