[HBR] Capacitor question

[Walt Hutchens]

Chris:

> I was wondering how hard it is to take plates out.  The capacitors
> I see appear to have slots in the axle into which the plates fit
> and have a small bakelite piece which drives all of the rotors at the
> same time.
> 
> How hard is it to remove plates?

It's usually easy to remove rotor plates to make reasonable reductions in
maximum capacity and a somewhat lesser reduction in the minimum.   (Less
reduction of minimum because of stray capacitance between stator and frame,
shaft, etc.)

You just grab each rotor plate firmly with your needle nose pliers, rotate
it away from the rest, then back most of the way, away ... a few times and
it'll usually pull right out.

There's a limit to how far this process can go:  While the math would say
you can get down to the range for an HBR (say a delta C of 15 mmf) it
wouldn't be a good performer because the plate spacing is too small for good
stability: A small warp anywhere as the unit changes temperature is a
relatively large change of capacitance.   With some units you can also
remove stator plates and if you take out suitable combinations of plates
from both rotor and stator you can increase the spacing too.   That'll work
but getting stator plates out without damage to the rest is much harder and
may not be possible with these staked together units.

A unit like this with a built in gear drive is good as a loading cap for a
transmitter or moderate power amplifier.   It would work to tune a general
coverage receiver, although if that minimum capacitance is right, it
wouldn't give you a very wide tuning range.  With the same reservation, a
two gang version would work well for the antenna and mixer tuned circuits on
a 'band imaging' receiver.  Actually, I have a very similar two gang unit
from which I did pull some plates to get a more appropriate range in a
receiver of that type.

(I'd expect the minimum for a cap of this sort to be under 50 mmf, maybe 35
or so.)

On the topic of the HR-10 to HBR project, I have added captions to the
photos at:

https://picasaweb.google.com/105255380483660395824/Walt2011Project

A successful project that needs just a couple of small tweaks and some more
coils.

Walt
KJ4KV