Heathkit VTVM probes

[Bob Bruner WB4TAJ]

The 1 megohm resistor in the Heathkit VTVM probe (and all other VTVM probes)
is there to provide isolation between the capacitance of the shielded cable
and the device under test.  There are many circuits where stabbing 80 or 90
picofarads of capacitance across the circuit would cause enough roll-off or
detuning to invalidate the DC measurement.  Reconfiguring the instrument by
putting the resistor inside would work for pure DC measurements and probably
audio work, but it could have important consequences for radio work.  Over
the years Heathkit (and others) used varying schemes for getting the 1 meg
resistor "out of the way."  The simplest and most reliable is using a
separate probe for DC and AC/ohms.  Early VTVMs of all manufacturers had
three probes.  Slide switches were later used as were mechanical arrangements
that rotated the head of the probe to switch the resistor. Older RCA VTVM
probes were a plug together arrangement where DC isolation heads, crystal
detector heads, peak to peak heads, etc, were plugged onto the direct probe.
If the original probe had a switch in it, the best therapy would be to employ
one's ingenuity to put a switch in a probe.

Bob Bruner
WB4TAJ/9

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