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4 P6249 probes passed through my hands that have 1 MHz oscillation. On one probe, this oscillation only shows up after a 5 minute warmup. The oscillation can be as large as 467 millivolts on one probe which makes it mostly useless. On another probe the oscillation is smaller and manageable. The probes that have oscillation meet rise-time specs and have no dc offset. I have one very expensive P6249 probe that has no hint of oscillation.

I have two TDS 694C 3 GHz oscilloscopes and needed an active probe with higher impedance than my 500 ohm passive probes. The TDS 694C is wired to power the P6249 4 GHz probe with +/-5V and +/-15 supplies and a data and clock pin interface to the TDAS694C.. I have the Tek manual which only shows a block schematic for the part that connects at the oscilloscope but not what is actually in the active probe tip. The oscillation shows up on all eight of the inputs for the two TDS 694C scopes I have.

Is there any information about what is in the probe tip such as a schematic? Is there any way to open the sealed probe tip? Would reducing the +/- voltage to the probe tip at the scope connection, which can be opened, help the problem?
All I hear from repair places is that they do not work on this probe. Local sources of the probes are asking more for the probe than I paid for my TDS 694C scopes.

I have the 1 GHz and 1.5 GHz versions of the probe which do not use the +/- 15V supply and have no hint of oscillation, but I do need the bandwidth for my photon counting business.
www.marinaphotonics.com.
I designed the PV16 1.5 GHz 65db gain amplifier discriminator for my photon counter which is in a 29mm cube and does not oscillate, so I have some knowledge of electronics. At this point I admit that I am just stubborn and want to fix the probe problem rather than go to another probe.
Jerry the fizzilist
PS that's what happens to physicists when they get older

Hi,

Perchance a silly question, but as a fellow fizzilist, how long is the ground lead? Maybe a resnoant problem?

I wish it were so simple as a ground lead. If you put the probe tip on the
scope calibration ground, it still oscillates. I tried all of the
combinations of direct leadless ground connections to the calibration and
scope ground I can think of, and it has no effect on the oscillation. I
also tried all eight inputs on my two scopes and cleaned the connections on
the scope with de-oxit compound. No effect.

Eventually I will cut into the probe tip and probably destroy it. Perhaps
if I knew how it was put together I could avoid destroying it. I suspect
that the Fet input transistor gets damaged in some probes which leads to
oscillation. The fact that I have two probes without the oscillation
means that it does not have to happen.
In any case, this probe is far far superior to my old P6201 probes.

Any chance you are interested in math and continued fractions? I have
some interesting unpublished formulae.
Jerry

On Mon, Aug 12, 2019 at 4:27 PM Lawrance A. Schneider <
llaassllaaass@...> wrote:

Hi,

Perchance a silly question, but as a fellow fizzilist, how long is the
ground lead? Maybe a resnoant problem?

On Aug 12, 2019, at 1:36 PM, wjlentz@... wrote:

4 P6249 probes passed through my hands that have 1 MHz oscillation. On

one probe, this oscillation only shows up after a 5 minute warmup. The
oscillation can be as large as 467 millivolts on one probe which makes it
mostly useless. On another probe the oscillation is smaller and
manageable. The probes that have oscillation meet rise-time specs and
have no dc offset. I have one very expensive P6249 probe that has no hint
of oscillation.

I have two TDS 694C 3 GHz oscilloscopes and needed an active probe with

higher impedance than my 500 ohm passive probes. The TDS 694C is wired to
power the P6249 4 GHz probe with +/-5V and +/-15 supplies and a data and
clock pin interface to the TDAS694C.. I have the Tek manual which only
shows a block schematic for the part that connects at the oscilloscope but
not what is actually in the active probe tip. The oscillation shows up
on all eight of the inputs for the two TDS 694C scopes I have.

Is there any information about what is in the probe tip such as a

schematic? Is there any way to open the sealed probe tip? Would reducing
the +/- voltage to the probe tip at the scope connection, which can be
opened, help the problem?

All I hear from repair places is that they do not work on this probe.

Local sources of the probes are asking more for the probe than I paid for
my TDS 694C scopes.

I have the 1 GHz and 1.5 GHz versions of the probe which do not use the

+/- 15V supply and have no hint of oscillation, but I do need the bandwidth
for my photon counting business.

www.marinaphotonics.com.
I designed the PV16 1.5 GHz 65db gain amplifier discriminator for my

photon counter which is in a 29mm cube and does not oscillate, so I have
some knowledge of electronics. At this point I admit that I am just
stubborn and want to fix the probe problem rather than go to another probe.

Jerry the fizzilist
PS that's what happens to physicists when they get older

Hi, there is a peculiar property with emitter followers and FET source followers. Under some circumstances they will provide a negative input impedance. This with a certain source impedance will start an oscillation. To prevent this from taking place a series RC link is normally placed from base or gate to ground. This might be a hint for you.
G?ran

Perhaps the input circuitry has been blown out by excessive voltage which
is a good hint for what is wrong.
I wish I had a schematic of what is in the unit before I cut into it. I
also wish I were not so stubborn about fixing it as it really is not worth
the time and effort.
Since the probe has a 4 GHz bandwidth (still) even though it oscillates, I
am not sure how to proceed.
Thank you for the suggestion.

Does the probe calibrate? I am new to the 694C and have two probes that calibrate and one which does not. All have oscillation similar to what you've described, with the more pronounced oscillation on the probe which will not calibrate (and I cannot effectively set offset according to documentation on this probe either). Does anyone have a screenshot of a 694C on the calibration pins at various scales they'd be willing to share, so I can use for a comparison? I really don't know what to reasonable expect from a working vs. a non-working P6249 probes.

I believe I've narrowed the issue to the probe tip, as you've suggested. Seems hitting it with a shot of cold air will calm the oscillation for a short period of time. The probe tip seems to be an amplifier microcircuit in the P6249 with 4 capacitors. The microcircuit had some red/rust color material on the traces, and I am leaning to corrosion as a potential contributor to the problem. Capacitors or microchip degradation may also be contributors. The tip is very well contained within the housing, and easily destroyed in attempting removal. There is a small, maybe 1/8" hole in the housing covered by a piece of tape. The traces of the circuit can be accessed at this point, with the remainder of the microcircuit well contained in the housing. It does not seem to be easily repairable. I have not been able to make further progress, and am not willing to sacrifice my remaining two probes, as even with the osculation, they are usable at higher frequencies.

'Osculation' ????

(Sorry, couldn't resist it !!!)
Tim


On Wed, 9 Sep 2020 at 00:52, fauffing via groups.io <fauffing=
[email protected]> wrote:

I believe I've narrowed the issue to the probe tip, as you've suggested.
Seems hitting it with a shot of cold air will calm the oscillation for a
short period of time. The probe tip seems to be an amplifier microcircuit
in the P6249 with 4 capacitors. The microcircuit had some red/rust color
material on the traces, and I am leaning to corrosion as a potential
contributor to the problem. Capacitors or microchip degradation may also be
contributors. The tip is very well contained within the housing, and easily
destroyed in attempting removal. There is a small, maybe 1/8" hole in the
housing covered by a piece of tape. The traces of the circuit can be
accessed at this point, with the remainder of the microcircuit well
contained in the housing. It does not seem to be easily repairable. I have
not been able to make further progress, and am not willing to sacrifice my
remaining two probes, as even with the osculation, they are usable at
higher frequencies.

Maybe he's of the KISS lobby :)

While this topic is related to the P6245 and repair, I think there is some included information I think is relevant to this post: