vtvm test lead

[Malcolm Leonard]

If we are talking about putting the 1 meg inside the VTVM case,connected to 
the internal rotary switch this completely defeats the whole purpose of the 
1 meg resistor.It *must* remain in the probe tip because its function is 
twofold as follows:

1.It greatly reduces the effective overall  input capacitance presented to 
the circuit under test by the probe.
2.It compromises part of an r-c input filter which filters out any high 
frequency components that may appear in the dc circuit under test.

The above functions are deleted if the resistor is put inside the VTVM as 
the instrument must "see" that 1 meg resistor in the tip itself first.
We have discussed this in the past in this listserv,quite some time ago.

Please see page # 37 in:
"How To Use Test Probes"by Ghirardi and Middleton.Rider books,1954.
This information is often given in VTVM owner's manual,also and has appeared 
in many magazine articles in years gone by.
Malcolm Leonard
----- Original Message ----- 
From: "Ron" <ra.ah at rcn.com>
To: "Malcolm Leonard" <radiorepair at MINDSPRING.COM>
Sent: Tuesday, May 03, 2005 8:56 AM
Subject: Re: [HEATH-TEMPE] vtvm test lead


>
> Malcolm is correct in his advice to use two low cost
> probes instead of the switching probe. However, it
> is also possible to use just one such probe, and
> build the 1Mohm resistor into the unit, in a way
> that switches it in and out of the input path by the
> function switch (I'll be happy to post detailed
> instructions if anyone wants them). This halves the
> cost of the probes and makes measurements easier.
>
> -- Ron
>
>
> Malcolm Leonard wrote:
>> For ordinary (not  RF demodulator)you can use two home brew probes.One 
>> contains a  1Meg resistor at its tip.It is used for dc only.The second 
>> probe has nothing at the tip.Use it for AC and ohms.
>>
>> Mouser sells the correct hollow probes that will accept RG58/AU cable.Do 
>> **not** use RG58 without the "AU" designation.Much too stiff to use on 
>> the bench.
>>
>> Phone jacks need to be ruggedized military PL55 types to get the proper 
>> spacing between tip and body to avoid arcing.Fair Radio Sales and Surplus 
>> Sales of Nebraska used to sell these military types;probably still do.
>> Malcolm Leonard
>> ----- Original Message ----- From: "Jack Schrader" <w1jhs at MIDMAINE.COM>
>> To: <HEATH at LISTSERV.TEMPE.GOV>
>> Sent: Monday, May 02, 2005 10:50 AM
>> Subject: [HEATH-TEMPE] vtvm test lead
>>
>>
>> Thanks for reading this note.
>>
>> I recently purchased a Heath IM-13 without the test lead. Anyone in radio 
>> land have a test lead they would like to sell. In particular one that 
>> could be used for rf with the switch in it and a 1/4 " phono plug on one 
>> end.
>>
>> Thanks Jack
> 

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