Unplug your Machines when not in use

If your machine has a bakelite foot controller you should never leave it plugged in when not in use. 
Singer tells you this in (some of) their manuals.

From Singer 201K manual

How they Work

These controllers use a stack of thin carbon discs which are compressed by the button you step on to allow current to flow. A friend of mine in Canada has a blog which explains this in detail: http://www.archaicarcane.com/singerattheraces/

Why Unplug?

There are three good reasons I can think of to unplug your machines:
1. Over time current starts to leak and this can make them hot. This heat can build up enough to start a fire, which can happen when you aren't home or when you're asleep in bed.
2. The higher voltage ones (220V/240V) were fitted with a capacitor to prevent AM radio interference. When this breaks, it short circuits and the machine suddenly goes at full speed. Yes it can happen when you're not at home! 
3. Your machines are fascinating to kids, who love to play with them. If the machine is plugged in, kids are more likely to be injured. When my machine isn't in use I unthread it, place a piece of fabric under the foot, foot down and needle down.

You can certainly tune bakelite controllers so there's less danger, but the best thing you can do is to get into the habit of unplugging them when you're finished sewing. Don't be frightened of these. They're original to the machines and perform very well, so there's no reason to get rid of them. Knowing how they work is valuable.
A few years back I did an entry on fixing them up. Not as comprehensive as archaicarchane's, just removal of the capacitor.

What do they look like again?

Not all carbon disc controllers look like this:

Singer Bakelite Foot Controller

Some of them look like this:

Husqvarna controller from early 1970s

Even clam shell controllers use carbon discs: My daughter's 1970 Singer has them, and a generic Taiwanese replacement I opened up also had them.
I was surprised they all didn't use coiled (resistance) wire, but they don't.
That's all. You've been advised to do it and told why, the rest is up to you. Have fun!

fixing a bakelite foot or knee controller

I bought another 201k on the weekend. It's aluminium and a knee controlled beast. Just couldn't resist it for $29. Pretty sure the guy who sold it to me was lying when he said he didn't know how to use it, but the motor ran at full speed as soon as the switch was flicked. This is caused by the capacitors shorting out. The capacitors were used to prevent any interference from the motor affecting things like TV sets and radios. These days they're not really necessary since most devices have supression built in. Both foot and knee  controllers have the same bits inside so instructions are the same.
So, just remove the four screws holding the base on, take the capacitors out like I did and your machine should be back to working. If yours is a knee controller, remove it from the case first. It's obvious how to do this.