Re: VTVM versus new DVM

Thanks, I appreciate the reply. I think your site inspired me to do most of my own repairs within the last few years. It is truly a fantastic help for us aspiring electricians and DIYers.

JR

No - a DVM will be fine. In the days of standard mechanical meter voltmeters, the only way to get a high impedance input was a valve - suitable ss devices didn't exist.

--
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    Dave Plowman        dave@davenoise.co.uk           London SW
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It doesn't even make sense why they'd specify a VTVM, given that turntables are mechanical devices.

Oh, I suppose it's one of those new-fangled turntables that have electronics to drive the motor in them, and they specify the VTVM so as to not load down the voltages when measuring.

Michael

[.... ]

Sorry Asimov, I find that questionable at best. For even in 1950 - 60' any 'industrial workhorse' like the tube-type HP 410B used 100 Meg ohms input as a standard input It was only when the DVM came on the scene that most inputs appeared to drop to 10 Meg Ohms.

------ I realize that physical reality is not governed by majority rule within a small group of human beings on an "insignificant planet [orbiting] a humdrum star lost in a galaxy tucked away in some forgotten corner of a universe" but I was wondering what a "straw poll" here might look like.

It was a rare VTVM example that wasn't 11 Meg, 1 Meg in the DC probe.

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N

Just for fun I looked up that old HP 410B. I still have one unit here, for nothing in the modern digital world can replace it with just 'one' unit!

The HP410B input as listed in the manual (complete with circuit diagram) was 122 Meg Ohm. Now, lets compare that to today's DVM, first of all, where are the circuit diagrams? (true, one cannot find custom IC's anyway, so it immediately becomes junk)

As for RF measurement does the modern DVM with a max reading of

100 KHz or if lucky maybe 2 MHz, really qualify or compare to that old tube-job (RMS 1 - 350 Volts) that was stable to 700 MHz and even useful to 1000 MHz? DVM still have some catching up to do...

Thats why I like and still hang onto my HP-410C which the owner was happy to sell at a fleamarket (because it wasn't digital!). The only little mod I made was to solder a sewing needle to the tip with a small piece of spagetti over it so that I can reach into close areas with that big AC probe (which is kind of nice on a cool day even :)

"Right On" with the comment about the included schematic - I often muse that those companies which produced "quality" equipment (whether it was test or otherwise)...."weren't afraid of their products" and their attention to detail in the manuals and literature backed that up - I don't care what the modern excuses are.

Gord

No, VTVMs designed for repair shops were 10 Meg on the AC ranges and

11 Meg on the DC ranges. The extra 1 meg was in the DC probe to reduce loading in tuned circuits to minimize the effects of probing the circuit.

You didn't see HP VTVMs in repair shops.

--
Former professional electron wrangler.

Michael A. Terrell
Central Florida

Actually 2 Meg was a common AC impedance.

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N

Most "shop quality" VTVMs had 11 Meg DC and 1 Meg AC input impedance, but there's another issue that could be much more of a problem.

When used for AC, all meter circuits *measure* some attribute of the signal, and *indicate* an attribute which may or may not be the same one.

A D'Arsonval meter (Simpson 260, say) is average measuring, RMS indicating.

A VTVM is most likely peak measuring, RMS indicating (a good VTVM will have a "peak" or "peak-to-peak" scale, too).

A DVM can be anything, including RMS measuring, RMS indicating (and that was *very* rare prior to DVMs.) Check the user's manual.

As long as the waveform you're measuring is pretty close to a sine wave, the indicated voltage won't be too far wrong, but if the signal is very "spiky", the different meters will give very different readings -- perhaps *none* of them very significant. Use a 'scope if you don't know what to expect.

Isaac