John - a couple of points on the Centurion. The first has nothing to do
with your problem, but concerns the metering. As you may know, there's
a mini-pot in series with every meter function source voltage, and the
mini-pots tend to change value over time. Maybe due to vibration or
oxidation? At any rate, in the two Centurions I've worked on, all of
the meter readings were off and required re-calibration. Calibration of
the multi-meter can be difficult if you have one of the Centurions that
was shipped with a meter with an incorrect full-scale current rating.
The intermittent nature of your problem suggests a tube problem as
someone else mentioned. But, the Centurion is prone to failures in the
bias circuitry and any HV fault is likely to zap one or more components
in that circuit. Such faults include a tube fault and discharging the
HV via the cover interlock switch. You could tell if the bias circuit
is damaged by checking to see if there is any plate current with the amp
in Standby - the meter should show zero current. With the meter switch
in Operate, I see a plate current of approximately 125 ma. when
grounding the relay line.
73, Floyd - K8AC
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