Not the left, not the right, but the middle symbol, it looks like a crooked "T" going thru a non-polar capacitor

 

Both Capacitors are adjustable.
It looks like they are simply different "styles" of adjustable-Capacitors.
I would guess that this partial Schematic is very old.
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The symbol on the right is that of a variable capacitor, one that the user would turn to select a specific frequency.
The symbol in the middle is that of a trimming capacitor, one that would be adjusted with a screwdriver in order to calibrate the tuning dial. Usually, the trimming capacitor would have a value that is a fraction of the main variable capacitor, maybe 5% to 10%.

 

 

Some older air-gap tuning capacitors had trimmers on the side. Notice the two screw holes on the solder tabs. Tightening the screws would increase the value of the trim capacitor.




Newer capacitors with plastic dielectrics had trimmers built-in.

 

Thank you!

 

Not the left, not the right, but the middle symbol, it looks like a crooked "T" going thru a non-polar capacitorView attachment 341686

That "T" shape in the middle might indicate a temperature compensating capacitor, or a 'fixed" adjustable capacitor.

 

That "T" shape in the middle might indicate a temperature compensating capacitor, or a 'fixed" adjustable capacitor.

No, it does not indicate a temperature compensating capacitor.

 

Here is a ganged tuning capacitor with trimmers on the side.



Why two or three sections of capacitors that move together?

In a classic superheterodyne radio receiver, there would be two tunable sections, the RF amplifier and the local oscillator. Both sections need to track equally at different frequencies in order to select the one radio station. The trimmers on the side are adjusted to ensure that the sections are tracking well across the entire frequency range.

With a triple section variable capacitor, you can have two RF stages. One could tune the antenna stage and the second would be for tuning the RF amplifier.

 

That "T" shape in the middle might indicate a temperature compensating capacitor, or a 'fixed" adjustable capacitor.

OK, and that was a guess about temperature compensation, and it was a bit off, of course.

But there are both trimmer caps and padder caps, as I now recall. One has the larger effect at the minimum capacitance end, while the other effects the maximum capacitance end of the range more. Some of the mil surplus have both of them and now I am trying to recall which was which. So trimmer or padder depends on the application.