VTVM probe modification

[Bob Bruner WB4TAJ]

The reson they went to the trouble of building the 1 megohm resitor into  the 
probe body is to isolate the circuit under test from the capcitive effects  
of the probe and cable.  Measurements of high impedance circuits and  tuned 
circuits where a VTVM is advantageous would be the  most affected by  removing 
the resistor from the probe.  Further, if you add this resistance  internally 
inside the meter, then any accessory probes, such as RF detector  probes, high 
voltage probes, etc, that you use with the instrument will have  their 
calibration affected by this resistor to the tune of nearly 10%  error.  These probes 
are designed assuming they will be looking into the 10  megohm basic 
resistance of the instrument, not 11 megohms.  
   DC probes on VTVMs are always made of shielded wire, because  stray pickup 
on the wire ln lower ranges can cause inaccuracies in the  readings.  
"Straight through leads" will not provide the necessary  shielding.  The 1 Megohm 
resistor is necessary inside the probe to isolate  the capacitance of the 
shielding from the circuit under test.  Instrument  Isolation is so important that 
oscilloscopes, where bandwidths and  sensitivities are even greater,  are almost 
always used with a TEN  megohm or greater resistor in the probe.
   The slight disadvantage of having to put the resistor inside  the probe is 
more than offset by a number of advantages, and removing it can  cause 
practical problems which will result in imporoper operation of your  VTVM.
 
Bob Bruner
W9TAJ

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