Practice amp hums even through headphones with no input

[jafo]

Not the ideal place to ask this, but definitely the least bad place; this forum has never steered me wrong.

I have a lousy Fender Frontman 241 practice amp, the one with transistors and op amps. Not a good amp, but it's what I have. Anyway, one day there was a popping sound, followed by no sound other than mild cursing. Seems to have been the fuse. Replaced it with one of the same rating, and it works, except that there is a fairly loud humming sound coming from it whether or not there is input. The hum appears on the speaker, or headphones (in which case there is no sound in the speaker), so I assume it's something in an output stage. Nothing looks funny in there, and there is no ozone or burnt smell. I'm terrified to touch anything with mains power, and as I've amply demonstrated here, my understanding of electronics is hopelessly inadequate.

The amp is basically a toy, so I should throw it out -- but it's still useful (and my only amp) and I'm on a very constrained income. And at the very least, it sounds pretty good on bass with the cab opened.

So, what should I look for? What can cause this problem?

Any help is appreciated, as always. TIA

[MikeA]

Given the clues that the fuse blew, but held when you replaced it with the same value, and now the amp works, but there's a hum at all times: I'm betting a power filter cap has failed, shorted (which blew the fuse) and is now open (which allows power-up and operation), and you're not filtering the power.  If so, look for one or more large-ish EL caps near the power input, might or might not be swollen or burst.  Observe all power precautions, discharge the caps, etc.