I picked up some super jobs at Dayton the other year for $1 each. 12volt, about
25 rpm, lots of torque. There's a lot of these motors around, especially if you
don't want a production run's worth. I got some lovely miniature 12 volt motors
in surplus over here for about $12.
My remote tuner has a vacuum variable for the vertical and a 'toast rack' for
the '600 ohm' feeder. I've found it worthwhile fitting a position indicating pot
to each cap; the vac variable has a lead screw (1/4-20) and microswitches as
limit switches. One disadvantage of the DC motor is that you really need to
short circuit it (or put a 5 ohm resistor across it) when you switch off. That
way, it gets rheostatic braking and stops quickly. My first motor controlled vac
variable was a bit of a kluge, as I hadn't really got the facilities for metal
working then, but this time, it was easy. I did start cutting the gears for the
gearbox, but then I found some, which is much easier. I took Rich's tip and
mounted the vac. variable vertically - whether it makes any difference, I don't
know, but it was easy to do.
The barbeque spit motors I've seen are DC permanent magnet jobs, and so are
reversible. The plastic gears can't stand much torque, though.
A stepper motor driver has attrractions, especially for a multi turn item like a
roller coaster or a vac variable. However, you do have to bear in mind that the
control circuits can be in a pretty big RF field, and that can give problems.
(Funnily enough, that's why I haven't commented before - I was in Canada last
week trying to sort out a customer's intense RF field problem!) Trying to slow a
DC motor in an antenna tuner with a pulse width modulator has given me a similar
problem. One useful trick is a series bulb - a 12v 0.3 amp bulb in series with a
12 volt 1/4 amp motor works fine. You get a big initial pulse of current to
overcome stiction, and then the bulb heats up and regulates the current. Needs
some playing with to get right, of course.
If you want an autotune system, then some thought is needed. A DC motor might be
easier if it's an analogue servo system; if it's digital, then a stepper motor
is probably the best approach. For example, if you want to tune a PA for a dip
in plate current, then a simple algorithm in a PIC will look for the dip, and a
stepper motor drive is a logical approach. If you use a phase detector, then the
analogue approach may well be easier.
Building the gear box can be great fun too - a drill press is advisable as the
minimum machinery though. A decent workshop makes the whole job easy, especially
a milling machine - although a lot of milling it can be very time consuming, as
G3SEK will testify.
73
Peter G3RZP
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