Electronics > Repair
Tektronix TCP202 current probe repair - Schematic and suggestions needed
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esc:
As for my measurements, after taking the probe head apart, I realized that my probe head has a newer transformer, designed for the TCP305 PN  120-2032-XX.
The transformer has 04-20-44-40 on it. Maybe that's why the discrepancy is there. 
MarkL:
Your 120-2032-XX is the same part number as my TCP202 #1.  Within Tektronix part numbers, the NNN-NNNN are the main part number, and the -XX is usually the revision.  According to the TekWiki page, the last number could also be a quality or a speed grade.

But given that these both came from TCP202 probes with the same ratings, I think it's less likely that the same major part number would have such different resistance readings, and by implication that the problem is elsewhere other than the Hall sensor.

So at the moment I think the Hall sensor is the prime suspect.  However, I can't come up with a good explanation of the strange frequency response.  Maybe the lower impedance has affected parts of the active circuitry that it's connected to.

Instead of just resistance, I will see if I can come up with an out-of-circuit dynamic test or two so you can compare to your Hall sensor.


vtp: Thanks for the nice pictorial on the Hall effect.
MarkL:
Attached is some frequency characterization for the Hall sensor.  And although you don't seem to be having a problem with the higher frequencies, I also did the coil up to 1MHz since I already had the sweep set up.

The Hall sensor was supplied by +/-5V with each leg in series with a 220R resistor, as previously described by user vtp.  I also confirmed I have the 220R resistors on both my probes.  The output voltage was measured differentially across the two outputs.  The outputs have an offset above ground of about +236mV, and between them about 4.7mV.  No load was placed on the output except the probing.  A test of a 1k load did not affect the output significantly, so it seems the Hall output is a fairly low impedance.

Likewise, the coil output was measured differentially across its two outputs, but because it's floating it won't matter if you do it single ended.  The coil output was loaded with a 50R resistor.  I don't know if that's the actual impedance seen by the coil, but at least it will serve as a basis for comparison.

The signal generator was set to 16Vpp with offset 0V.  One leg of the output was looped through the probe transformer halves and terminated in a 50R load.

I don't have a Bode plot on my scope, and I didn't feel like writing one.  So, what you see horizontally is a full sweep at the noted start and stop frequencies, and then a closer look at the start, stop, and 3dB point.  The scope has an RMS ratio function between two traces, so it was easy to find the 3dB point by spinning the horizontal delay until the ratio was down 3dB from the maximum.  The sweeps include 20kHz, which is where you were having the most severe drop-off.

The DC response of the Hall sensor was 44.4mV/A, measured at 1A.

Hopefully this info will help you evaluate your Hall sensor.

One thing you should check, if you haven't already, is that your TekProbe interface has all the +/-12V and +/-5V supplies working. You could also verify the Hall +/-2V supply at the head.  But given the lower impedance of your sensor, it's probably going to be low.

Also make sure the Hall and coil are isolated from the sensor's metal shell.


EDIT#2:  Ever have one of those days?  I discovered my connection to the 50R load was bad throughout the AC measurements.  I've removed all the previous screen shots and re-posted updated full sweeps for the Hall and coil, both 100Hz to 100kHz.  I didn't mention it before, but it should be obvious the frequency on the X-axis is logarithmic.

I'll just tell you the numbers derived instead of posting all those detailed screen shots again:

         Frequency at reduced signal
           level from maximum (Hz)

           -3dB    -6dB   -10dB
          ------  ------  ------
  Hall      2.3k    4.0k    8.1k 
  Coil      2.3k    1.3k     775 

  Hall @ 100kHz = -21dB
  Coil @ 100 Hz = -24dB

The sweep envelopes now look right and the numbers make a lot more sense.
esc:
Thanks MarkL for the info.

Since your transformer in TCP202#1 and mine are both the same part 120-2032-XX, the resistance measurements should be close, and they are not. So I agree, it is likely that the Hall sensor in my transformer is the problem. The interesting part is that my probe measures DC accurately. Maybe the small resistance between the +3V and -3V and the two 220R series resistors causes a too small supply voltage for the Hall sensor, and even though it "works", but causes the anomaly at 20KHz?
Tantratron:

--- Quote from: esc on October 17, 2020, 06:36:40 am ---Maybe the small resistance between the +3V and -3V and the two 220R series resistors causes a too small supply voltage for the Hall sensor, and even though it "works", but causes the anomaly at 20KHz?

--- End quote ---
If you have an anomaly circa 20KHz, this could be related to the electronics amplifier and mixer, namely the 2 responses (current transformer and hall sensor) which are fused or hybrid. If you look the service manual of the A6302 and AM503A, it mentions the cross over region. It seems clear visually that the TCP202 probe head is same technology or design than the A6302, maybe check more circa pages 3-6 of the manual. In the case of the TCP202, the compensation box contains the amplifier and probably the crossover electronic management.

System Behavior in the Crossover Region: When the frequency of the current being measured is in the vicinity of the crossover region (approximately 20 kHz), part of the measurement appears at the Hall device and the other part appears across the coil. The waveforms in Figure 3 8 illustrate how these two components combine to make the total measurement. Waveform (a) shows the current being measured. The fre quency of the current is in the crossover region of the probe. Waveform (b) shows the low frequency bucking current produced from the Hall device output. Waveform (c) shows the high frequency component that is induced directly into the coil from the current source. Waveforms (b) and (c) are summed together in the probe coil and amplified by the AM 503A amplifier. The amplifier output is connected to a measurement instrument, such as an oscilloscope, where a waveform similar to waveform (d) appears. Waveform (d) conforms to the original signal shown in waveform (a).
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