FN3 is simply a shortend way of saying SX-FN3. They are one and the same.
All Technics instruments are designated SX-something, so when referring to the model number many folks just drop the SX part. Even the factory designation, which appears on the control panel of your organ, shoud be simply FN3. However, the sticker on the rear should state the full model number as SX-FN3, SX-EX30L, SX-G100C, or whatever. The serial number of the unit is usually found on the underside of the keyboard.
The most common cause of low volume is a broken or disconnected cable that connects the lower part of the organ to the upper part, inasmuch as the wires from the expression pedal are contained in the common cable. Actually, organs equipped with such a cable cannot be played at all unless it is firmly connected.
Other posters are absolutely correct; used organs in today's market are practically worthless. Here in Southern California I would be expect an FN3 to sell for $500 or less. Sad, but true.
Dick Barker