[sdiy] variable capacitor projects?

[James Patchell]

You could use it to make a sine wave oscillator (wien bridge 
oscillator).  They are a little tricky.  You need to stabilize the gain of 
the amplifier to exactly 3 to make it oscillate.  You can find circuits in 
National Semiconductor app notes.  Instead of using a fixed cap, sub the 
variable..(you need two sections that have the same capacitance)...

What would interest me out of something like that is the old IF 
transformers (for radio projects).  Finding old IF transformers is very 
difficult these days.


At 10:32 PM 4/30/2004 -0500, rkmoore at memphis.edu wrote:
>Hi y'all,
>
>I just got my hands on a very old (I'd guess about 1940's)Philco
>radio/amplifier.  I saw it on the curb for the trash collectors and
>couldn't resist the urge to tinker.  It is sufficiently damaged to make
>it not worth repairing, but there's a slew of interesting old parts.  I
>hope to take the power and output transformers and throw together some
>sort of a 10 or 15 watt guitar amplifier.
>
>Another part that intrigued me was the large variable capacitor that
>was used in the tuning circuit. I know that variable capacitors are not
>generally used in synth DIY applications because of cost, size, and
>availability, but I would like to use this for something.  Maybe some
>sort of interesting filtering module or instrument preamp could use
>such a capacitor.  Has anyone used one of these in a synth or preamp?
>Any ideas for application?
>
>Thanks,
>Richard

         -Jim
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