URM-127 variable capacitor data

Frank Gentges fgentges at mindspring.com
Sat Sep 23 16:21:37 CDT 2006


Nice info.

Karl W4KRL wrote:
> Frank/Andre & the group,   Frank gave me a URM-127 signal generator 
> with the variable capacitor tuning last night at the VWS meeting. I'll 
> be able to post a schematic in a few days. Meanwhile you may be 
> interested in what I found out about the capacitor. It appears to be a 
> 10 to 540 pF dual-gang unit made in England by JB. Each gang has 
> circular plates but I've forgotten what that means in terms of 
> capacitance versus rotation. Each rotor is composed of 17 plates. I 
> couldn't get an accurate count on the stator but 16 would seem right. 
> The spacing is VERY small. You could use this unit only at very low 
> power.   Angle of rotation is 180°. There is an antibacklash gear 
> drive connected to the front dial so that the dial has a larger angle 
> of rotation. The gear is not part of the capacitor unit.   The 
> capacitance range for dial readings of 20 to 200 goes from 500 pF to 
> 24 pF. 
Nice range.  My casual look left me with the impression that it has two 
sections.  Also, the capacitor frame is floated so it will work in 
circuits where this is needed.  The unit has some good potential for use 
in RF test equipment designs.
> I figure it is padded with 29 pF to get the correct 10:1 range. 
A 10:1 capacitance range should give a sqrt(10) = 3.16:1 frequency 
tuning range.  Not bad, not bad at all.

> The frequency range selector switches resistors of 16M, 1.6M, 160k, 
> and 16k for the X1, X10, X100, and X1k ranges.   73 Karl W4KRL
Thanks for the good work Karl.

Frank K0BRA

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