Fender amp hums when i turn it on and pops when I turn it off. I can still get signal through it.?

Question

What causes this problem and how do I fix it?

Answer 1

The hum is probably coming from one of two things - either there is a bad ground somewhere in the amp or, if it is an older amp, it is probably a degraded electrolytic filter capacitor.

These filter capacitors are prone to "drying out" and losing some of their ability to filter out the 60 Hz line frequency.  

Check grounds first, capacitors second.