Multiple identical-looking VTVM's.

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

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  1. N2EY

    N2EY XML Subscriber QRZ Page

    Let's do the math!

    A 748 ohm resistor causes a voltage drop of 12 volts. What's the power?

    Power equals voltage squared divided by resistance. So we have 144 over 748, or about .193 watt.

    Works for me!
     
  2. K9STH

    K9STH Ham Member QRZ Page

    The fumes from a failing selenium rectifier are definitely poisonous! They won't instantly kill someone. However, the fumes affect the kidneys and, if you get enough fumes, within weeks your kidneys will fail.

    Especially during the late 1940s and well into the 1950s and even early 1960s, when televisions had selenium rectifiers, when there was a fire in either a store that sold televisions, or, a television repair shop, many fire departments did not have "air packs" at all, or, only a small number of air packs, that the firemen, who went into the area in which there was a fire, developed severe kidney problems within weeks of the fire. This happened even when the fire was out.

    It took a lot of investigation to find out the cause of the kidney failures before it was discovered that the fumes from the selenium rectifiers that was the direct cause. After that, a warning went out to all fire departments telling firemen NOT to enter any location that had a number of units, with selenium rectifiers, without having breathing apparatus.

    In the mid 1950s, the local fire chief started a youth program as a search and rescue group. I was one of the original members of this organization. The group met weekly and the chief also taught the group fire safety including what could happen in a fire. The selenium rectifier situation was imparted as part of the instruction.


    As for voltmeters, I have a Ballintine Model 300 which is a vacuum tube AC voltmeter. The design is from the 2nd half of the 1940s:

    Ballantine 300.jpg Ballantine 300-1.jpg Ballantine 300-2.jpg

    I also have a novelty "volt meter" that doesn't have any active components:

    fields-1.jpg

    Of course, this is just a 1.5-volt battery tester.

    I really need to restore the Ballintine unit. Looking on the Internet, I discovered that the Ballintine Model 300 can be used to calibrate the output from signal generators! That is, the accuracy is "good enough" to get a very accurate reading from the piston type attenuator used on most of the older signal generators. Since I recently had to replace the oscillator tube in my TS-497B (military Measurements Model 80), I really need to check the readings on the attenuator against a standard.

    Glen, K9STH
     
  3. AF6LJ

    AF6LJ Ham Member QRZ Page

    Nice job, another VTVM that won't poison its owner.
     
  4. KB1WSY

    KB1WSY Ham Member QRZ Page

    Finished! I do enjoy little jobs that can be done in one afternoon. (By the way, in the picture is looks like the resistor is touching the meter movement's green connector wire, but that's just the angle of the photo. There's a good quarter-inch space between the two, in reality.)

    Like many projects on vintage gear, it was mechanically a bit challenging because the nut underneath the chassis that held the selenium in place was very hard to reach (buried under a sea of hookup wires and components) ... and that was also the chassis hole that I re-used to fasten my terminal strip. But, with a certain amount of cursing and judicious use of a plastic "nut holder," I managed it in the end. Why weren't we humans born with three or four hands? Actually, I suppose we were, and then we had the silly idea to become upright....

    Replace_5.JPG

    The VTVM is back in the test equipment rack. I tested it with some known voltages and some known precision resistances, and the readings were spot-on. So it looks like I won't have to re-calibrate it, even after changing the rectifier.

    Replace_6.JPG

    Thank you (it was at the back of my mind to do some of the math). Even the simplest of projects has useful educational results. Of course in this case it wasn't 12 volts ... but rather less, regardless of whether you look at the change in the B+ or the change in the "B-".

    Oh wait a minute I think I've understood what you've done. You've added the voltage drops at both ends of the filter cap: 8 + 4 = 12. Is that right? Or put it another way, the DC output of that filter circuit is actually 56 + 100 volts, or 156. It's been shifted with reference to ground, for reasons to do with the way the dual-triode setup works in this case.

    Edited to add: looks like the voltage-shifting is done by R24 (68K):

    VTVM_5.jpg

    See, I'm still very green about this stuff. But I'm learning....
     
    Last edited: Sep 17, 2014
  5. KB1WSY

    KB1WSY Ham Member QRZ Page

    What I didn't realize is that this one's still being made. For instance, http://www.amazon.com/Simpson-260-8...23&sr=8-1&keywords=simpson+260#productDetails.

    Is that the one you mean, or should I be going to vintage equivalents? It's pricey, but OTOH if cool analog gear is still being made, it's something I'd be interested in having.

    But I really should remember to set it on a nice high voltage range after use! (Mind you, the above-linked model has something called "current resistance connections safeguards" so perhaps it would protect itself against WSY-idiots who try to measure a B+ voltage having left the meter in the resistance range....).
     
  6. K8ERV

    K8ERV QRZ Member QRZ Page

    And NEVER leave it set up for current!

    TOM K8ERV Montrose Colo
     
  7. KC9UDX

    KC9UDX Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    The new ones are pretty much identical to the old ones, from the best of my memory. (I don't have a new one anymore)

    At least, the new 260-8P is the same as the old 260-8P. There were a bunch of model variations that aren't available anymore.
     
  8. KB1WSY

    KB1WSY Ham Member QRZ Page

    Hah! It's actually the second time I've broken that DMM. The first time, it's because I left it on the current scale ... but that blew a fuse inside the DMM that was easy to change. Mind you, the new fuse cost almost as much as the $3 DMM!

    They are also, according to the product description on Amazon, made in the USA. That's unusual. (I'm an international type myself, so I don't attach a particular "value judgement" to the place of manufacture. But it's unusual.)
     
  9. KC9UDX

    KC9UDX Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    The fuse in my Fluke 117 costs considerably more than your DMM! :eek:
     
  10. W1GUH

    W1GUH Ham Member QRZ Page

    'WSY:

    I've got a Triplett 630-NA here, and it's pretty much a clone of the 260, except for one thing. The range switch has an "off" position that (I believe) puts a short across the meter to dampen its swing as the meter is moved. I see that that's missing from the Simpson. Or do Simpsons have a different kind of meter that doesn't need something like that?

    triplett630.jpg

    The one I have has an added feature - a cal sticker from LEOS, Loral Electro-Optical Systems. Got it at the Redondo hamfest ($35.00) and that makes it a souvenir of that wild and wacky company I loved and sometimes hated so much - Loral.
     
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