I finally got around to trying a good 'ole fashion VTVM probe with a Fluke
DMM. Recall that most VTVM probes have a switch that allows for either DC
Volts in one position, then AC/Ohms/mA in the other position. The DC
position typically has a 1-meg isolation resistor, highly useful for VT grid
measurements. Without value compensation, a modern DMM cannot use such a
probe and is otherwise useless for serious tube receiver work.
I took a Simpson VTVM probe with a BNC connector and connected it to a
Pomona BNC-to-Banana adapter with standard 3/4" centers. With the adapter,
the probe easily connects to a DMM. My first measurement was a precise +12V
source. When using the VTVM probe in the DC position (series 1-meg
resistor), the DMM displays +10.93V. So, +1.07V is being dropped across the
1-meg resistor. Assuming the resistor is close to 1-meg in value, the input
Z of my Fluke 8060A calculates to 10.215 meg. Essentially, a 10:1 voltage
divider is being created between the 1-meg iso-resistor, and the internal Z
of the DMM. The drop is creating the value discrepancy. VTVMs are
compensated in design and manufacture for this.
I have several Fluke DDMs, including 8060A (my favorite DMM), and an
advanced model 189. However, peering through the manuals, I see no setup
routine to create a user-defined DC offset. What I want is the ability to
measure a precise DC voltage, then enter a menu that allows me to assign a
new display value to compensate for the voltage drop across the
iso-resistor. I have a lab-grade Keithley bench-type DMM that does allow
for such an offset, but hauling it around is a pain.
So, does anyone know of a DMM that allows for DC voltage offset? This is
different than the "Relative" button seen on many DMMs. Relative is used to
"zero the display" for any input value. I want the same thing but instead
of zero, assign a new value of my choice.
Paul, W9AC
_______________________________________________
Drakelist mailing list
Drakelist@zerobeat.net
http://mailman.zerobeat.net/mailman/listinfo/drakelist
