RF VTVM

Discussion in '"Boat Anchor" & Classic Equipment' started by KI9R, Jul 11, 2011.

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  1. KF5LJW

    KF5LJW Ham Member QRZ Page

    Sure I have, use to have a VTVM 30 years ago and it was 20 year old design then. Today I use a FET VM or an oscilloscope.Today you can buy brand new FET VM's, but there are no new VTVM's that I am aware of. You have to take your chances on Flea Bay or swap meets if you can find one.
     
  2. KF5LJW

    KF5LJW Ham Member QRZ Page

    Huh? I did not call anyone a relic. Don't make it personal. Use a FET VM as they are still made and available for sale.
     
  3. W9GB

    W9GB Ham Member QRZ Page

    Even if it needs work (like the Heathkit IM-18 that I picked up), it is a matter to reversing the 'solid-state mods';
    cleaning the corrosion from a D or C battery cell left inside, and replace bad components.

    The advantage of the Heathkit VTVM units -- the manual is available so you can rebuild it and virtually every part is still available
    from the original OEM suppliers or numerous parts unit.
    A good approach, even for the novice/apprentice.

    w9gb
     
  4. KE3WD

    KE3WD Ham Member QRZ Page

    Many Digital Multimeters also feature an analog bar graph scale that is to be used for peaking or nulling, just as you would use the needle on the older meters.


    73
     
  5. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    They do, and that helps. But it's not anywhere near as good as an analog meter because there aren't nearly enough bars!

    If they had 120 bars or more, it would be almost as good as a "meter." But usually, they don't...they have a much smaller number so you cannot possibly see a 1% or 2% change in anything. Insufficient resolution, but better than nothing.

    I can see a 0.2% change (two tenths of one percent) on an analog meter.
     
  6. KF5LJW

    KF5LJW Ham Member QRZ Page

    I agree they can be used. But as brought up sample rates and latency can be problematic for some. But a good slow steady hand will overcome that.

    There is also a problem with too high of an input impedance resulting in what is called Ghost or Phantom Signals by some in the electrical trade. Mostly observed by electricians testing to see if circuits are turned off or not as the DMM's will read stray phantom voltages that are not really there. So most Electricians use a meter called a Wiggy (solenoid volt meter)
     
  7. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Those are fine, since again, you're only making a measurement and not adjusting anything.

    If one has to peak or dip an adjustment, where a 1% change is critical, you really need an analog meter, a scope, or a very high class digital instrument with very fast sampling and no latency. That's not found in $5 DVMs, or even $150 DVMs.
     
  8. KE3WD

    KE3WD Ham Member QRZ Page

    The bargraph on my Fluke responds fast, like an analog meter needle. It also has quite good resolution, which surprised me at first. I think it may automatically prescale to what you are doing as well as changing with the scale selection. At any rate, it is not a problem to use at all.


    73
     
  9. W1GUH

    W1GUH Ham Member QRZ Page

    'STH said:

    Good news from the hamfest circuit -- 'scopes are appearing regularly for < $100. And, just this past weekend, I saw a Tek 465 in great shape, and complete and the guy was asking $250. If I coulda, I would grabbed that one. And it's true that for "just" measuring a peak or a null, a 'scope can be overkill -- but it's something that comes along "for free" with a 'scope...so if you've got a 'scope, may as well. And, if you don't have a 'scope, it's well, well worth it to seek out and acquire one. You can spot a problem in one glance at a 'scope that could take days of hair-pulling otherwise.

    Wonder how many total of those Heath/Allied/Eico/RCA/etc... VTVM's are out there? They seemed to be universal in shops and shacks. And still mighty useful, especially since a lot of Voltage and Resistance charts in manuals are referenced to an 11 megohm input VTVM.
     
  10. WA9SVD

    WA9SVD Ham Member QRZ Page

    Unfortunately, those multimeters simply have a digital system with a pseudo-analogue display; the latency and sampling rate ARE the same as that of the digital display, so have the same deficiency in attempting to detect a true peak or null. It's no substitute for a true analogue meter, (unless the resolution were increased by a factor of perhaps 100; [as stated by WIK previously] AND the sampling rate were increased be a factor of 10 or 100, to match the response of a mechanical analogue meter.)


    That said, does anyone have a good circuit for a FET-VM. I have a dead Knight-Kit MM that has a decent size analogue meter (the insides were fried when I got it) and would like to re-build it as a FET-VM. (I WAS going to use it as a darkroom exposure meter, but that was eons ago:( (before digital photography was developed)
    A simple voltmeter is all I would really need, for the reasons described in this thread. I have digital meters for most other purposes.
     
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