At 01:40 PM 2001-04-12, you wrote:
> > You can get a good idea of the vacuum by just grabbing
> > the drive mechanism and seeing you can pull it straight
> > out. The same as reducing the capacitance but without
> > turning the screw. A bad vacuum will be easy to pull--
> > only a couple of pounds of force max. A good cap will
> > require 30 or 40 pounds of force depending on the surface
> > area of the bellows.
>
>It will depend on the cross-section area of the shaft. (Think about it).
>Anyway, 95 percent of atmospheric pressure is hardly a vacuum at all!
It is not the shaft area but the area of the bellows that is important.
Most caps have a larger insert that is drilled and tapped for a
1/4 inch shaft. The air leaks around the shaft and around
the insert and fills the air-side of the bellows. It is this pressure
on the area of the bellows that the tuning mechanism works against.
Any vacuum cap that I have ever seen that has a leak comes
to equilibrium with the atmosphere at some point. Old
caps are either very easy to pull open (when the cap
is full of air you are not working against the vacuum/
air-pressure differential but just the natural "springiness"
of the bellows) or are quite tough to pull apart when you
are fighting that air-vacuum differential. I have never seen
anything in between--if it leaks even a little bit it soon
fills with air.
Nothing beats a high-pot test, but this quick test eliminates
the real dogs quickly if there is only a very small return force
when trying to pull open the vacuum capacitor in question,
and you can move on to other units without wasting a lot of time.
John W0UN
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|