Multiple identical-looking VTVM's.

Discussion in '"Boat Anchor" & Classic Equipment' started by W1GUH, Sep 16, 2014.

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  1. W1GUH

    W1GUH Ham Member QRZ Page

    There were VTVMs from at least Heathkit, Knight-kit, Eico and RCA that looked identical on the outside. Were they identical on the inside?
  2. N2EY

    N2EY XML Subscriber QRZ Page

    They may look identical, but there were variations, both inside and outside.

    That said, there were only so many ways to build a VTVM and still be in the price ballpark.
  3. W1GUH

    W1GUH Ham Member QRZ Page

    Anyone care to rate the four I mentioned? I have no clue which worked better. I always favored the Knight-kit because I generally liked Knight-kits over Heathkits, but that's based purely on looks. And, of course, the RCA Voltohmyst had RCA on it to ad to it's cachet.

    Thanks for the info, Jim.
  4. WA7PRC

    WA7PRC Ham Member QRZ Page

    My Eico 232 has always been a good unit. I still use it, from time to time.
  5. KL7AJ

    KL7AJ Ham Member QRZ Page

    I remember buying aged tubes for those. :)
  6. AF6LJ

    AF6LJ Ham Member QRZ Page

    I have one of those in the garage, never powered it up.
    I did remove the battery.........
  7. KB1WSY

    KB1WSY Ham Member QRZ Page

    Inside, the circuits are very similar, at least for the "medium-price" models from Heath, Eico, RCA, ... as I discovered a couple of years ago when I bought and rebuilt my Eico-232.


    Usually there are two tubes. For DC measurement, a dual triode. When no voltage is applied to (for instance) the "left-hand" grid on the schematic, equal currents flow through both sections and the cathodes are at the same voltage. The meter is connected between the two cathodes and thus reads zero volts. When a DC voltage is applied to the left-hand grid, the "balance" of the tube is disturbed and a current flows between the two cathodes, causing the meter needle to budge upwards.

    As explained in ARRL manuals, for the scale to remain reasonably linear, the range of operations is limited to about 3 volts. To measure higher voltages, you must use a resistive voltage divider. Commonly, the ranges on a VTVM increase in steps of about 1:3 rather than the 1:10 that you might see on a VOM. That's why you have (for instance) a 1.5 volt scale, then a 5-volt scale, then a 15-volt scale, then 50 volts.... This is such a standard progression that almost all VTVMs follow it, and usually that rotary switch is on the right. On the left, the Function switch with the DC, AC and resistance positions.

    Yes indeed, that switch layout is so common that it might imply all VTVMs are the same!

    At this price point, almost all VTVMs have the same DC resistance of 11 megohms. Almost all of them also have a 1-megohm resistor connected in series with the probe when measuring DC voltages. To reduce measurement error this resistor is mounted inside the probe. This means you either need to use separate probes for AC and DC, or use a probe with a switchable built-in resistor such as the (pictured) Eico Uniprobe.

    To measure AC voltages, a second tube rectifies the current to DC, and then passes it on to the dual triode mentioned above. Typically the rectifier is a dual diode. One half of the diode rectifies, and the other acts as a balancing device. The output of the rectifier is a "peak to peak" reading (not RMS) which means that the AC readings on the VTVM can be tricky if the waveform is complex and not a pure sine wave. For instance, the readings could be different if you reverse the probes. The scale on the meter is RMS but only holds absolutely true if the waveform is a sine wave.

    To measure resistance, the system is similar to a VOM, with an internal source of low-voltage DC power (either a battery, or a rectified current from the power supply) that is fed across the measured load.

    Being tube devices, VTVMs also have power supplies, frequently using a solid-state rectifier for the B+ to the tubes. Mine uses a selenium rectifier that's a ticking time bomb ... really need to get around to replacing it with a silicon rectifier.
    Last edited: Sep 17, 2014
  8. KL7AJ

    KL7AJ Ham Member QRZ Page

    Yep...the configuration of those was the first common implementation of a differential amplifier. I have a beloved RCA Senior Voltohmyst
  9. KB1WSY

    KB1WSY Ham Member QRZ Page

    I forgot to mention something that I did not understand at first, until N2EY explained it to me two years ago. VTVMs measure voltages between the measuring point, and ground. They are hard, or impossible, to use if you are measuring a voltage between two points that both float above ground -- to do that you need a VOM. In practice however, the vast majority of voltage readings relate to ground, rather than being "floating."

    If you are looking for less standard VTVM circuits, I suspect the top end of the HP models was more sophisticated. In particular, there were models with a much higher DC resistance: 30 megs of more, much higher than the 11 megohm that was standard in the less exalted models. I'm not sure how HP achieved that.
  10. K8ERV

    K8ERV QRZ Member QRZ Page

    Does anyone remember the first (?) commercial VTVM? I think it was McMurdo Silver.
    Had an input Z of 10 megs, and was so sensitive the needle would go wild if you touched the test probe.
    Servicemen were not used to that and thot it was defective.
    Don't think it was a big success for that reason.

    TOM K8ERV Montrose Colo
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