RF VTVM

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

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  1. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    LJW:

    Most FET voltage meters don't have a digital readout. As such, they are analog and NOT a DVM as you stated in your first reply.

    A FET voltmeter is just an extension of the old vacuum tube meters. That is, a VTVM in which the tube has been replaced with a FET. In fact, in many cases, it is possible to modify a VTVM to use a FET.

    Glen, K9STH
     
  2. KM1H

    KM1H Ham Member QRZ Page

    Its very rare to find one with a RF probe and the AC probe is often shorted. The vacuum diode in the RF probe is also getting hard to find.
    If you got it all for $10 you did very well.

    Carl
     
  3. KM1H

    KM1H Ham Member QRZ Page

    If he got that price he was laughing all the way home. Known good ones, turned on and running are $100 or less at Nearfest, add $25 if it has the DMM option. I paid $125 for mine with DMM, 2 probes and carrying case. An older 453A, with probes , bright and sharp trace, and case was $25 at ESS in Manchester a bit over a year ago.

    One bad thing about a DMM is it wont show ripple on a PS while even an old 260 will have needle flutter; no matter how small it can be seen unless youve been in the sauce first[​IMG]

    Carl
     
  4. WA7PRC

    WA7PRC Ham Member QRZ Page

    Hi Larry,
    Take a look at National's AN32 (FET Circuit Applications). There's a FETVM circuit on page 3. That application note has many other classic JFET applications as well. :)
     
  5. K4VLF

    K4VLF XML Subscriber QRZ Page

    I always like to keep a Simpson 260/270 around for those critical measurements. (not RF, of course)
     
  6. WA9SVD

    WA9SVD Ham Member QRZ Page

    Thanks for the hint. I didn't think of looking in the National Data Books that I have hidden away somewhere. I hope the transistor types are still available!
     
  7. W1GUH

    W1GUH Ham Member QRZ Page

    I sure appreciate the information -- makes me glad that I couldn't do anything that..in your opinion I would've gotten screwed.

    OTOH...given what that scope does, and the value that his price represented....it's still not a bad price, considering everything.

    But your words are useful in bargaining.

    But I gotta add....sounds to me like "He know the price of everything and the value of nothing."

    $250 for that capability is STILL a great price?
     
  8. KF5LJW

    KF5LJW Ham Member QRZ Page

    I never said they did, nor have I ever seen a digital readout on a FET VM. My whole point was VTVM's are antiquated, not manufactured anymore, and have been replaced by FET VM's.
     
  9. KM1H

    KM1H Ham Member QRZ Page

    About 5 years ago it was. Lately they are being dumped, mine originally came from a company in RI that surplused a bunch of them dirt cheap to the guy I got mine from, still had a current cal sticker.

    In 2009 ESS had a nice 475 with DMM for $175 but they also had a loaded R7704 for $150 which I grabbed. However its bulky, buried under gear so not moveable, noisy, and I dont always need its capabilities so the 465 came next. Having several scopes helps when doing an alignment or trouble shooting a HRO-500, RA-17, TS-940 and similar complicated radios.
     
  10. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    LJW:

    Unfortunately, if you go back and read your original post, you specifically state that VTVMs have been replaced with DVMs. However, a DVM is not really suited to be used with an r.f. probe as several people have pointed out!

    You have to be very careful stating absolutes, especially here on QRZ.com because there are a number of people who will correct mistakes in a "New York minute"!

    You also have to remember that the majority of amateur radio operators do not have the "latest and greatest" test equipment available. I do have some pretty good test equipment. However, for obtaining "peaks and nulls", I definitely use older test equipment that is well suited for that purpose. Using my Motorola TEK-7A I can accomplish a peak or a null in a couple of seconds whereas if I try to use one of my digital units it will take a lot longer and generally will not be as accurate.

    Now I can use my Tektronics oscilloscope for obtaining peaks and nulls. However, I don't usually have it turned on and would have to wait for it to warm up for about a minute while by using my TEK-7A I am "long gone" and on to the next stage in alignment.

    I have a couple of service monitors which are digital and I use them at least 99 percent of the time. But, there are situations which require more signal level than either service monitor is capable of providing. Therefore, I also have a military TS-497B (military version of the Measurements Model-80) on my test bench. I do have to use an external counter to determine the frequency because the frequency calibration on the signal generator is not good at all. However, the TS-497B can put out at LOT more r.f. than either service monitor. As such, especially for those on a limited budget, older test equipment is capable of doing an excellent job. It is just the fact that, in many cases, getting high accuracy on the older equipment does require a little more effort.

    Glen, K9STH
     
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