If you buy an old amp, deck, ... or just take something out of your attic, there's dust inside the machine (that you should clean of course), and the rear panel connectors are dirty.
RCA connectors are exposed, so they tend to accumulate dust and tend to become less conductive.
Here's what you need to correct this ...
First, you must remove the surface dust with a cloth.
Then, spray some DeoxIT D5 on a cotton swab, use it to clean the outter part of the RCA. If it's really dirty, you'll have to do this a couple of time, until you don't see any residue left.
The surface should be shiny, not perfect, but shiny.
After that, wrap a non-fluffy cloth on the cotton swab (you can remove the cotton partially to accomodate the diameter of the RCA jack), and spray DeoxIT D5 on it.
Use this to clean the inner part of the jack.
Repeat the process until the cloth comes out the jack as white as when it wents in.
Allow some minutes to let everything dry.
To have a perfect result, and really shiny connectors, you can use some 0000 steel wool to polish the outter part.
After this step, you have to spray on more time DeoxIT D5 on you cloth and wipe off the connectors.
Now your RCAs are clean, shiny, like new and protected.