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