[AMPS] ARCING MORE

[Rich Measures]

>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   


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