In case anyone was interested, I finally fixed the NC-109 that I asked
some questions about here awhile back. The symptom was that it would
fade out at the bottom of each band.

After checking the stuff suggested in this group like the tuning cap
shorting I borrowed a scope from a friend at work and found that the
local oscillator output was dropping radically at lower frequencies,
to about 10% of what it was at the higher frequencies. Also, I found
that the resistor to the screen grids on the mixer tube was cooked,
50% over its rated resistance.

Anyway, the two problems were apparently conspiring to produce the
symptoms. Replaced the resistor and the 6C4 tube in the oscillator
and it is working nicely, except for a hum. I'm also going to replace
the filter caps. Probably going to end up sinking more into this
project than the price of a new radio, but what the heck.

A word of warning, the actual wiring departed from the schematic I
found on the net on several points. Mainly the mixer and IF circuits
were running at a higher B voltage (not the lower voltage off of the
gas regulator tube), with appropriate resistor changes to compensate.
Also the B power filter caps were larger values. I suspect that at
some point they started using a bigger filter cap, which reduced the
ripple on the higher B voltage enough that they could use it in the
mixer and IF. A guess anyway...

--arne