Hi!
This kind of fault nearly always indicates a missing power–supply voltage, which may NOT be simply missing from the psu circuits, there may be damaged or open–circuit pcb tracks, burnt–out supply feed resistors, particularly if surface–mount, or even defective op–amps with an output stuck near a supply rail!
Good Will circuit–diagrams are not easy to come by, but open up your gen and write down the type number of all the main devices used in the unit, then look up their datasheets online – the datasheet will indicate where (and what in the case of multi–supply devices like op–amps or Analogue–Digital converters) the power supplies are for each device are.
In a F.G., there's usually +5 for digital control and display functions, and ±12 or ±15V for analogue processing and waveform output.
Have a look round inside and let us know what you find!
Chris Williams