This can occur if the parameter unit window display is set to "Frequency setting value".
If the screen display selection (first monitored window) is set to "Frequency setting value" instead of "Output frequency monitor", the setting value (voltage between the terminals 2 and 5 to which the frequency setting potentiometer is connected) is read and a value other than 0.00 Hz can be displayed without the STF signal. 　
You can see that the display is the frequency setting monitor rather than the output frequency monitor because the frequency unit (Hz) is shown in white on the black background.
The first monitored window is one that appears upon power on and use it to display a certain item, such as voltage, first.
To return to the output frequency monitor that is the initial setting:
(1) Press the "HELP" key, place the arrow at the next to "f monitor", and press the "READ" key. 　
(2) Ensure that "Hz" is shown in black and press the "WRITE" key. 　
This resets the first monitored screen to the initial setting of the output frequency monitor.