Could a rectifier diode have gone open (as in it's now a half-wave circuit)? If so the filter caps are at 60 Hz. Just a thought... Roy K6XK I have a KPA500 with a low HV reading. It is on the center tap and AC voltage is 115v No load and 110v full load. The HV goes from 77v NL to 56v FL (500W output) Is that something I should look into? Doug K6JEY