RF VTVM

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

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  1. KI9R

    KI9R Ham Member QRZ Page

    Good evening,
    I own a Kenwood TS-520 and I'm in love with this radio. Well I've been going through the service manual to get a better understanding of working on this rig, and I keep coming across an RF VTVM. Could anyone please shed some light on what this is?

    Thanks,
    Basil
    N9KOG
     
  2. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    That would be a VTVM with an r.f. probe. There have been instructions on how to make an r.f. probe for decades in the ARRL Radio Amateur's Handbook.

    Glen, K9STH
     
  3. KF5LJW

    KF5LJW Ham Member QRZ Page

    A Vacuum Tube Volt Meter with a diode and filter capacitor as a voltage probe. Pretty much a relic from yester year. Digital volt meters pretty much replaced VTVM's 30 years ago.

    VTVM's had fairly high input impedance, but they cannot match DVM's today, even a $5 Flea Bay China made unit. However you can still buy or make RF probes.
     
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I wouldn't say that at all. Most decent VTVMs from the 60s were far better instruments than cheap DVMs today are, and when it comes to "making adjustments," DVMs are nearly useless because of the display update latency. It's very difficult to peak or null an adjustment using any kind of digital meter, even if it's a $500 Fluke. An analog VTVM is much better for that.

    RCA, Heath, Eico, et al. who made affordable VTVMs all had accessory RF probes you could buy for them. Those are still available surplus, but it's fairly easy to make one that will cover a decade or two of operating frequencies. Precision RF voltmeters from HP, Boonton, et al. are much better and provide true calibration for RF work, often to about 1 GHz (down down to very low frequencies as well) and they're not expensive on the surplus market.

    I wouldn't be without a good VTVM, and good RF voltmeter (millivoltmeter) on the bench; I have very good DVMs but they won't work for many things that require adjustment. They're more appropriate for just making a "measurement," but not critically adjusting anything.
     
  5. W9GB

    W9GB Ham Member QRZ Page

    Derek -
    Watch what (or who) you are referring to as Relic ..... the same could be said for the Kenwood TS-520.

    Actually I have a Heathkit IM-28 w/RF probe (B&K VOMatic and Heath IM-18 are in "to fix" backlog) ...
    as well as Fluke 45, 87, 77 DVMs; Motorola Millivoltmeter and many other bench tools.

    For Basil's usage there are advantages of an analog meter versus an LCD readout.

    w9gb
     
  6. WA7PRC

    WA7PRC Ham Member QRZ Page

    My VTVM is an Eico 232. DC input resistance is 10Meg in the instrument + 1Meg in the probe. AC input resistance is 1Meg, shunted by 60pF. The manual says it can measure RF up to 250mc with the optional PRF-11. A simple RF probe can be easily fabricated.
     
  7. KM1H

    KM1H Ham Member QRZ Page

    My VTVM of choice is a Hickok 209A and for a RF VTVM its a Boonton 91CA. I have others including SS but these are more fun to use with BA gear.

    Carl
     
  8. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    LJW:

    Obviously, you haven't tried to peak or null a circuit using a digital meter! There are none that I know of, at least at a price that the average person can afford, that sample fast enough to detect a sharp peak or a sharp null. Eventually, you may be able to get a satisfactory adjustment (although I suspect that even then it will usually not be optimum). However, an analog meter is extremely fast as well as being extremely accurate for peaks and nulls. Now for the absolute measurement of the peak or null in terms of volts, then, in general, the digital meter can be more accurate. However, for achieving that peak or null in the first place, an analog meter is considerably faster!

    I have several digital meters ranging from Flukes down to Harbor Freight. They are very handy for measuring voltage and resistance. But, for determining peaks or nulls I have, among other units, and old Motorola TEK-7A which is an analog meter the design of which is well over 50 years old. It works great!

    Of course, you can also use an oscilloscope when peaking or nulling. However, that is using an instrument that often costs in the hundreds of dollars when something that costs under $10 can do the same task. In many cases a GDO (grid dip oscillator) can be used to detect r.f. peaks and nulls. But, again, a GDO is an analog unit.

    Modern ways of doing things are great under most conditions. However, there are definitely situations where modern, digital, equipment just cannot do the job as fast as, and in many cases no where near as, well as "olde fashioned" analog.

    Glen, K9STH
     
  9. K7MH

    K7MH Ham Member QRZ Page

  10. WT9S

    WT9S Ham Member QRZ Page

    HP-410B,the best $10 I ever spent at a hamfest.
     
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