I have a fair number of medium-sized (i.e. big enough for a 100-watt
amp or antenna tuner) variable caps that have one problem:

The spacing of the rotor does not match the spacing of the stator. I
can line up the front end to be nicely spaced, but then the back end
is too close. Or I can do the back end, and the front end is too
close.

I can compromise by choosing a point halfway between the two
alignments, but:

Were these old capacitors (many of them from the 20's and 30's) really
so poorly constructed that the rotor spacing did not match the stator
spacing?

Or is there some long-term "swelling" of the metal parts that results
in this misalignment?

I could correct the slightly-off spacing by pulling out the spacers on
the rotor and grinding them down just a little bit each (literally,
just a thousandth would be too much), but I think there must be
something more fundamental that I'm missing :-).

Tim.

wrote in message
ps.com...
I have a fair number of medium-sized (i.e. big enough for a 100-watt
amp or antenna tuner) variable caps that have one problem:

Were these old capacitors (many of them from the 20's and 30's) really
so poorly constructed that the rotor spacing did not match the stator
spacing?

Or is there some long-term "swelling" of the metal parts that results
in this misalignment?

I could correct the slightly-off spacing by pulling out the spacers on
the rotor and grinding them down just a little bit each (literally,
just a thousandth would be too much), but I think there must be
something more fundamental that I'm missing :-).

Tim.


Pot metal. It suffers from grain boundary distortion caused
by moisture. The problem is very common in some tuning
caps used in TRF Majestic radios made in the early 1930s.

One problem you'll is that the capacitance will be much
higher than it should be if the plates aren't centered. The
capacitor change isn't linear--that means that equally
shifting the distance between a rotor plate and two
stators from center causes an increase in capacitance.
Intuitively, one would believe the changes would
cancel each other.

Pete k1zjh