I  have an old Princeton 65 SS amp. It is so noisy it's unusable. I've  resoldered and then replaced the power caps and no change. I've  resoldered all of the connections on the PCB and it made no difference  ('refreshed' the solder). I've tried several guitars, cables, etc, and  can not change the noise that way. It's 60-cycle hum. I have conditioned  power to plug it into, a shielded studio (HAM radio operators made that  a necessity. I basically have a Farraday cage in my basement  HAHAHAHAHA!), have tried it in several circuits, in several locations  (inside and outside of my house), and no change.

   I've also  swapped out the input jacks and have not had any luck. Fender SS amps  are 'noisy' out of the box but this is insane. ANY help would be  appreciated. I was an electronics tech in the Army and work in the  industry now, so I'm not a novice, but I simply cannot figure this thing  out! I love the amp (when it works) but am about ready to set it on  fire! Thanks in advance!  
'73 Strat
'87 Laney AOR Pro Tube 100
Digitech RP-1
Digitech RP-10
Home Made 4 X 12 Cabs
22 Home Made Electric Guitars
Life doesn't get any better!
If you get rowdy 60hz hum even with a humbucker guitar or with a Strat in the out-of-phase position, your transformer is likely fecked.  Not worth repairing IMO as there are a lot better amps around for $200 used.
"Your sound is in your hands as much as anything. It's the way you pick, and the way you hold the guitar, more than it is the amp or the guitar you use." -- Stevie Ray Vaughan

"Anybody can play. The note is only 20 percent. The attitude of the motherfucker who plays it is 80 percent." -- Miles Davis

Guthrie on tone: https://www.youtube.com/watch?v=zmohdG9lLqY