Repair - Calibration - tektronix P5210 differential high voltage probe

[Tantratron]

This is a new thread under the repair section which attempts to summarize different information, topics from two earlier threads discussing P5205 and P5210 probes under test gear section https://www.eevblog.com/forum/testgear/tek-p5205-hv-differeantial-probe-teardown-btw-what-are-the-red-and-brown-wires/ and https://www.eevblog.com/forum/testgear/repair-tektronix-p5210-high-voltage-differential-probe/

As of today, there does not seem to be any service manual on internet explaining how to calibrate these probes but only a manual showing performance verification. Both P5205 and P5210 seems to have about similar amplifier circuit except the main High Voltage part, see attached different pictures comparing both probes (top and bottom PCB).

One of my P5210 was calibrated prior purchase to USA at Advanced Energy Industries, Inc and the second P5210 purchased at eBay-Turkey had a problem. Initially with the 2nd probe there was no signal at all then later I did found a soldering issue in the high voltage potted part so did reflow the connector line. Good news: it was then probing OK except the calibration issue discussed on test gear thread. In short, I could not get same gain output between the two when sensing 250 KHz induction 250W ballast output. At that time, I was only testing both probes with my tek 2465 oscilloscope to discover the mismatch gain but never thought to try out other frequencies.

This week-end I thought to use my good old tektronix PG506 section, specifically the High Ampl mode to generate circa 18V at different frequencies (100Hz, 1KHz, 10KHz, 100KHz and 1MHz), both probes with 100x mode. The probe which is working OK seems to offer a flat square output at these frequencies, see the result from my TDS540C screenshot attached. However with the partially failed or needing complete calibration probe, some screen shots shown partial overshooting or non-flat frequency response, what is clear the gain is not flat for some reason.

Ch 1 is direct output from PG506, Ch 2 the good P5210 and Ch 3 the partial fail P5210

Unless another part or component of the circuit is failed, it seems there are a total of 4 variable small brown capacitors, Common Mode Rejection blue potentiometer, white gain potentiometer and two other grey potentiometer (no idea what are their use or function) for the calibration. Would any of you know the exact steps to calibrate the probe or what troubleshooting method to follow ?

Thank you in advance, Albert

[Tantratron]

This morning I've redone the 50Hz testing and circa 250KHz testing where both P5210 probes are connected together to the generators, the display of both channels output is now taken thanks to my TDS540C.

Ch2 is the good calibrated P5210 probe and Ch3 the P5210 in need of calibration and/or repair.

As mentioned before, there are a different sets of potentiometer and variable capacitor, I only tuned the white potentiometer (R360 in the P5205 attached schematics) to have same gain at 50Hz which is confirmed in the attached scope shots. However we do see the mismatch of gain at 250 KHz of 10% on the 2nd scope shot where as a reminder, pushing to min or max the white gain pot will not solve the issue.

In my previous post and pictures using different PG506 outputs, we see the frequency response not to be flat so the million dollar question, what other potentiometer or calibration sequence should be done.

Just for the sake of the discussion, I'm attaching the P5205 reverse engineer schematics done by a member long ago where more or less the schematics is pretty similar.

On a side note, it is quite sad to find the service manual of the P5210A proving the calibration steps but no way to have the same information for the P5210. The P5210A does have more potentiometer so I guess its PCB and BOM to be slightly different. Maybe some of you has a PCB5210A which could be open in order to make a picture of its PCB for comparison.

Thank you in advance for any clue how to solve this legacy P5210 calibration

[Tantratron]

I'm attaching the extract pages of the P5210A service manual, obviously the P5210A does have bit more tuning potentiometers and variable capacitors as opposed to the legacy P5210 (no A suffix). In particular it states a DC gain (low range) and DC gain (high range) but no idea of what would be the procedure for the P5210. If someone being the owner of a P5210A, maybe peeling off the label then picture of the PCB would help to compare the circuit, what tuning pot or caps would be highly appreciated.

[Tantratron]

I've decided to make a new test being inspired from the P5210A service manual (adjustment part), here the idea is about shorting both probes leads together then connecting the single port to the PG506 High Amplitude output running respectively at 100KHz, 10KHz, 1KHz and 100Hz.

Both P5210 inputs are shorted and mutually connected to the single BNC output of the PG506. Attached two pictures, the first with no signal and then 2nd showing each signals outputs of the P5210 displayed on one TDS40C. Both P5210 have been choosen with their highest sensitivity, namely 100x attenuation.

The CH2 is correct calibrated P5210 so we see an acceptable AC-CMRR but on the other hand, the CH3 connected to the un-calibrated P5210 shows problem with clear ringing.

Question 1: do you think this side-effect could explain non-flat gain frequency response

Question 2: from the P5205 reverse engineered schematics, what potentiometer or variable caps would you suggest to decrease the AC-CMMR issue

Question 3: what are the function or use of both potentiometer R160 and R165

Thanks, Albert

[Tantratron]

Ok this seems to be a monologue but I've might almost solved the issue.

What I've done, do not ask me for the logic since this was blind tuning without order of preference... anyway I've progressively trimmed the 4 variable capacitors (C311 C312 C711 and 7400) to make the CMRR noise at 100KHz as low as possible. After that, I've trimmed the white gain pot to match the PG106 at 1KHz Standard Amplitude (100V) then later check voltage at 250KHz between the induction lamp ballast and the induction bulb.

So roughly it is quite clear now the CMRR ringing would void any attempt to properly match the gain.

I still do not have access to the authentic tektronix calibration manual of this P5210 probe but both probes seems to behave similar now with the hypothesis the first one has a valid calibration seal.

N.B. If anybody could explain the role or effect of both grey potentiometer, it would be great

[David Hess]

You might get some insight from the calibration instructions in the service manuals for the Tektronix 7A13 differential comparator and 7A22 differential amplifier.  Also check out the calibration instructions for the P6046 differential probe and maybe P6135A and P6055A differential probes.