BarryFrom:
https://www.electronicdesign.com/technologies/test-measurement/article/21146730/multimeter-measurements-explained
"Measuring resistance with a multimeter can be done a couple of different ways,
depending on the level of accuracy needed in the measurement. Multimeters
measure resistance by injecting a small current into the circuit, and then
measuring the voltage drop across those points in the circuit. The known
current, and the resulting voltage drop are then used to calculate the
resistance using Ohm’s Law, V = I × R. Since even wires have resistance, the
wires of the probes can actually add to the observed resistance measurement.
For this reason, there are two different modes for measuring resistance: 2-wire
mode and 4-wire mode."
In most analog meters, the current injected into the circuit is not tightly
controlled. This leads to the compression of the meter reading at the high
resistance end of the scale. A constant current source eliminates this
compression of the resistance scale but greatly increases the cost of the
analog meter. This problem is not seen in a DVM. So to answer your question,
input impedance of the meter is not a concern in measuring resistance.
Measuring voltages is a different story since the meter may load the circuit
you are trying to measure. The higher the meter's impedance, the less the
loading on the circuit being measured. So the original meter reading used to
specify voltages in the circuit may read lower then when you verify the
voltages using a DVM.
Not always specified or easily found in most equipment manuals but up to +/-
20% of the specified voltage reading would be an acceptable reading in most
circuits. In other words, the voltages given are average readings or bogey
numbers.
From Wikipedia:"A bogey is a published value for a parameter of an electronic
component, such as a vacuum tube, that is average or typical of devices that
will be sold, and which the device's manufacturer is attempting to achieve."
Hope this helps,JimLogic: Method used to arrive at the wrong conclusion, with
confidence. Murphy
On Monday, September 2, 2024 at 09:31:33 AM CDT, Barry <[email protected]>
wrote:
Did Collins specify what input impedance a meter should be to properly read
the resistance values in the manuals? I did see where a TS-352 was mentioned
but that's selectable between 20,000 ohms/volt and 1,000 ohms/volt.
I started checking the RF Deck in my R-390 and, starting with V201, I noted a
few values that are significantly high. I was using a modern DMM with 10M
input impedance so not sure if that could be the problem but I doubt it would
account for that much difference but who knows.
The voltages were off a bit as well but not nearly as much as the resistance
values. I suspect the radio will still work with wacky voltages but the R
values really have me thinking I have work to do.
Thanks,
Barry - N4BUQ
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