[sdiy] variable capacitor projects?

[Colin Hinz]

On Fri, 30 Apr 2004, James Patchell wrote:

> 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)...

Ages ago, I made a few Hartley oscillators using some of these big old
variable capacitors, some hand-wound coils (wire-wrap wire, nothing
fancy like proper electromagnet wire), and some germanium transistors
which were salvaged from some ancient IBM boards (which I used because
(a) I had them, and (b) they had reasonably decent medium-power
ratings). The coils had steel cores which were moved to and fro through
the windings by mechanical means.

They worked nicely as punchy LFOs. I ended up calling them my
"Hartley-Davidsons" on account of the sound quality. I partly
dismantled them (they were built up on salvaged tag-posts from
1960s military gear) in order to re-case them but I haven't managed
to do that quite yet. Soon, real soon.

> 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.

Inevitable but nonetheless kinda painful to think about. When I was
in my early teens I dismanted a *huge* quantity of discarded old
TVs, to the point where I was throwing unwanted parts at concrete
walls just to watch them disintegrate on impact. When you've
already got a shoebox of old coils and transformers, who needs any
more of them?

When I moved east I left a huge pile of electronic detritus in
my mother's garage. Luckily, she discovered a geeky schoolteacher
who hauled the stuff away in something like 6 carloads. She quite
sensibly realised that while I thought this stuff was too cool to
haul off to the dump, I wasn't about to move all this tonnage an
approximate 2000 miles. In this case, I think everyone won.
There was a *small* amount of stuff in that tonnage which I
occasionally wish I hung on to, but given that (a) I'd poked
my way through that pile at least twice and (b) I've now got
another pile of stuff I haven't gotten around to doing anything with,
I'm not exactly losing any sleep over a possible tragic loss.

- Colin Hinz
  Toronto, Canada