Most probes have a discrete low-value resistor built into the probe tip extremity, located at the tip in front of the 9M\$\Omega\$ divider resistor and x1/x10 switch. I measured the end-to-end resistance of some probes (in x1 setting) and found values in the range 180\$\Omega\$~ 270\$\Omega\$.
This is something that I was unaware of and does not seem to be factored into the Tek diagram of the probe, but if true of the P6062B, then it might explain the reading of 278.88 ohm and would then seem to confirm that the probe is as per spec in all respects except for that shorting link.
Er, no. The explanation below hints at why scope probes are relatively fragile, and why I don't like seeing them compressed into a figure-of-8 with an elastic band.
It is easy to simulate the problems mentioned below with LTSpice and a lossless transmission line. Change it to a lossy line and the simulated results are markedly improved.
From Tektronix "Oscilloscope Probe Circuits, by Joe Weber in 1969, pp14-15, my
emphasisI have ignored one aspect of lossless 50ohm cables.
A lossless cable, as its name implies, is a circuit
with a high Q. A high Q circuit will oscillate
when it is excited by a burst of energy such as a
fast rise pulse. The situation is shown in Fig. 2-5.
The output is a series of sine waves with a frequency
around 60 MHz for a 3.5 foot 50 ohm cable. These
oscillations, often called ringing, will eventually
damp if no further energy is applied to the cable.
If the bandwidth of the oscilloscope is relatively
low, say 30% down at 5 MHz or less, the ringing
will not be seen on the CRT display. If the 30%
down frequency is 10 MHz or more, the oscilloscope
is capable of processing the ringing. Then the CRT
display will include the distortion introduced by
probe cable.
We find the circuit of Fig. 2-3 is unacceptable for
measurement systems with 30% down frequencies
greater than 10 MHz. About a generation ago, 10 MHz
was the state-of-the-art for high speed oscilloscopes.
The design of passive probes had to change to
provide an acceptable device for coupling signals to
these oscilloscopes. We can suppress the
oscillations by adding resistance to the circuit
which lowers circuit Q. Since we want good
transient response, the resistance must provide
critical damping. Ergo, let us place an unbypassed
resistor in series with the cable. The location of
the unbypassed resistor will affect transient
response. Placing the resistor at the oscilloscope
end of the cable results in a slow transient
response. Placing the resistor at the probe end of
the cable or using equal value resistors at both
ends of the cable gives some improvement in transient
response.
We can obtain the resistance required for critical
damping in another manner.
Let us replace the high
conductivity center conductor of the 50 ohm coax with
a resistance wire such as a nickel-chromium. Assume
the resistance required for critical damping is
350 ohms. Then a 100 ohms/foot wire will satisfy the
resistance requirement of a 3.5 foot cable. We now
label this coax as an R cable to differentiate with
a 50 ohm or Z0 cable.[1] When a Z0 cable is replaced
with an R cable, critical damping is realized and
we
will have a faster risetime than is obtained with
series damping resistors.
[1] R cable is also called lossy cable to differentiate
from lossless or Z0 cable
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