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'[PICLIST] varactors vs. air variable caps and LED'
2001\08\09@150006 by Douglas Butler

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About knobs for the air variables:
I have used 1/4" diameter spacers that have 2-56 threaded studs with
that type of capacitor.  Just screw the spacer into the cap with a
little locktite and use a regular 1/4" shaft knob.  Beware that excess
locktite may attack the plastic of the capacitor body.  Also I usually
use a panel bearing for the shaft to reduce stress on the capacitor
body.

Sherpa Doug

> {Original Message removed}

2001\08\09@152104 by Mike Kendall

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Doug,
   Thanks for the reply.  Where might I get the panel bearings at?? There
is a local family  owned hardware store (Lonestar Hardware, after all I'm in
TX) that I was already planning on bringing one down to to try just that.
Was figuring they'd be metric.  All Electronics has the male to female
circuit board standoffs for 100ea at 8 clams ($).  These units have what
looked like mica but  probably plastic like you said spacers between the
plates, I'll just use super glue from the dollar general store for the same
affect.  I've experienced  the affects of nail polish on RF tuned circuits
before when setting something after an adjustment. Missed out on a buy it
now on Ebay for 800 pieces of these caps  for $100 as I wasn't so sure if
this would all work out.
----- Original Message -----
From: "Douglas Butler" <spam_OUTdbutlerTakeThisOuTspamIMETRIX.COM>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Thursday, August 09, 2001 6:49 PM
Subject: Re: varactors vs. air variable caps and LED windows


> About knobs for the air variables:
> I have used 1/4" diameter spacers that have 2-56 threaded studs with
> that type of capacitor.  Just screw the spacer into the cap with a
> little locktite and use a regular 1/4" shaft knob.  Beware that excess
> locktite may attack the plastic of the capacitor body.  Also I usually
> use a panel bearing for the shaft to reduce stress on the capacitor
> body.
>
> Sherpa Doug
>
> > {Original Message removed}

'[EE]: varactors vs. air variable caps and LED win'
2001\08\09@152715 by Mike Kendall

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I origionally  accidently didn't  include the brackets around the "EE" so am
re-sending this message with the correct subject.

Have a project I've been working on for months  now.
1st question-  What is a good  source for the plastic used for LED windows
for the red 7 segment LED's?
2nd question-   Polling for input on Varactors vs air variable capacitors.
I recently purchased 16 RF tuner capacitors at $1.25ea from Electronix
Express only to find that there is no such thing as a mating knob for them
in the USA (they have two flat sides on the tuning shaft and a threaded hole
in the center of that.  Also, the shaft is extremely short).  I've
considered using a varactor tuned oscillator circuit with the 120pF variety.
Saw a web page that uses two varactors chained together with the tuning
voltage at the  tie point.  The circuit web page mentioned the low Q of
varactors and  I'm  concerned about this affecting my circuit negatively.
I'm aware of homebrew amateur 2meter transcievers using varactors and their
usage in radio receiver tuners.
Regards,
Mike

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2001\08\09@155348 by Douglas Butler

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To get a few panel bearings you can use the threaded collars from old
potentiometers.  I have a supply of them I have salvaged from old
military gear.  I would think any supplier of small fittings would have
them but the first few I looked at came up dry.

Sherpa Doug

> {Original Message removed}

2001\08\10@044528 by Alan B. Pearce

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>2nd question-   Polling for input on Varactors vs air variable capacitors.
>I recently purchased 16 RF tuner capacitors at $1.25ea from Electronix
>Express only to find that there is no such thing as a mating knob for them
>in the USA (they have two flat sides on the tuning shaft and a threaded
hole
>in the center of that.  Also, the shaft is extremely short).  I've
>considered using a varactor tuned oscillator circuit with the 120pF
variety.
>Saw a web page that uses two varactors chained together with the tuning
>voltage at the  tie point.  The circuit web page mentioned the low Q of
>varactors and  I'm  concerned about this affecting my circuit negatively.
>I'm aware of homebrew amateur 2meter transcievers using varactors and their
>usage in radio receiver tuners.

I think you should check out the Hyper-Abrupt junction Varactors that are
available. I think you will find that these give a higher Q than older
varactors. You do not state what frequency range you want, but if you are
talking in terms of 120pF then I suspect it will be HF rather than broadcast
or VHF. To get this you may need several diodes in parallel to get the
capacitance you need, but it is a while since I looked at varactor data
sheets so am not sure of the capacitance ranges available these days.
Certainly every TV and Video tuner has them in.

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2001\08\10@145546 by dbengtson

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On Thu, 9 Aug 2001 14:31:49 -0000, you wrote:

>I origionally  accidently didn't  include the brackets around the "EE" so am
>re-sending this message with the correct subject.
>
>Have a project I've been working on for months  now.
>1st question-  What is a good  source for the plastic used for LED windows
>for the red 7 segment LED's?
>2nd question-   Polling for input on Varactors vs air variable capacitors.
>I recently purchased 16 RF tuner capacitors at $1.25ea from Electronix
>Express only to find that there is no such thing as a mating knob for them

What are the specifics of the Circuit that you are tuning? It may be
limited by stuff other than the Varactors. Although you might need a
DAC to drive a Varactor, they might be a lot easier to deal with.
Dave

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