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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