I recently bought a second-hand tuner, an Akai AT-550. The condition of the mains plug pins was the worst I have ever seen. Even though the connections pass relatively high voltages, they still need to be clean or sound quality will be affected. I am particularly fussy about the earth pin connection. Here is a picture of the tuner:
To clean the brass pins of corrosion and dirt, I use a fibreglass stick but sandpaper or even a knife can be used. If you have the confidence, it’s worth taking a look inside the plug to make sure all three connections are good and that the fuse connections are not corroded or lose. This shows the plug:
I cleaned all three pins, not just the earth and when finished, they were all shiny.
If anyone needs an explanation of how to connect a mains plug properly, please do ask me and I will upload a post showing how to do it – not everyone knows.