vtvm question??

[Deane D McIntyre]

In message  <34F770AF.BFC45201 at sprynet.com>  writes:
>
> greetings
>  does anybody know why heathkit on the
> im-13 vtvm put the extra 1 meg resistor in the
> probe and not in the unit,without the probe
> the vtvm is only 10 meg

This is actually true for most VTVM's. Having the 1 meg resistor in the
probe helps to isolate the probe (and this the circuit under test)
from the VTVM. The resistor is only used on the DC volts ranges.

My IM-13 came without the proper probe, (as did my RCA Voltohmyst).
It is possibe to adjust the DC cal control to compensate for the lack
of the 1 Meg resistor, but a better solution is to make a probe.
In the case of the RCA Voltohmyst, which has seperate inputs for
the DC Volts probe and the AC Volts/ohms probe (which does not use
a resistor) I made a DC volts probe by hiding a 1 Meg resistor
inside a standard probe. For the IM-13 this will not work as only
one input is used, with a switch on the probe to short the 1 meg
resistor for AC Volts/ohms measurements, unless you make up a
seperate set with 1/4 inch jack for DC vots measurements. In my
case I "borrow" the probe from my IM-11 if I want to use the IM-13
(a better instrument than the IM-11 by the way).
>
> when reading articles about rf probes
> they mention the probe will work with
> 11 megohm vtvms however without the probe that
> comes with the im-13,the input is only 10 megohm
> which will affect the reading,
> maybe the 1 meg should be put into the rf probe
> when using the im-13...

This should work fine

73, Deane D McIntyre VE6BPO
dmcintyr at acs.ucalgary.ca

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