vtvm test lead

[Ron]

Malcolm,

Since this has been the subject of a previous discussion,
I shouldn't rehash; however, since I didn't take part in
that discussion, let me have my say, after which I forever
will hold my peace...

Your 1st point: the yes, moving the 1Mohm resistor does
increase capacitance seen by the circuit under test, but
not 'greatly' and only in DC MODE, where it makes little,
if any, difference. If a low capacitance measurement is
needed, a Heathkit VTVM is the wrong instrument to use,
regardless of where the resistor sits. This could be a
valid point in maybe 0.01% of measurements...

Your 2nd point: if the resistor is part of an RC filter,
then my IM-13 schematic must be wrong, since I don't see
the 'C' part (unless you are referring to the cable
capacitance, which is too puny to matter). The input
filter, made up of 3.3M and .005uF at the 12AU7 grid, is
unaffected by the mod.

A 3rd point, unmentioned, is of some interest: the VTVM
is rated to measure up to 1500V, DC and AC. One must
assume Heath chose input components (probe cable, phone
jack and function switch) that withstand 1500VAC. However,
at DC, only 10/11 of those 1500VDC (about 1370V) are
actually applied, while after the mod the full 1500VDC
are applied. Again, I consider this a nit; I doubt the
additional 130VDC will 'break the camel back'. And who
uses a VTVM to measure high voltages, anyway?

To summarize: If an original probe is handy, it should be
used. However, the mod is an acceptable substitute and
-- mentioned issues notwithstanding -- does improve the
convenience of using the VTVM.

Just my $ 0.02

-- Ron


Malcolm Leonard wrote:

 > 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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