first of all you have to know where to start.
best method to track down the problem is to find the source
there are 3 possibilties:
1. bad preamp (built into the piano section with volume, EQ and tremolo knobs)
2. bad cable
3. bad power amp (built into the loudspeaker cabinet)
Bad power amp:If you own a later build cabinet (e.g. FR7710) you are pretty lucky.:
Use the external/internal connection possibilities of the main amp modul.
Connect another source (e.g. I-POD, CD player, turn the volume down :x ) to the power amp input jacks
or use the headphone jack to connect a headphone.
If all speakers/channels are working. Fine. it is not the power amp !Otherwise ... you know where to zoom
If you own an older cabinet with a panel without external jacks you could do this to check if it is a main amp or pre amp related problem:
Warning:
Be aware that there is the danger of an electrical shock when working inside the cabinet. Even if you have unplugged the cabinet (which you should do at first) some electrical parts (capacitors most likely) may have some power loaded which might cause harm to you. So dont touch any metal contacts on the printed circuit boards.1. unplug the suitcase piano (light socket)
2. open the cabinet (loudspeaker box)
3: there are 2 RCA cables (cinch connectors) going from the connection panel to the 2 power amps. switch them.
4. bring the cabinet back in playing condition and plug it in.
5. play it.
If the same channel/loudspeakers are bad (hum, dead, etc.) it is a bad main amp. If the other channel is missing, you should check cable and preampbad cable:change it or check if all 4-5 pins on both connectors read 0 ohm or have continuity (you will need an ohmmeter/ continuity tester).measure the same pins on both jacks, meaning one tip of your measuring device on pin #1 of the preamp jack the other on pin #1of the cabinet jack.
bad preamp: if its not the cable or the power amp it is the preamp.
Otherwise you can use the preamp output/external amplifiers jacks of the later cabinets and connect the preamp to another amp (mixer or guitar amp or even a recording device) to be sure.
In many cases following this "tracking down method" you will have to measure (voltmeter, oszilloscope), swing a soldering iron and read schematics. If you dont know how to do this ask a tech or someone else skilled to help you.
@James: how about a sticky behaviour for this post ?