Hi All, KPA500,S/no:1778 was being operated on RTTY at approx.320W. Antenna is a steppir beam. Heard a loud DHOOP sound and the amp.went off. Fuses intact.Opened the top and left side panel,nothing seen as far as I can make out. What should I be looking for? 73 and HNY. Nandu VU2NKS ex-vu7ag,vu4kv