Electronics > Repair
Tektronix TCP202 current probe repair - Schematic and suggestions needed
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MarkL:

--- Quote from: Weston on July 04, 2020, 09:30:53 pm ---...
I saw the "System Aberrations 10% p-p", so  guess this is technically in spec? I guess I am just surprised its so bad.  Talking about this issue with a friend got them to check their AM502 + A6302 and they were seeing a similar response, although not as bad. Based on that + your measurements I guess its just bad compensation and not any damage to any of the active parts. I wonder if something drifted over time.
...

--- End quote ---

I dragged out my old AM503 + A6302 and looked at that too.  I think it has a bad core or a bad mating between the transformer surfaces.  Squeezing the surfaces improves a 10% overshoot problem by several percent.  So, mechanical problems can cause bad waveforms too.



--- Quote from: Tantratron on July 05, 2020, 04:54:18 pm ---...
So far I've never owned any hall-Transformer probe plus as mentioned before I got unlucky with a purchase on eBay-Austria. I've been refunded thanks to PayPal protection but do you guys confirm acquiring TCP202 is still good investment for laboratory use now we find this non-flat bump response ?

--- End quote ---

The small level of non-flatness I'm seeing in my TCP202 was surprising.  I guess I'm going to have to temper my opinion of this probe a bit.

However, I'll also say that in the 20 years I've been using it I never noticed it.  The resulting waveform was always close enough to what I expected to see, and I never required a high level of precision.

The aberrations may be a good question for the TekScopes mailing list on groups.io.  I also don't know if the behavior is typical of other manufacturers' DC current probes.

Weston:
I always considered these probes the gold standard in current sensing so it is a bit discerning to see this behavior. I am still trying to figure out if my probe is within specs or is defective. Based on what others are seeing, if this is the case I am surprised its not a well known thing. I guess, similar to how I have not noticed until now, most people do not probe square currents where this behavior is most evident?

In favor of the argument that these probes are all that bad, this post shows the weird crossover behavior in simulation; https://www.eevblog.com/forum/testgear/p6042a-tek-current-probe-replacement-circuits-an-61-lineartechnology/msg3085619/#msg3085619

On the other hand, upon taking apart my probe and making sure that the surfaces were clean and in contact with maximal force, I was able to reduce my LF overshoot by a just barely perceivable amount. I have heard that the cores are fragile, it may be possible that we all have at least slightly damaged transformers with these ebay acquired probes.

The research group I work in has some more modern current probes based on the same architecture. Next time I am in lab I will have to test and see if I can observe similar behavior.
MarkL:
I think you're right that most people are not usually looking at perfectly square current waveforms.  I know I'm not, which is why I never noticed.  Nevertheless, I still think it's a good test of the probe's signal integrity.

My TCP202 was purchased new by me in 2000 and has never been mistreated.

The A6302 was an ebay purchase, along with two AM503's.  The A6302 has the same behavior on both AM503's, so I'm going to say the issue is with the probe.  The AM503 has four compensation adjustments, but none can bring the output flat.  Or even affect the waveform significantly, for that matter.  I'm going to have to go through the official cal procedure for both of them.

I have a second TCP202 which I will retrieve and test.  It was purchased from another user on this forum in the Buy/Sell section a couple of years ago.  It hasn't seen as much use, but again I never noticed anything because I wasn't testing square current waveforms with it.  It's a newer version with a black cable instead of gray, and the serial number is on a tag on the cable instead of on the probe.  But it's still labeled as a TCP202.

I may also try swapping the comp boxes on the two TCP202's to see if the issue moves.

It's also interesting to note there's no response flatness in the verification procedures or specifications for either the TCP202 or A6302/AM503.  Only BW, rise time, and DC accuracy.  You could have a completely bizarre waveform and still meet the verification (and still be within 10% "aberrations").

The Agilent/Keysight N2893A (15A/100MHz) is similar in ratings to the TCP202.  The specifications are even sparser.  Accuracy is +/-1% of reading +/-1mV (DC or 45 Hz to 66 Hz), and rise time 3.5ns.  They include a Bode plot showing (typical) flat response out to 10MHz and then a rolloff to -3dB at 100MHz.  The amplitude scale is in 10dB increments, which is hardly enough precision to show anything interesting.


If anyone else has a DC current probe from Tek or another manufacturer, it would be great if you could take a few minutes to test it and post a screen shot or two of a square current waveform.  It could help answer the question if non-flatness is par for the course, or if all of us so far have messed up probes.
MarkL:
Ok, here's some screen shots.

Since I mentioned it, first up is the A6302 overshoot.  Screenshots included of normal pressure and then significantly more pressure applied to the transformer halves.  The surfaces are absolutely clean, and under a microscope everything looks perfect.  Maybe there's a tiny crack in the transformer core where it's potted that I can't see.

Second up is some screenshots with the newer TCP202 that I purchased used two years ago compared to the older TCP202.  At a low frequency it's absolutely flat.  I couldn't ask for much better than that.  But a closer look at the edge reveals a ringing problem, and by the looks of it at *two* different frequencies.  Great.  It's still within +/-10% aberration, but barely.

All three current probes are clamped onto the same wire.  The order doesn't affect the waveform shapes.  All the differences appear to be introduced by the probes, and confirmed by the shot that also includes Vin (scope measuring across the 50R load which is creating the current draw).  I must admit it's a bit disappointing.
MarkL:
Here's some more screen shots of a couple other DC current probes, again clamped onto the same wire as the TCP202/AM6302 tests above.

The first is an A6303 which is rated for 15MHz/100A and is also used with the AM503 amplifier.  It has three tweakable compensation adjustments on the probe body and it's possible to get the response very accurate to the actual current.  The rise time is showing a BW of only 13.5Mhz so a little more tweaking is in order.  Perhaps an examination of what these pots do can be applied to the the TCP202 and/or the A6302.

I'm not for a moment suggesting anyone use the A6303 for small circuit probing.  The thing is huge.  I was interested in seeing its response since it uses the same Hall/transformer design.

The second is an Aim-TTi iProber 520 which is rated for 10A but is only 5Mhz.  It's normally used open-ended to measure current in PCB tracks, but also comes with a horseshoe shaped attachment (presumably ferrite) to capture the flux around a wire.  It uses a fluxgate magnetometer instead of a transformer/Hall device.  It has a completely flat response apart from some settling after the edge.

That's it for me.  I'm out of DC current probes.  Screen shots from other DC current probes from Tek and other manufacturers welcome for comparison.

EDIT: Oops - had the iProber set on 500Hz BW limit.  Screenshot fixed.
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