VTVM probe modification

[Malcolm Leonard]

Thanks for bringing this up.I had a discussion about this with the original 
poster who had suggested moving that resistor but I failed to convince him 
that such a modification was not advisable.
I also expressed my concern with that mod on this listserve but something 
went wrong with my post and apparently,it did not appear.
I had envisioned dozens of Heathkit VTVM owners making this change and that 
thought kind of shook me up a bit.
Malcolm Leonard
----- Original Message ----- 
From: "Bob Bruner WB4TAJ" <RABRUNER at AOL.COM>
To: <HEATH at LISTSERV.TEMPE.GOV>
Sent: Saturday, May 07, 2005 1:48 PM
Subject: Re: [HEATH-TEMPE] VTVM probe modification


> 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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Products bought, sold or traded here is the responsibility of the
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for losses or misrepresentations of any kind.  Buyer beware!

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