[Boatanchors] Heathkit IM-18 VTVM probe

Fri, 13 Feb 2004 19:15:12 -0800

Actually, the issue is the "impedance" of the circuit being measured. If the
point the probe touches is low impedance compared to the input impedance of
the probe, and the probe is "compensated" (that is frequency flat in the
range of interest), then the measurement will be correct. If the circuit is
high impedance, and you remove the isolation resistor, then your readings
will be swamped by the shunt capacitance of the probe cable. Use a scope.
Regards,.....Larry

-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Philip Atchley
Sent: Friday, February 13, 2004 6:58 PM
To: [email protected]
Subject: Re: [Boatanchors] Heathkit IM-18 VTVM probe

Hi,
The only problem with this mod is that it no longer provides "isolation" at
high frequencies.  The 1M resistor was installed in the probe of this and
other VTVM's to isolate the tip from the high capacitance of the probe lead.
This allowed you to read voltages right at the grids and plates of
oscillators etc with minimal effect on circuit operation.  Elimination of
this isolation makes this probe useless for many tasks that you would want a
"real" VTVM for.  You might as well use a DVM (Digital multimeter).

73 from the "Beaconeers Lair".
Phil, KO6BB

>
>
> > Hello, list.
> >
> > I have a Heath IM-18 VTVM that I'd like to obtain a probe for, or
> > perhaps make.  This is one of those probes that has a little switch on
> > it to switch a 1 MOhm resistor in/out of line (similar to the RCA
> > Voltomyst probes).
> >
> > Any suggestions/ideas?
> >
> > regards,
> >
> > Mahlon - K4OQ
>
> Hi Mahlon,
>
> One of the neatest suggestions that I've seen in a long time can be
> found on
> http://www.theheathkitshop.com/vtvmprobefix.html
> Using the step by step instructions, you can easily do away with the
> "special" probe and use an ordinary straight-through probe instead.  The
> Heathkit probe was always a source of problems for me and this solves
> them all.
> Hope you find it interesting if not helpful.
>
> 73,
> Ken K4XL
> [email protected]
> *** BoatAnchor Manual Archive ***

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