F-2B hum, channel B

[crysdal]

Sorry for the slightly OT question. I have a F-2B  
that I've been using for my Chapman Stick. I love the sound, but recently have aquired a HUM in channel B and mono output. No unusal noise from channel A. I did the usual diagnostics (cables, rerouting, etc.). The tubes are secure in the mount and no loose wires or shorts seen.  Has anyone else had this problem?  
Are repairs needed or is there a fix I can do?  
 
Thank you.

[mica]

Try swapping the tubes and see if the sounds is persistant in channel B. Also, check that the nuts holding the input jacks are nice and tight.
 
If the F2B is older and made with the blue chassis and black front and back, make sure the left side (when looking at it mounted in a rack) front plate screws are very tight - the right side should just be snug.
 
Other than that, you can have a tech look at it locally (schmatic is pasted on the inside lid) or send it here for testing.