Documenting this fix, since it was not obvious to find. I got some helpful hints from “buzz” at fieldphones.org.
The Feldtelephone Modell 1950 (FTf 50) is a field telephone used in the Swiss Army. It was developed by Albiswerk Zürich AG (the Swiss Siemens subsidiary). Its design was inspired by the US EE-8 field phone. At only 2.6kg, the device was very lightweight for its time.
The device was also used by the Dutch army as “Veldtelefoontoestel TA-3001”.
The ringer induction coil is actuated by a hand crank. On this particular device, the crank refused to turn. The generator has a centrifugal switch that shorts the coil when at rest, but with time the ball bearings that constitute this switch can become stuck, so the coil is always shorted, making the crank very hard to turn.
To get at the centrifugal switch, the two screws keeping the device in its canvas housing, the crank, and the three security screws need to be removed. It’s easiest to also remove the bell assembly (kept in place by the two screws on the bottom of the phone).
After unsoldering the three wires connected to the generator assembly, the entire assembly can be taken out.
On some models the assembly is housed in a transparent casing, on others the casing is black. There are two screws on the side opposite of the crank (in this case the screws were secured with black glue that had to be scraped off first). When these screws are removed, the generator can be taken out of the housing.
The centrifugal switch is the round black plastic widget. Normally it rotates along when the hand crank is turned. It has a lid that is kept in place by a lever-like contraption held in place by one screw.
Inside are the ball bearings that form the centrifugal switch. After cleaning away the dried old grease with naphtha, and re-lubricating with fresh bearing grease, the generator turns smoothly again.