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Discussion in 'Electronic Repair' started by [email protected], Mar 22, 2013.

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  1. Guest

    Hi Folks,

    I'm working on an old tube radio (Zenith) and could really use a VTVM. I don't have much money at all. I'm looking for something very basic and cheap. I had an old Heathkit but the meter/dial broke.

    Gabe Read
    Providence, RI
  2. You can get a cheap DVM for a couple of bucks at Harbor Freight.
  3. tm

    tm Guest

    And the reason it needs to be a VTVM and not a $25 Radio Shack digital
    multimeter is?
  4. the input impedance on throw away (and even good bench) meters can be
    really really low on some ranges, like AC.

    I use a Leader FET input? meter for stuff like this. These replaced VTVMs
    but retain the essentially zero load on the test circuit.

    One use is for takings readings with the 40kV Fluke probe. You'll get
    dangerously low readings with pretty much any digital meter on AC ranges.
  5. tm

    tm Guest

    Good point. However you will pay an accuracy premium on the DC and Ohm
    ranges. Most VTVMs will only give 3% FS vs <1% with even the cheap DVMs.
  6. Because a VTVM can be much better at RF frequencies. Also, your old service
    data gave alignment and measurement instuctions referencing a VTVM.

    However, by the time you add in an RF probe (missing from most VTVM's you
    see on eBay) on a good working VTVM, you could probably buy a decent working
    100mHz 'scope instead.

    Back in the day, a VTVM was way cheaper - most people had no hope of buying
    a 100 meg 'scope.

    MarkZ .
  7. Guest

    The little HF LCD display multimeters seem to have pretty high input
    impedance, but I've never actually tried to measure it. Certainly
    much higher than the old Simpson multimeters.
  8. tm

    tm Guest

    The little HF LCD display multimeters seem to have pretty high input
    impedance, but I've never actually tried to measure it. Certainly
    much higher than the old Simpson multimeters.


    Now, the 100 meg scope would be my choice for working on the old tube
    Should be able to pick up a good analog 100 meg scope for (way)under $100.

    A good Tek 475 will do 250 megs. Just set up a search on ebay and wait.

  9. Phil Allison

    Phil Allison Guest

  10. Bill Gill

    Bill Gill Guest

    I worked on tube type radios for many years and never needed
    a VTVM. A normal multimeter worked just fine.

  11. tm

    tm Guest

    Same here. That plus a good scope (2465B). Meets 90% of the needs. A useful
    tool that helps save time is a good ESR meter to do a fast check of the
    filter caps.
  12. That was mixed. One minute it would be a high impedance point, like the
    agc bus in a receiver, and the schematic would warn about using a VTVM.
    But in other cases, the schematic would indicate that the the measurements
    were made with a Simpsons 260 VOM. The high impedance point needed the
    high impedance meter, but the VOM points expected the load of the VOM, so
    using a VTVM meant the readings would be high. I thought the schematics
    with the VOM measrued voltages were because the serviceman was more likely
    to have VOM, so might as well list what they'd see.

    VTVM weren't so great at RF. Generally you needed an external RF
    detector. Yes, the 410B has a wonderful RF probe, but that's ane
    exception. The average VTVM had an "AC probe" but had the diode in the
    case, so there was that long cable to it, not making it suitable for RF,
    even if the diode was okay for it.

    The same with DMMs, lousy for RF if for no other reason than the diode
    isn't at the probe. One could build an RF probe, plug it into the DC jack
    on the DMM, that used to happen a lot and it makes no difference whether
    the meter is a VTVM, a DMM or a VOM (except for the loading of the last).

    The problem with DMMs is that the readout is digital. Not good for
    peaking or watching treands. Once upon a time, decades ago, one company
    had a DMM with a small analog meter for peaking purposes, but that never
    was a trend. Instead, the best we get is a bargraph on an LCD readout, but
    the one I have is so slow that it's not useful for peaking.

    One might as well dig out a decent size meter (or even a tuning meter off
    an old stereo) add a jfet input op-amp, and build a relative DC meter for
    peaking. Don't fuss about calibration, the DMM is for absolute voltage.
    You don't even need an stepped attenuator with precision resistors, just a
    few divider resistors or even a high value pot. Since it's all relative,
    you just need something to ensure the input doesn't deflect the meter too
    much. And then use it when you need to peak.

    As for finding a VTVM, a couple of years ago I was walking along a street
    I'd not been on for a long time, lots car traffic though less foot
    traffic. I look down, there's a cardboard box on the sidewalk, and inside
    is an RCA Voltohmyst VTVM. Seems intact, I've never gotten around to
    plugging it in. That was all, no other neat stuff no other garbage, and
    no obvious store where it might have come from.

    It becomes all a more interesting story since I probably was the only one
    walking along there who'd recognize what it was and bring it home. If I'd
    not gone that way for some extraordinary reason, the meter would have gone
    to the garbage dump.

  13. tm

    tm Guest

    Yes, that is why I sometimes wrap a resistor on the tip of the probe to make
    the measurement.
    Of course with a scope probe, that's not a problem.

    Still, there is much one can do with a Harbor freight $10 DMM.

    I wonder, if you could only have one instrument, what would you choose? If
    cost were the main consideration?

  14. The voice of experience! Thanks!

    Mark Z.
  15. If you could only have one instrument, what would you choose?

    Easy. One of those super-whammy digital LCD scopes that can measure just about

    A budget model of one of those.
  16. Jamie

    Jamie Guest


    A Hantek DSO 8060

  17. Mike Cook

    Mike Cook Guest

    the input impedance on throw away (and even good bench) meters can be
    Would you explain please? Is the FET input impedance so much higher that it
    loads less than even a high-quality DMM?

  18. cjt

    cjt Guest

    On 03/22/2013 09:43 PM, wrote:
    The "Simpson multimeters" were generally VOMs rather than VTVMs, no?)
  19. Jamie

    Jamie Guest

    My Fluke 289 will measure correctly with the 40k probe doing
    AC or DC. So won't a couple of older flukes I have.

    THe only problem with AC and a HV-probe is you need to keep
    the frequency down, otherwise, the small amount of cap present
    on the input of the meter is going to attenuate it some.

    It's very possible cheap meters on AC mode may not behave correctly
    with a HV-PROBE and show great losses in readings.

    I also have a scope meter that works very well with the HV-probe.
    I just need to remember to scale the input properly.

  20. But there is another factor here that no one has considered. The OP
    may be used to working with an old tube type VTVM of the kind he no
    longer has and would be very comfortable with a similar replacement. I
    still have and use my RCA Senior Voltohmyst that I built in 1962. And
    it still keeps good time. Lenny
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