Question: re ....Work around to 1 meg in Heath IM-28 probe?

[Ed Richards]

Tom;
I was faced with the same situation and solved it by using two probes,
one straight through for AC and OHMS; and one with a 1 meg resistor in
the tip for DC measurements. You can buy the probes, phono plugs,
alligator clamps at Radio Shack. They come one red and one black. I use
the red for AC and OHMS and the black for DC. I use shielded wire or coax
with a single wire coming from the phono plug for ground. I use the
original wire from the probes. Be certain to get the probes you can take
apart. Some are moulded. A bit of a nuisance having to change probes, but
better than putting the 1 meg resister in the meter.

You are correct. There is a reason for putting it in the tip of the
probe....to get some isolation right away before the capacity of the
cable loads down the circuit under test. Good luck.

Ed Richards

On Thu, 2 Mar 2000 00:46:55 -0500 Tom Bridgers <tarheel6 at EMAIL.MSN.COM>
writes:
> Hi....
>     Just found out that instead of having to use the probe with the
> built-in
> switch that permits the switching in a 1 meg resistor for use on DC,
> that a
> probe without a switch can be used (in other words, using a direct
> through
> probe).  What you do is modify the connections to the function
> switch,
> internal to the IM-28, so that the 1 meg resistor is switched in the
> circuit
> whenever the VTVM is switched to either DC+ or DC-.
>     My question is this:  I thought having a 1 meg resistor in the
> probe was
> necessary for the probe to be "isolated" from the VTVM...  What is
> the
> theory behind this?  And at day's end, will the VTVM function
> properly (give
> just as accurate results) with the 1 meg resistor now internal to
> the VTVM
> and approximately 36 inches from the probe?
> thanks for any info you could shed on this..
> -tom   KE4RHH

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