How a trimmer can be trimmed..

Sometimes one has made a tuning circuit and the frequency range is not exactly as one wished for.
Here are some tips on how to solve the matter. It is so simple that you could have thought of it yourself.

Basic knowledge: What is a capacitor?
Nothing more than a little box to temporarily store electrons in.

Next, we need to remember what exactly makes a capacitor a capacitor:
A capacitor is made of two conductors, separated by an isolator.
 The Capacitance is determined by:
 - Area of the plates;
 - Distance between the plates;
 - Thickness of the plates.

In case of a varcap, plates are positioned parallel, so together they provide more area, e.g. more electrons can be stored.

There are a few more properties (temperature, moisture, material, dielectric, a.s.o.), but these are not of interest for this article.

Cutting down the Cap    
In the situation one has a varcap of 150 pF and one needs a varcap of 100 pF, the solution is to remove some of the vanes. This is how:
 - Measure the capacitance of the varcap:
let us assume the meter indicates 148 pF.

 - Count the number of vane pairs:
We count 20.
This means 148 / 20 = 7,4 pF per vane pair.

We can also count the total of vanes: 40.
Which means that one vane equals 3,7 pF.

We want a varcap of 100 pF.
So 100 / 3,7 = 27,027 vanes have to remain on the varcap, 12,973 vanes have to be removed. This implies two things: an impair number of pairs will remain and we won't come exactly to the value we want.

Take off 11 vanes and take a measure. Than decide if you want to go on removing that extra vane. You can't put them back on!

Tip: Take a good look which vanes form a pair!

Increasing the Cap    
Let's imagine you have a 100 pF varcap but need a 150 pF varcap. Remedy is simple, again: Go to the Paraset Warehouse and buy a new one.

Trimming the Cap    
Bringing the value slightly down:
Increase the distance between the last two vanes. On tuning capacitors in commercial broadcast receivers one can observe that the last vane is cut in parts to facilitate easy trimming.

Bringing the value slightly up:
Add some material to the last vane to make more room for electrons: apply a few soldering drops. This won't work in case of aluminium vanes.

Looking at the photo's to the right, you can see two varcaps I deliberately ruined for this tutorial. The upper one has aluminium vanes, the one at the bottom has silver plated brass vanes.

Varcap Vane Value Calculator    
I made a little calculator in Excel. You can download it here, if you like.