Re: P6137 scope probe repair

Tom Lee

Hi Jared,

You've interpreted those measurements the same way I would. That doesn't mean that they're right, but in the worst case you at least have a comrade in wrong. The 20pF reading you got from the probe end is probably not quite quantitatively reliable, but we'll take it as "much bigger capacitance than 3.3pF", which is all we really need. Your break is indeed at the scope end, so that's good news.

The low-frequency capacitance per length of scope cable is typically (and very roughly) around 100pF/m, so a 3.3pF reading implies that the break is somewhere within about an inch of the scope end, give or take. You should be able to fix it without having to cut off more than about that much cable, which should not materially degrade the probe's response.


Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070

On 1/16/2021 04:07, Jared Cabot via wrote:
I have a small TDR unit, but it i mainly for longer runs, I'll give it a go though. I also have a 4276A LCZ meter I can use though.

Using the 4276A, I get 3.3pF at the scope end, and the probe end is not really able to be read properly, the 4276A freaks out a bit but shows 0.02nF, and my 34461A goes out of range.
I remember reading that oscilloscope probe cables are deliberately resistive or something, maybe that could be causing the weird reading. If that is the case, I would assume that the break is near the scope end of the cable as there isn't enough of the cable there to throw readings off.
It would make sense too, if the cable was pulled while attached to the scope, that is where the strain would likely be concentrated.

Sound like a reasonable assumption?

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