WTB VTVM

You can get a cheap DVM for a couple of bucks at Harbor Freight.

And the reason it needs to be a VTVM and not a $25 Radio Shack digital multimeter is?

the input impedance on throw away (and even good bench) meters can be really really low on some ranges, like AC.

I use a Leader FET input? meter for stuff like this. These replaced VTVMs but retain the essentially zero load on the test circuit.

One use is for takings readings with the 40kV Fluke probe. You'll get dangerously low readings with pretty much any digital meter on AC ranges.

Good point. However you will pay an accuracy premium on the DC and Ohm ranges. Most VTVMs will only give 3% FS vs

Because a VTVM can be much better at RF frequencies. Also, your old service data gave alignment and measurement instuctions referencing a VTVM.

However, by the time you add in an RF probe (missing from most VTVM's you see on eBay) on a good working VTVM, you could probably buy a decent working

Back in the day, a VTVM was way cheaper - most people had no hope of buying a 100 meg 'scope.

MarkZ .

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The little HF LCD display multimeters seem to have pretty high input impedance, but I've never actually tried to measure it. Certainly much higher than the old Simpson multimeters.

The little HF LCD display multimeters seem to have pretty high input impedance, but I've never actually tried to measure it. Certainly much higher than the old Simpson multimeters.

Now, the 100 meg scope would be my choice for working on the old tube radios. Should be able to pick up a good analog 100 meg scope for (way)under $100.

A good Tek 475 will do 250 megs. Just set up a search on ebay and wait.

Regards, tm

Wot a tool.

... Phil

I worked on tube type radios for many years and never needed a VTVM. A normal multimeter worked just fine.

Bill

Same here. That plus a good scope (2465B). Meets 90% of the needs. A useful tool that helps save time is a good ESR meter to do a fast check of the filter caps.

That was mixed. One minute it would be a high impedance point, like the agc bus in a receiver, and the schematic would warn about using a VTVM. But in other cases, the schematic would indicate that the the measurements were made with a Simpsons 260 VOM. The high impedance point needed the high impedance meter, but the VOM points expected the load of the VOM, so using a VTVM meant the readings would be high. I thought the schematics with the VOM measrued voltages were because the serviceman was more likely to have VOM, so might as well list what they'd see.

VTVM weren't so great at RF. Generally you needed an external RF detector. Yes, the 410B has a wonderful RF probe, but that's ane exception. The average VTVM had an "AC probe" but had the diode in the case, so there was that long cable to it, not making it suitable for RF, even if the diode was okay for it.

The same with DMMs, lousy for RF if for no other reason than the diode isn't at the probe. One could build an RF probe, plug it into the DC jack on the DMM, that used to happen a lot and it makes no difference whether the meter is a VTVM, a DMM or a VOM (except for the loading of the last).

The problem with DMMs is that the readout is digital. Not good for peaking or watching treands. Once upon a time, decades ago, one company had a DMM with a small analog meter for peaking purposes, but that never was a trend. Instead, the best we get is a bargraph on an LCD readout, but the one I have is so slow that it's not useful for peaking.

One might as well dig out a decent size meter (or even a tuning meter off an old stereo) add a jfet input op-amp, and build a relative DC meter for peaking. Don't fuss about calibration, the DMM is for absolute voltage. You don't even need an stepped attenuator with precision resistors, just a few divider resistors or even a high value pot. Since it's all relative, you just need something to ensure the input doesn't deflect the meter too much. And then use it when you need to peak.

As for finding a VTVM, a couple of years ago I was walking along a street I'd not been on for a long time, lots car traffic though less foot traffic. I look down, there's a cardboard box on the sidewalk, and inside is an RCA Voltohmyst VTVM. Seems intact, I've never gotten around to plugging it in. That was all, no other neat stuff no other garbage, and no obvious store where it might have come from.

It becomes all a more interesting story since I probably was the only one walking along there who'd recognize what it was and bring it home. If I'd not gone that way for some extraordinary reason, the meter would have gone to the garbage dump.

Michael

The 1 Meg resistor in the probe for DC measurements in the RF sections. It prevents detuning by isolating the cable capacitance from the circuit you're testing.

Yes, that is why I sometimes wrap a resistor on the tip of the probe to make the measurement. Of course with a scope probe, that's not a problem.

Still, there is much one can do with a Harbor freight $10 DMM.

I wonder, if you could only have one instrument, what would you choose? If cost were the main consideration?

The voice of experience! Thanks!

Mark Z.

Easy. One of those super-whammy digital LCD scopes that can measure just about everything.

A budget model of one of those.

Yeah

A Hantek DSO 8060

Jamie

Would you explain please? Is the FET input impedance so much higher that it loads less than even a high-quality DMM?

Thanks.

The "Simpson multimeters" were generally VOMs rather than VTVMs, no?)

I think those were the most common. In my lab I have one Simpson VOM and two Simpson VTVMs. ;)

Cheers

Phil Hobbs