A 600v cap in an old Teaberry boat anchor. Was working fine I was listening to some locals on 11 while wiring up a mic for it to test TX and all the sudden BOOM! What could cause this, it had only been 8 years since the radio had last been regularly used? Fortunately I grabbed the plug within seconds after and it kept receiving the entire time. The foil that shot out across the board was about a foot long.