I also had 5 to 7 volts AC at the transfer switch but that is not what provides the exciter current. Normally not much exciter current is needed as a residual magnetic field provides enough induced current but sometimes it loses some of the residual field and needs a dc current applied to excite and build the magnetic field. If you start the generator and simply continue to hold the start button for 3 to 5 seconds it allows the circuit from the voltage regulator to provide the dc current needed. I decided to install the boost kit to get a reliable field boost regardless of how long I pushed the start button.
Try simply holding the start button for a few seconds after the engine starts.
Quote from manual: The Controller Circuit Board houses a field boost
diode and resistor that are not part of the automatic
choke circuit. These two components are part of a
“field boost” circuit (Figure 2.4). During engine cranking
only, a positive DC (battery) voltage is delivered
through a diode, resistor, brushes, slip rings, and the
generator rotor. Application of this voltage to the rotor
“flashes the field” whenever it is started. Flashing of
the field each time the generator starts makes sure
that a sufficiently strong magnetic field is available to
produce “pickup” voltage in the stator windings.