Recently my vintage stereo has developed a new problem. After it has had time to warm up, about 15 to 20 minutes. Static starts coming through the speakers. I'm at a loss as to what it could be. Once it starts the static starts to slowly get louder. It doesn't matter if I change inputs or volume (even mute), move the speakers from output 1 to 2, the static volume will not vary at all. It will just continue to slowly get louder. Even if I turn the speakers off I'll get the same static through headphones. Occasionally it will also make a popping sound. If I smack it the static will go away for a few seconds to minutes. Here's what I have done. - I've cleaned the pots with Deoxit - I visually inspected the capacitors and found no sign of leaking or swelling. - Checked and tightened all connections. This is a Harmon Kardon HK3500.