I have a little Fender Frontman 15 that has a steady hum when turned on, & doesn't change with control settings. It works, but hums.?

If it hums with nothing plugged into the inputs, you have a bad filter capacitor or connection to that filter capacitor in the power supply. These filer out the 60Hz hum that is generated with the AC line is converted to DC. When that capacitor goes bad, that 60Hz hum rides on the DC power supply. Look for a large cylindrical component about 4" high x 1.5" in diameter (maybe bigger or smaller, may be a bank of more than one capacitor). The repair is to identify the faulty capacitor and replace it.

Most guitar amps had a hum switch. Does yours? Does it have a 3-conductor AC cord plug? Is it plugged into a 3-conductor receptacle?

Here's a schematic diagram source:

http://www.electronicstudio.net/au-schem-fender.ht...

Unfortunately my smartphone won't open any .pdf files, but maybe somebody else can.

This is a 15 watt, solid-state guitar amplifier. The power supply electrolytic capacitor is probably damaged from age and non-use.

You just have to locate it, identify its capacitance and voltage rating and replace it.

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60 cycle hum. You have a grounding problem between guitar and amp. Take it to an expert to fix. You may do more harm than good by trying it yourself.

It sounds like you're getting interference from somewhere. Have you tried replacing the cable? Also, if you have another electric guitar, try it on the same cable and amp just to make sure that the guitar itself is at fault. If the problem is the guitar itself, you may need new pickups.

If it hums with the instrument unplugged, it may just be dirty, causing bridging between components on the SS board. Clean the input, output and SS components with contact cleaner and a soft brush or cloth.