>Hi again Jon...
>
>I agree with Carl 100% on this one. Based on experience I'd guess that
>there is virtually zero chance that in your case a defective parasitic
>suppressor either caused, or resulted from, the cap arc.
>
In my opinion, a vhf suppressor resistor (Rs) can not be damaged by a
Tune Capacitor arc. The things that can damage Rs are 10m RTTY
broadcasting, and vhf energy. . My guess as to what may have happened:
An unusual transient occured in the anode-resonant circuit as the result
of the wrong antenna being used . If the amplifier is not
unconditionally-stable, an intermittent oscillation might have taken
place at the anode-resonance frequency (c.110Mhz). To see if this
happened, measure the resistance of R1 and R2.
>As others have already suggested, the tune or load cap is usually the
>"weakest point" when rf voltage is excessive.
agreed. Better that aluminum plates pit rather than band switch contacts
evaporate.
>Maybe that's because cap
>plates often aren't perfectly aligned and spacing varies; a spec of dust
>between plates effectively narrows the gap and triggers the arc; often
>plates have sharp rather than rounded edges, and on higher freq bands
>(i.e., 21-28 MHz) where the plates many times are nearly "full-out" those
>edges "look at" each other, creating excessively high voltage gradients
>which increase the tendency to arc; corona is a function of frequency, so
>ionization of the air leading to arcing is more likely at the higher f's.
>Just some thoughts...
and if one smooths the corners and perfectly aligns the Tune C plates,
the next transient that comes along may instead arc and vapourize some of
the open contacts on the bandswitch.
>my guess is that all these factors contribute to rf
>arcs at one time or another.
>
>73, Dick W0ID
cheers
Rich...
R. L. Measures, 805-386-3734, AG6K
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|