[Amps] W8YX at Univ of Cincy

[Paul Christensen]

>"One of the first kits I built as a kid was an Eico VTVM, model 221-K. I
learned so much by playing with it." 

A VTVM is still one of my most-used bench tools.  It's tough to beat when
taking voltage measurements at high-Z points -- like a tube grid in a
receiver.  

I'm often asked why I don't normally use a modern DMM for those
measurements.  Although a DMM's input Z may exceed 10M-ohm, the 1M resistor
in a VTVM's probe tip plays a significant role in isolating the circuit from
the measurement.  I've made many measurements with a VTVM that could not be
made with a Fluke DMM.  

A few years ago, I wanted to see if it was possible to use a VTVM probe with
a common DMM, and achieve accurate results.  Although very little current
flows with a DMM DC sample, the inclusion of the 1M resistor in series with
the input Z of the DMM will show a large measurement error.  

A NOS Simpson 312 probe was modified with a precision *10M* series resistor,
followed by a shunt resistor at the DMM's input.  The shunt value was
selected so that only a mental change in the decimal point is needed.  For
example, a +12V measurement shows +1.2V on the DMM display.  The result is
DMM accuracy with 10M of isolation at the probe tip.  Although this
arrangement works well, I still gravitate to one of several VTVMs when
needed.  

Paul, W9AC