Tektronix 466 oscilloscope HF response

[David Hess]

I asked about the input resistance; so the oscilloscope inputs are completely normal and not damaged.

I have never even heard of a probe modified like that before.

[lyonsk]

This is something that I was unaware of and does not seem to be factored into the Tek diagram of the probe,

Why do you think the pdf is titled "The Secret World Of Probes". 
There's lots of links to probes stuff in tggzzz's sig. above.

If we make it bigger the voltage should go up so I tend to agree with StillTrying - it should get worse.

I'm glad somebody does!

Before you condem or dismantle the probes, get yourself a 1M resistor, connect directly between the 1kHz and the BNC input's center, and measure the 0% to 63% risetime of the edge to check the scope is ~20pF.

I'll post my attempt when I find it. - Found one.

I adjust the Y amplitude so that the height of the wave covers exactly 6 divisions and I can then judge the 63% amplitude position from the 4th up 66% grid line.

8.6us / 0.5M = 17.2pF which is close enough to the 17-18pF specs. for me!
You can also 'measure' the capacitance of probes this way.

I tried this approach and result is 12ns -> 12ns/0.5M = 24pf. That should be fine considering that I don't have anything to check the time base calibration with.

Now to get my paws on sime nice function gen...

Thank you guys for all your help.

[StillTrying]

12ns/0.5M = 24pf

us  24pF is in the right ballpark!

I can't think what device would have had 1M+~80pF of input impedance either.

This thread has provided me with an interesting insight into probes!

Did you see my attempts to make a DIY X1 High BW Passive Scope Probe, it's a "work in (slow)progress".
https://www.eevblog.com/forum/beginners/diy-x1-high-bw-passive-scope-probe/

[David Hess]

I can't think what device would have had 1M+~80pF of input impedance either.

A really good voltmeter input could have that low of an input capacitance.  The highest capacitance 1 megohm instrument inputs I have seen are 47pF and operate up to 1 MHz although they could be faster.

Maybe we are thinking about this the wrong way.  What if the probe was intended for a different input resistance yielding a different attenuation factor?

[StillTrying]

Maybe we are thinking about this the wrong way.  What if the probe was intended for a different input resistance yielding a different attenuation factor?

I think the LF and HF would be matched on a 3M/20pF scope giving a X3 probe.
Maybe all the extra HF capacitance was to be used on a very long probe cable of 10m 35ft, someone thought that was just a misprint and put the probes and boxes on a 1.2m 3.5ft cable!
After much googleing the purpose of the mod is still a mystery.