> > The doorknob capacitor in the Heath is within its current ratings,
> > assuming you know how to read manufacturers data. I suspect it might
> > be within the rating in the ETO also.
>
> Current rating has nothing to do with the ETO and Heath problems. They
> get too hot and change value. The power output on 160 meters changes
> after tune-up. The micas don't do this.
Hi Phil,
First, Ameritron does not use mica's in the tank of higher power
amplifiers. Never has, probably never will.
They switched to ceramic chip capacitors because the doorknobs they
had used for years suddenly failed to meet temperature vs capacitance
change ratings when HEC starting using tooling or materials purchased
from ITT-Jennings.
If the old ETO's used ITT-jennings parts, they likely do have drift
problems. The earliest i was involved in this was about 1980, and at
that time ITT had temperature-drift problems. HEC and others did NOT.
You are correct that it is NOT a current rating problem. Someone
either did not read or does not understand ratings correctly or
completely, and leaps to the conclusion that current rating is
exceeded.
The real problem is the temperature of the doorknob capacitor changes
(while still remaining well within the allowable temperature rise),
and the temperature coefficient is far greater than the factory
states.
The problem is solely in the 500pF (or higher value) loading padding
capacitors, because larger capacitance doorknobs are those with drift
greatly exceeding published specs. The 170pF padders are OK, because
the lower capacitance provides a more temperature stable component
and the parts are almost always within spec.
The result of this is not that the manufacturer picked the wrong
part, but rather that the component quality and performance changed
over time. While ITT-Jennings capacitors NEVER worked well, HEC
capacitors were fine until they purchased Jennings doorknob capacitor
line.
The ceramic chip capacitors are not perfect solutions. They give up
headroom in voltage for greatly improved stability. Many ceramic
chips are contaminated in the manufacturing process, and do not meet
rated voltage specs over time. The trade-off is extra cost and
higher initial failures for nearly perfect stability.
It would be very foolish to blindly change doorknobs to ceramic chips
**unless** the PA is actually having loading capacitor stability
problems.
I find it odd that the manufacturer continues to stamp parts N3300
when they are really much worse than that. Even so, it is bad advice
to recommend carte-blanche replacement of components when many behave
fine as is.73, Tom W8JI
W8JI@contesting.com
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