Is there a use for a VTVM today?

Discussion in 'DIY' started by N8Nagel, Jun 17, 2017.

  1. N8Nagel

    N8Nagel AK Subscriber Subscriber

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    I already have a Simpson 260-8P as well as two Flukes and a couple other cheap meters.

    Today I saw a Heath VTVM in a thrift store, was marked $15 but I think it might have been 50% off. Movement was marked as "made by Simpson Electric"

    Should I go back and get it or is it pretty much a curiosity at this point?
     
  2. Blue Shadow

    Blue Shadow I gotta get me a new title

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    Need analog to watch the meter move instead of waiting for an update, sometimes. There are uses but DMM is much more commonly used.
     
  3. N8Nagel

    N8Nagel AK Subscriber Subscriber

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    yeah but if the analog movement is the main appeal, I already have a 260-8P...
     
  4. arts

    arts Super Member

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    Ya know,sometimes working on classic equipment using classic test gear is just so cool:)
     
  5. hrballenger

    hrballenger AK Subscriber Subscriber

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    I like using a VTVM for head alignment on RTR and Cassette machines.
     
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  6. N8Nagel

    N8Nagel AK Subscriber Subscriber

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    OK, interested - how do you use it, and why would it be better/more suitable than a Fluke or Simpson?
     
  7. gadget73

    gadget73 junk junkie Subscriber

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    They're stellar when you need to look for a peak or a null in something. Digitals are quite simply crap for that task. Scrolling numbers just won't be as easy to id as a needle movement.

    Depending on the circuit, a standard analog might load it down too much. A Simpson 260 is 20k/volt on DC and I think 5k/volt on AC. Most VTVM's are 10 megohm on DC, maybe only 1 megohm on AC. The frequency response also tends to be wider with a VTVM. I usually use mine for tuner alignments. That wants very low loading, and you're adjusting for a peak most of the time. FM you adjust for a zero, but you can very handily set "zero" to be the middle of the scale so its very easy to land it there.

    I have one, I use it once in a while. Its great for certain things, and sometimes just having an additional meter available. For that price I'd pick it up.
     
  8. ChrisMarantz

    ChrisMarantz AK Subscriber Subscriber

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    +1 for both of these comments - This is exactly how I use my Simpson 260 - Chris
     
  9. NAD80

    NAD80 Super Member

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    I would too. Sometimes the DMM doesn't catch the actual up and down voltage changes in real time.
     
  10. hrballenger

    hrballenger AK Subscriber Subscriber

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    Gadget73 said it much better than I could've:

    "They're stellar when you need to look for a peak or a null in something. Digitals are quite simply crap for that task. Scrolling numbers just won't be as easy to id as a needle movement.

    Depending on the circuit, a standard analog might load it down too much. A Simpson 260 is 20k/volt on DC and I think 5k/volt on AC. Most VTVM's are 10 megohm on DC, maybe only 1 megohm on AC. The frequency response also tends to be wider with a VTVM. I usually use mine for tuner alignments. That wants very low loading, and you're adjusting for a peak most of the time. FM you adjust for a zero, but you can very handily set "zero" to be the middle of the scale so its very easy to land it there. "

    Hurley
     
  11. crispycircuit

    crispycircuit AK Subscriber Subscriber

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    I like the db scale for setting feedback on amps...
     
  12. OMGCat!

    OMGCat! AK Subscriber Subscriber

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    The reason to use one has been explained perfectly well but make sure to use an isolation transformer with it or you have a chance of sparks and damage if not used with one.
     
  13. audiodummy

    audiodummy Active Member

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    I have an autorange FETVM (actually opamp) analog meter. When I had a VTVM it was weird why it had a +/- setting but eventually made sense. The FETVM obsoletes that selection by having an LED that turns on when the probes are swapped (and automatically reversing input probes, just like DMMs with a negative symbol).

    I think the input impedance of the FETVM is the same as the VTVM at 10M ohm for most ranges (3V and up) but the 300mV range is only 1M. About the only thing that the VTVM could do that the FETVM could not was center the meter (I'd have to settle for watching the LED and point at 0V) and it had a Rx1M range. I will have to use my 20M or 2G range on one of my DMMs to check those resistors...

    Also that VTVM had "reverse ohms" where infinite ohms was full scale. The FETVM I have works like most VOMs for resistance - full scale is 0 ohms.
     
  14. N8Nagel

    N8Nagel AK Subscriber Subscriber

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    OK OK... if I get out of work at a reasonable hour today I will see if it's still there... I mean for $7.50 if I'll use it once it'll pay for itself
     
  15. gadget73

    gadget73 junk junkie Subscriber

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    The ohm scale on my RCA is the same as my Simpson 260, zero ohms to the right, infinite to the left. I have seen a backwards one somewhere, but I couldn't tell you where. Getting used to setting zero and infinite needle positions takes a little getting used to though. I changed the pots in mine out to 10 turn, it makes that a much less fussy job. Also the original pots were dying anyway. The only time the 10 turns kind of suck is when you want to center scale zero it. Lots of turning involved.
     
  16. audiodummy

    audiodummy Active Member

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    It would require a constantly powered meters to have the "reverse" ohms so likely only going to see this in VTVM and FETVM (and implicitly, DMM... when was the last time you saw a DMM where it was zero when you had infinite ohms? no?).

    I'm not sure why one would need a 10 turn for the ohms adjust? Most of the "unpowered" passive ohmmeters (i.e. VOMs that don't have constantly running power sources) have not been too hard to zero. Dirty probes, however, can make it tough.

    BTW, the FETVM I have does not have zeroing knobs...it self calibrates just like a DMM. The VTVM did, just like my regular VOMs; it had two zeroing knobs for both ends of the scale (Center/left zero for all scales, and right infinity for ohms).
     
  17. Omegaman

    Omegaman Ultra Super Member Subscriber

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    If you are checking the tracking of a potentiometer then an analog meter is best
     
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  18. H/K crazy

    H/K crazy Active Member

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    I have one ...rarely use it, but for what they cost used, it's not a bad idea to own one, they come in handy from time to time as others have mentioned here.
     
  19. gadget73

    gadget73 junk junkie Subscriber

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    I used them for the ohms and zero adjust. The originals were in bad shape and it had a lot of problems with drifting as it warmed up. New single turn pots may have fixed it just as effectively, but I had the 10 turns salvaged from some other piece of gear and no other use for them. It doesn't have drift problems anymore either.
     
  20. audiodummy

    audiodummy Active Member

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    Ah yes, the VTVM indeed drifted (due to warmup time, past the heater warmup). But strange that a pot would drift unless it itself actually warms up over time for whatever reason. Regular VOMs the drift seems marginal if at all, though re-zeroing is frequently expected hence it's on the front panel instead of being a trimpot.
     

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