To answer a couple of questions about the posting below:
Yes, the step start does engage the HV circuit ... the HV transformer is
current limited for a 3 seconds by a resistor in the primary. The failure
occurred AFTER the resistor in the primary was bypassed by a time delay
relay. Everything seemed normal up to that point.
RF drive was not applied. Plus, the amp has never shown an inclination to
oscillate.
Upon closer inspection, R3 (looks like B- to ground) is burned open.
Obviously too much current going somewhere. The capacitor bank appears
okay visually, and with an ohmmeter, but this says nothing about what
happens with working voltage applied.
Ideas? Other things to try before taking a wild guess and assuming it's
the capacitor bank?
Dennis, K7FL
==== begin original message ====
Hi Folks:
I finally got around to wiring the shack in our new home for 220VAC, and
plugged in my vintage SB220 for 48 hours of CW contesting. After the
step-start circuit disengaged, there was a load pop and flash from the
high voltage section behind the meters. The amp had been off for six
months. I checked the
AC mains, and found 245V! The old QTH had 220-225V ... and the amp had
run flawlessly at this voltage for 15 years. My theory (and I am asking
your opinion) is that the old filter caps probably decided the 10%+ extra
voltage was too much! BTW, the diode board was replaced last year with
one of Harbach's.
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