[AMPS] How to control a vacuum C ??

[Jim Bryant]

I own both of the MFJ transmitting loops, and their technique seems to work pretty good.

For fast tuning, they have a constant positive or negative 12VDC to the cap motor.

For fine tuning, they shoot short pulses of the voltage to the cap motor.

I'm assuming that the short pulses don't develop enough of a field to keep the motor running for long, but still enough to just bump 
it in the correct direction.

The overshoot is evident at the fast-tuning speed.

Also, a zero-backlash motor is crucial to doing things right.

73 de KC5VDJ

Verner Topsøe-Jensen OZ5TG wrote:

> I am trying to build a controlcircuit for a major PA with an YC156.
> I am using a pair of Jennings 10-1000PF motordriven capacitors.
> 
> Problem :
> The integrated DC-motors requires 18volt to work reliably, but the
> end-to-endtravel takes only about 3 seconds i.e. 330PF/sec.
> The controlcircuit has a delay at about 25mSec i.e. the cap. stops
> 8 - 10 PF off the wanted value. No problem on 160, but on 10, it
> is a matter of success or failure.
> 
> The controlcircuit using variable resistors, cannot, even with a very little
> hysteresis, stop the motors without overshoot. 
> 
> I have tried to chop the DC for the motors. That was an improvement,
> but far from enough.
> 
> Any ideas about how to slow down the RPM and maintain the torque ?
> 
> I checked the Jennings URL, but they do not have any information about 
> how to do.
> 
> 
> Any help, any idea is welcome.
> 
> 
> OZ5TG, Verner
> 
> 
> --
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> 

jim
-- 
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