[R-390] Measuring Resistance Values

[Jim Whartenby]

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 <n4buq at knology.net> 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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