[Amps] issue with AL-1200 output -UPDATE NUMBER 2-

[Merv Schweigert]

That small piece formed a arc gap,  it prevents the tuning capacitor 
from arcing
under some fault condition of mistuning etc.
It is suppose to arc there instead of the tuning cap arcing and 
destroying the plates
its usually set just a little smaller gap distance than the gap between 
the cap plates.

You take your chance,  either set it to the correct gap and have a 
saftey measure,
or take your chance at arcing the capacitor some time when the amp is 
mistuned.

If yours arced all the time at 400 watts,  it would appear the spacing 
was set to small.

> Hello all,
>
> Good news, I guess.
>
> That mystery board is a sophisticated T/R board assembly that was 
> introduced many, many years ago.  It still is not documented in the 
> schematic on the Ameritron page.
>
> That aside, I found the problem.  I will try to describe as best I 
> can, but if anyone wants a pic, let me know.
>
> Anyway, on the PI/PI-L input cap, C1, there was an issue.  On the top 
> of these air variable caps the aluminum blades are fixed (stator.)  
> The main aluminum frame mounts to the chassis and the movable blades 
> of the capacitor (rotor) are mounted on a shaft, which, in turn is 
> mounted to the main capacitor frame.  The fixed blades are mounted on 
> a two threaded brass rods and separated by aluminum spacers.  These 
> brass rods, with fixed blades attached are mounted to ceramic 
> insulators, and these insulators are then attached to the main frame.  
> This makes a solid assembly that will allow the rotor to accurately 
> variably mesh with the stator blades.
>
> Now for the issue.  The ends of the threaded brass rods are used to 
> make mechanical connections to the rest of the plate tank circuit.  
> The nut is removed and the copper strap, wire, etc is placed over the 
> end of the brass rod and the nut replaced.  90% of us have probably 
> seen this.    The front end of one of the rods, had a piece of silver 
> plated copper strap about 5/16 inch wide x 3/4 inch long, secured on 
> the shaft.  It went no where.  It was positioned coming off the brass 
> rod in such a manner that it spanned across the ceramic insulator and 
> was parallel to the fixed alum end of the capacitor.   The way it was, 
> there was only about .050 of an inch between the silvered copper strap 
> and the cap body, which is at ground potential.   The RF voltage in 
> that cap at anything over about 400 watts out was high enough to arc 
> and keep arcing as long as power was applied.   The copper strap was 
> significantly melted and cap frame had burn marks all over it.   I 
> have no idea why that one small piece of copper was there in the first 
> place.  I could not find any purpose for it.
>
> I took it off, fired the amp up and in small increments brought it up 
> to almost full output, about 1400 watts.
>
> Down side is the tube seems 'well worn'.  Maybe the arcing didn't 
> help.   100 watts in, 700 ma plate and 250 ma grid into a dummy load = 
> 1400 watts out on 15 meters.
>
> Thanks for all the suggestions on getting to the bottom of this.
>
> 73 de Steve, NR4M
>
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