Bad probe-what's wrong?

[et...@whidbey.com]

I got my new to me BK precision 4001A function generator and I
thought I would see what it does. Tried looking at square waves but
they weren't square. The probe is a Probe Master 100 MHz. So, trying 3
other probes the display shows nice square waves. Using the output on
the TEK 465B 'scope itself also shows nice square waves.
The display should show horizontal lines 1 division long and
separated by 1.5 divisions, according to the dial settings on the
'scope. But instead the lines sweep up and down from the center and
the closest points of the lines are only about 1/5 of a division
apart.
I tried adjusting the probe by turning the adjusting screw with an
insulated screwdriver, and although it does flatten the shape some it
does not fix it.
So the probe must be trash but what could cause this behavior?
Thanks,
Eric

[4ctest...@gmail.com]

Do you mean you're seeing more of a triangle wave with 1/5 div peak to peak?

[Paul Drahn]

Perhaps the prob has been repaired and the wrong coax used. Probably
should be 95-100 ohm.

Paul

[Mike]

First thing to do is describe the experiment in enough detail to think
about it.
What model probe master? link to specs?
3 "other" probes? More description might be helpful.

Divisions??? Frequencies are important. Sweep rate?
Not at all clear what you mean by separated by divisions and up and down
from center. Are you saying that the DC levels using the good probe
are not centered around ground but the bad probe is?
AC coupled looking?

What do you measure OHMS from the tip of the probe to ground with the
probe connected to the scope with the input set to DC?
What OHMS do you measure across the BNC with the tip shorted? Tip Open?

Can you get the BNC end apart to measure the cable resistance?

TEK scope probes had a resistive center conductor, but it wasn't
a huge number relative to the input impedance. probe master is unknown.

I'm betting that you have an open connection or a fried resistor
somewhere in the tip or compensation box.

If you know anybody with a TDR, they could make short work of it.

Check the 9Meg resistor in the tip. Depending on what's in the compensation
box or whether the probe has selectable 1X 10X, you might be able to
measure it without disassembly.

If you can disconnect the center conductor of the cable at both ends,
measure the capacitance at each end. If not the same, the ratio of
capacitances give some idea where the break might be.

[et...@whidbey.com]

On Thu, 24 Jan 2019 13:27:38 -0800 (PST), 4ctest...@gmail.com
wrote:

What I mean is that if the wave was a proper shape there would be two
horizontal lines 1.5 divisions apart. These lines have a shape that is
a curve that starts fairly steep and then gets closer to flat. At the
point where the curves are closest to flat I would expect to see 1.5
divisions separation. but it's more like 1/5 of a division.
Eric

[et...@whidbey.com]

On Thu, 24 Jan 2019 13:38:54 -0800, Paul Drahn <pdr...@jodeco.com>
wrote:

I bought the probe new many years ago and have never seen any reason
to repair it. There is no obvious damage. Could dropping it have
caused the problem?
Eric

[Terry Schwartz]

Sounds like it's AC coupled.

[gghe...@gmail.com]

On Thursday, January 24, 2019 at 2:23:49 PM UTC-5, et...@whidbey.com wrote:

Hmm well I'm not much of a probe guy, but your probe has to be compensated
for the input impedance (R and C) of the 'scope. It could be that the
probe is not designed to work with the input of your scope.
Is it a x10 probe?

George H.

[gghe...@gmail.com]

Oh, it worked in the past? Or maybe you've never had a good square wave
to check it with?

GH

[tabb...@gmail.com]

So if I guess correctly the probe is suffering poor hf response. Without any info from you on your test freq or the probe's specs what can one say? Some probes have very low f limits, and it varies greatly from x1 to x10.


NT

[et...@whidbey.com]

Yeah, it worked in the past using the square wave output on the
'scope.
Eric

[Ron D.]

1. The adjustment only works in anything but x1 mode.

2. The adjustment corrects the squareness of a square wave. It makes the divider resistive.

3. Probes are designed for a certain input Z e.g. 1 M 22 || pf and freq response
Make sure the scope isn't 50 ohms.

4. For the adj, the scope should be DC coupled.

5. Scope could be in UNCAL mode. Freq or voltage.

6. Check voltage with a DC source. Freq: Try the AC line for frequency.

[et...@whidbey.com]

That DC couplng migt be the problem. It was mentioned in another post
too. Tomorrow I will check things again with the 'scope properly set
up and report back.
Thanks,
Eric

[et...@whidbey.com]

It is. DC didn't seem to change things but I don't know. Tomorrow I
will have time to set everything properly and will report back.
Thanks,
Eric