VTVM DC (Isolating) Probes

[Michael Donnelly]

Gang, a few weeks back (before I had E-mail and list access) there was a
discussion about the DC probes for VTVM's. The discussion talked about
the need for the 1 MEG resistor in the probe and why not put the resistor
'inside the case'. And in fact I guess someone said there was a Heath
factory authorized modification to do just that. I am somewhat surprized
this was indeed done. From what I can find out the resistor is in the
probe for 2 important reasons, besides just for impedance match.

I'm going to quote verbatim from a book called 'The VTVM' and written by
Rhys Samuel and published in 1956 by Gernsback....

"Isolating probes are used for the measurement of DC voltages. For
example, DC voltage measurements from 0 to 1000 are made with a shielded
cable terminated with an isolating resistor. The shielded cable
eliminates the pickup of any stray fields about the test bench, which
might otherwise result in inaccurate indication on the VTVM. The
isolating probe resistance prevents the shielded cable from acting as a
capacitive shunt across tuned and high-impedance circuits, such as the
local oscillator in a receiver. If it were not for the isolating effect
of the resistor, connection of the VTVM test leads might detune the test
circuit so greatly that circuit operation might cease altogether. The
cable capacitance is isolated from the probe tip by the 1 megohm
resistor, which cuts down the effective value to approximately 1 or 2pf.
This is a large reduction, as compared with the approximately 75pf of
cable capacitance. Hence, the DC probe can be applied across a local
oscillator circuit without disturbing circuit operation. "
"The equivalent circuit is also a low pass filter. This is an essential
feature, since DC voltages at the grids and plates of tubes often must be
measured in the presence of large AC voltages. If the AC signal is passed
along to the bridge circuit in the VTVM, it might be rectified and cause
an erroneous DC reading. Most VTVM's include another
low pass filter at the grid of the VTVM tube to make certain that no AC
voltage reaches the grid."

So.....I dunno about putting the resistor inside the VTVM. Even modern
solid state oscillators might be affected by the cable capacitance if it
were not for the isolating resistor. I hope I haven't offended anyone but
if I have in any way violated list rules or etiquette please let me know.
I just wanted to pass on some info that I ran across.
BTW....I was able to make up a DC probe for my Heath V-5 VTVM using
readily available Rat Shack parts. If anyone wants that details feel free
to E-mail me.


Mike Donnelly
MJATS at JUNO.COM

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