variable Capacitors

Discussion in 'General Technical Questions and Answers' started by G3EGY, May 16, 2018.

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  1. G3EGY

    G3EGY Ham Member QRZ Page

    Some years ago had quite a number of old circuits, all given away or 'binned'. Looking for a circuit, to measure a box of old Variable caps' , some pretty obvious, but quite a few, not so .. Any help, greatly appreciated
    Many thanks
  2. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    The easiest solution is to buy a VOM multimeter that includes capacitance measuring function or buy a dedicated LCR (inductance, capacitance, resistance) meter.

    Peak Atlas is a UK company that sells excellent hand sized meters:


    Many DMM meters now offer capacitance measuring and inexpensive dedicated LCR meters are too numerous to mention. Let your budget guide you.
    KK5JY and NH7RO like this.
  3. GM4BRB

    GM4BRB Ham Member QRZ Page

    That's an old, old callsign!
    I have such a thing but honestly I think, if time, or patience, is an issue, just pick up a capacitance meter off fleabay. This is mine and is of a different, more classical style.
    This one measures from 200pF to 20milli-Farads.
    Then there are of course the old Marconi Universal Bridges which are more similar in concept to any old schematic designs you'd find in ye olde magazines ...
    TF2700 Universal Bridge
    TF2700 Universal Bridge.jpg
    I also have one of these. It remains on the shelf ... with one of the big old batteries inside.
    Does work but unintuitive, and difficult enough to set up.
    I shall be keeping it however, as it does for Inductance too.
    Best to get up to date, especially I think, if you're a bit long in the tooth and like me, in a hurry.
    Last edited: May 16, 2018
  4. G3EGY

    G3EGY Ham Member QRZ Page

    Thanks KB4QAA for the reply and the suggestion. Appreciate your input but , at this stage in life, hardly worth the price . Some years ago maybe ! Thanks again es 73's
  5. G3EGY

    G3EGY Ham Member QRZ Page

    Hi GM4BRB, Yep' you're quite right, old callsign for an old 'codger'.. I guess it shows in my 'old-fashioned thinking'. Now that's the meter I am l0oking for, ( YF-150 )or similar, Been on Amazon, thru' E-Bay , but it seems all the meters I see are for' fixed caps' stuff . You mention time and patience, I suppose they are closely related; the former is definitely true in my case Hi.. When I go QRT, whoever is interested will have to sort the values out for themselves ! but it would be nice to have at least, some idea what the 'not-so obvious' , variable
    caps are .. Many thanks for the info and pictures etc . 73's to you and yours
  6. KE0ZU

    KE0ZU Ham Member QRZ Page

    Here are a couple of possibilities. First the old Tektronix LC-130, an excellent meter for small value components as it measures down to as little as 3pF or 3uH full scale, great for RF work. The top end is limited to 300pF.


    Interestingly I have the same digital meter as GM4BRB. This little meter is the first I've seen that exhibits stable readings in the lowest scale, in that first off, you can "zero" the thing, and secondly it will stay there after several minutes. It also reads in fractions of a pF on the bottom scale. Nice little meter.


    And finally, a contemporary of the TF2700, the General Radio model 1650A. As the Marconi, it's long the tooth but an extremely versatile and accurate meter.

    Last edited: May 17, 2018
  7. G3EGY

    G3EGY Ham Member QRZ Page

    Hello Mike, thanks for your interest my, my, those pics certainly bring back memories,, Hi.
    Got some really ancient stuff here lying around but not what I need ! By the way , just been looking around for that YF-150 cap meter and when I queried if it would indeed measure Variable capacity, the answer was 'sorry, only fixed cap ' ??? Price was reasonable , £32.0 pity ! or did they advise me wrongly ? thanks again 73's
  8. 2E0ILY

    2E0ILY Ham Member QRZ Page

    Hi, I am in north Shropshire just south of Whitchurch, if you get over this way I can measure them for you with an in calibration Philips LCR meter. My e-mail is on QRZ.
  9. G3EGY

    G3EGY Ham Member QRZ Page

    hi 2EOILY, thanks for the offer ,will have to get my 'Pogo stick ' out Hi. OK on The Philips LCR meter , sounds a useful ,& great asset If you 'work' CW, be nice to hear you
    Thanks again es 73's
  10. GM4BRB

    GM4BRB Ham Member QRZ Page

    I had the same thought. That unless the nature of the Dielectric matters in any way to come out with an accurate reading, then I CAN'T SEE HOW IT WOULDN'T work on variables ...
    Principle of operation is generation of a low-frequency AC signal (maybe as low as 1Khz?) used to determine the capacitance by calculation within the meter.

    Maybe airspaced caps would pose a problem since they are not built specifically of rolled thin sheet of conductor separated by the dielectric. I have difficulty seeing the difference between a rolled film capacitor and an air-spaced one, electrically, when it comes to an approximate measurement.
    The formulæ for capacitance hold ... I would have thought.

    Having said all that. I did just find a 70pF wide-space variable and it MEASURES perfectly.

    It WORKS! Sufficiently well when the VANES are MESHED.
    The problem arrises as the vanes become further UNMESH. Then the capacitance meter appears to give erroneously high readings. Because, I presume, it's built to detect REAL capacitances, as opposed to 100% Air-Dielectric scenarios. I have a picture or two more.
    That's a 70pF cap, because it says so on the box, in my immaculately labelled categorisation system here in the work-shack.
    Last edited: May 17, 2018

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