[R-390] AGC voltage issue, erratic readings

[Tisha Hayes]

Well, your DVM is also interacting with the capacitor and the resistor and
cap are working together to create an RC circuit. You will not get a true
reading of the resistor because your meter is using a DC potential to make
current flow through the resistor and then measuring the current through
the DUT (device under test). The meter than takes this precision voltage,
along with the measured current (or voltage drop across a precision
resistor in the meter working as a shunt) to do ohms law and figure out the
resistance.

I bet if you watch it you will see the resistance starts as one thing and
then starts to climb or fall. That is the resistor charging up. Depending
upon if it is a series or parallel or complex circuit you can see all sorts
of weird things happening. The only accurate way to get a reading is to
test the resistor by itself, away from the influence of other components
that can take a charge or transform the energy. (meaning, magnetics with
inductors).

That is one of the reasons on why I really liked some of the old tube based
models of HP voltmeters. They used AC voltage to do the measurement and did
not exhibit this charging effect as readily
-- 
Ms. Tisha Hayes. AA4HA

*""It is not because things are difficult that we dare not venture. It is
because we dare not venture that they are difficult." -Seneca"*