the contact on the bulb protrudes more on some bulbs than others. sometimes they go cheap with the solder. the other thing that commonly happens is that little bent spring part at the bottom of the socket fails to make good contact. then you get arcing between the contact and the bulb. then the springy metal bit looses it's "springiness" the heat takes the temper out . this is called annealing. the result is that the springy bit is less springy and it gets flattened out. - see the vicious circle here? , now it makes bad contact because it is less springy.. and maybe more arcing.
the heat of the arc removes the temper. If it is blackened you know why it's blackened - because of arcing. You can replace the socket but you might like the old one in an antique. usually this springy bit is riveted on so it isn't easy to remove or replace you have to change the socket. so clean it up if you look at the bottom of the socket and see it's blackened.
If you bend it make sure power is off- of course! another solution - if the bulb maker went cheap on the solder , just use a soldering iron to re-melt it and add a bit more solder so it sticks out more. some bulbs dont protrude as much and this may also fix it.
in modern manufacturing they are leading away from lead , so you see more things that are riveted and crimped instead. sometimes they use stainless steel with tiny spot welds instead of soldering parts like they used to.. this creates an issue with repairs because you can't really solder to stainless steel very easily whereas traditionally they used more brass and copper.
I see this more and more on machinery and automotive applications , they are steering towards lead free products, even circuit boards.
to remove the blackened coating youcan do it by scraping, like with a pointy little screwdriver. or use fine sandpaper, or do it chemically. this is a good contact cleaner and it has chemicals in it that will clean the black stuff off chemically.
https://www.rpelectronics.com/dn5s-6n-d ... -caig.htmlthe same thing happens with old electrical outlets. they have springy parts in there to contact the prongs on a plug. when they get old and worn they don't contact as well. You can feel when you push the plug into a new outlet it grips the prongs, but when it gets old and used a lot then you get the exact same issue. the springy tabs get old , blackened , oxidised and this causes arcing and resistence which produces heat which removes the springiness..
I'm sure you have seen old outlets where you have to wiggle the cord. New outlets are cheap and I'd prefer to have good ones that work right and don't arc to vintage outlets..
If your house is a museum then maybe you want repros, or you can try to use ones that aren't worn and you can clean them with the contact cleaner.
those contact cleaners are also going towards "safer chemicals" but the old chemicals worked better. also they often have lubricants and for this purpose you don't really want the lubricants so I'd wash with alcohol after to remove any oily residue. If you use the cleaners on rotary switches and volume controls then you do want the lubricant so the switch doesn't wear out mechanically from lack of lubrication.
I see a lot of new made in china extension cords where they fail prematurely I've seen them go up in smoke because of this, its dangerous especially if you are also working with flammables. I found one recently in a wood shop where it was smoking and buried in an inch of sawdust on the floor. - fire hazard. I often cut the end of the cord off and put a new one on or just chuck the cord if it's all hardened up and old.
also you see the same circle blackened cord ends,, make bad contact with outlet springy prongs, prongs get hot ,, loose their temper or springiness, and that causes even worse contact even with a new cord now even your new cords get blackened. the problem transfers itself around. this arcing often isn't enough to actually blow the circuit breaker.
new wiring now requires a different kind of breakers for bedroom outlets "spark fault breakers" dont; confuse with "arc fault breakers" the reason for the code change.. people causing fires.. these new breakers are supposedly more sensitive to arcing I installed some and I haven't seen them tripping off even though I do have some old and temperamental lamps and things.. but the code change is for this reason.
all these parts are cheap(except those breakers are 100 apiece) when new so in my opinion it isn't worth it to keep old stuff in use. a typical outlet is only a couple bucks. - good insurance.
if you happen to have a old lamp, some can be valuable , like a tiffany so I think there is a point where you'd say No way am I putting a new socket in this old lamp ,, if it's worth a couple thousand then you want it safe but also originality is important.. so it's a trade off.
because I fix old radios lot I see a lot of bad cords. sometimes I put new on , sometimes I try to clean them and save the authenticity.. one source I have found is clothing irons in thrift stores, Often I can find old travel irons that have been put away and they have perfect soft supple cords because they were stowed away in bedroom dressers.. also things like hair dryers, toasters, fry pans, they have heavy duty rubber cords because they draw a lot of power.. sometimes I find these for a couple bucks in thrift stores.