[ARC5] Meter Calibration and HV probe

[Fuqua, Bill L]

  The IM-10 and HP-410 both have separate range dividers and calibration pots for AC and DC.
The DC probe resistance does not have anything to do with the AC measurements.
  Lets say you are checking to see if the AVC voltage is going to all the IF, and RF stages of a receiver.
If you did not have the resistor in the probe you would detune the grid LC circuit and in turn change the 
DC voltage you are measuring due to a decrease of signal to the detector. Or if you were adjusting the coil on
a crystal oscillator  in a receiver and had the probe on the grid, if the resistor was not there it could seriously
affect the end result or even kill the oscillation altogether. 

73
Bill wa4lav
 
________________________________________
From: arc5-bounces at mailman.qth.net [arc5-bounces at mailman.qth.net] on behalf of brianclarke01 at optusnet.com.au [brianclarke01 at optusnet.com.au]
Sent: Sunday, August 04, 2013 4:21 AM
To: Ken Gordon; ARC-5 List
Subject: Re: [ARC5] Meter Calibration and HV probe

The reason for the 1 M-Ohm in the probe was to give approximately the
same response to DC and AC - the 1.11 form factor of a sine wave comes
to mind. And the AC / DC switch was right on the probe to short out
the resistor.If you switched the resistor in and out, the difference
would be 10% not 1%, as hinted in your third paragraph.
73 de Brian, VK2GCE.

OnSat, 03 Aug 2013 21:58:03 -0000, Ken said:

 On 3 Aug 2013 at 16:38, D C _Mac_ Macdonald wrote:

 > Somewhere in the recesses of my memory I seem to remember that the
11
 > Megohm rating of so many VTVMs "back in the day" consisted of 10
Meg
 > in the instrument and another 1 Meg resistor in the probe!

 You're absolutely correct, Mac. I have two Heathkit VTVMs here, which
I use regularly, and
 both are set up the same way: 10 meg in the instrument and 1 meg in
the probe.

 Someone some time ago published on the web a modification to include
the 1 meg inside the
 instrument instead of in the probe. However, that turned out to not
be such a hot idea after
 all.

 Also, I have two Heathkit HV probes here (up to 30 KV) and a Simpson
version. I have used
 those with a couple of different versions of DMMs and the voltage
readout was always very
 close to the readout when connected to a Simpson 260. In fact, it was
close enough that I
 never bothered about "calibrating" it any closer. The difference
between 10KV and 10.1 KV
 isn't enough to worry about at that level.

 Ken W7EKB

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