[ARC5] VFO problem solved

[Ian Wilson]

I acquired a ham-modified ARC-5 TX a while back. Although I have several
TXs, I don't have any of the 7-9.1Mc variants. This particular unit is the
only one that will tune up to 7086kc (my weekly CW net).

A continuing problem has been the VFO. It had a tendency to 'warble' around
in frequency, and occasionally would jump a kc or two. I tried lots of
things including recapping, changing the dogbone ceramic capacitor in the
oscillator enclosure, etc. Cleaned the variable capacitors as much as
possible without stripping down the unit. Occasionally the VFO would settle
down but the problem always reappeared eventually.

I was suspicious that there was some partial insulator bridging the
capacitor plates, or making a track from the stator plates to ground. This
unit also showed some signs of having perhaps been exposed to muddy water
some time during its life (there was a light sandy deposit, and some
'tidemarks').

Eventually I stripped things down enough to remove the padder variable
capacitor, and to allow the oscillator tuning capacitor to be swung away
from the chassis on its flex drive connection. Cleaned everything with
isopropanol, toothbrush, 'flossed' between the capacitor blades, etc. I also
thoroughly cleaned the mating surfaces for the variable capacitors (since
these form the ground returns for the resonant circuit); also the oscillator
cover.

Glad to say that this cleared up the problem, finally. I am pretty sure that
the real problem was caused by poor ground contact between one of the
capacitors and chassis, will never be 100% sure of course.

Hope this helps if anyone else is unfortunate enough to run into this.

73, ian K3IMW