Old electric doesn't stay in tune, what to do?

You probably just need a new set of strings and a decent setup. I'd guess the bridge is probably floating (that or the neck is hanging on by one screw lol.) The nut probably needs proper filing, and action and intonation need to be set. Find any decent tech in your area, and they should be able to make it better than playable for around $50.

 

Most tuning problems are the strings binding on the nut, somtimes on the bridge too. Clean all these slots with fine sandpaper and maybe lube with pencil dust for good luck. If the strings are old loosen them a bit and stretch em and tune em and stretch em and tune em, see if they behave, remove corrosion with green scrub pad. {or just change em}

Only strat I ever saw that wouldn't tune had Fender's version of a FloydRose trem on it, made in Japan in the 80's. Also possible the tuners are beat but I doubt it.

Fixing cheap guitars is a funny subject these days. We've seen some really cheap Fender style guitars sell for like 200 whacks or less and can't find anything wrong with them

 

If it is an old Fender, the reason I am not a big Fender fan is because every one I ever had the neck would change a lot. I would get it intonated and well set up, and the neck would change and I would have to do it all over again.
And the change would always happen while playing, knocking it out of tune and out of intonation.
I have VERY warm hands and some guitars the necks would change as my hands warm the neck up. Fenders seem to do that a lot.
Interestingly, I have never had it happen with Squier strats.

 

If the bridge floats...that's normal and inherent in the design, keep tuning.

 

Take it to your local guitar shop for a "set up." They'll put fresh strings on it and they'll adjust the truss rod, action and intonation. They'll likely tighten the tuning keys and float the bridge correctly. May lube up some pivot points. Regardless of what they do, they'll make your guitar playable and better functioning. They'll tell you if something is broken.
Good Luck,
Barnzy

 

If the bridge is actually floating, you need to adjust the springs correctly. The spring(s) on the bass end need to be a bit tighter than the treble springs and it's a bit of trial and error to find the best setting. You can set it to bend up in specific intervals on the E, G and low E strings if you play with it enough. Once you have the springs set correctly, you'll want to tune the lower strings first and work you way to the top ones. I have a Strat Plus and only use two springs, one tighter than the other- I can go nuts with the whammy and it's always in tune.

Once the springs are set correctly, you should be able to use the whammy bar to your heart's content and it should stay in tune, as long as the strings have been stretched when installed and aren't left on for too long. Old strings that have been stretched to their limit don't stretch further, they just snap because they have reached the limit of their elasticity. You can test this with a rubber band- it will stretch well until you stretch it really far, and then it doesn't stretch much, at all. The next thing that happens if you exceed this point, is breaking.