PMK German Probes X1000 strange defect

[jonpaul]

Bonjour, We have issues with some HV probes from the German probe firm, PMK.

https://www.pmk.de/en/products/5_mm_hochspannungstastkoepfe
Use two  PMK PHV probes, X1000 2m 2800V CAT II PHVS-2000.
One probe is perfect, but the matching second probe has distorted  waveshape (even on 1 kHz CAL square)  and about  100 ratio not the marked X1000!

We thought a loose or oxidized connection but it is not.
The resistance at the bad probe BNC is flaky, varying and lower than on the good probe.
We tried these   on several Hameg scopes (HM203, HM204) and channels 1/2, it is definitely the probe.

We tried to adjust the LF/HF/DC trim on the compensation box, the LF/HF   work but does not fix it.
The DC trim (middle) is flaky.

PMK Germany has offered an RMA and good  support.
Any PMK probe users or PMK staff contact, your advise is appreciated.

Many thanks


Jon PAUL

[Gyro]

Surely a higher than expected amplitude, coupled with a lower than expected resistance at the BNC can only point to breakdown and reduction in value of the main tip series resistor. That sounds like a very bad thing in a 4kV pk [EDIT: I only see 2kV and 4kV pk rated probes on the linked page, not 2.8kV - Which is it? A 2kV probe that has been exposed to 2.8kV or a 4kV probe?]. If this is the case then it is not very surprising that the trimming does not function correctly.

I think return / scrapping is your only safe option.

[ebastler]

jonpaul's description, "MK PHV probes, X1000 2m 2800V CAT II PHVS-2000" suggests part no. 870-722-A00. Hence 4kV peak/pulse, but only 1kV CAT II.

[Gyro]

Thanks ebastler, I didn't look that closely at the page beyond noticing the 2kV and 4kV models. I don't think it changes my diagnosis (tip resistor starting to break down), that should never happen.

[TurboTom]

To me, it rather looks like a flakey  ground connection of the BNC or the probe cable. Please try connecting the ground at the probe tip and have a good look at the segmented annular grounding contact of the BNC. It may be necessary to carefully pry the segments slightly outward so they have a decent spring force against the BNC(F) ground "tube".

I've got some SMA/BNC adapters that only after this procedure work properly (of course, the were of the "el-cheapo" variety...)  .

[jonpaul]

Hello all

thx for the notes:

Further tests   

COMP LF/LF trim on comp boxes work as expected.
The bad probe scale factor adjust (middle) trim ofCOMP box changes 
shape of wave with  change of attn factor :  intermittent and scratchy  as you  trim.
on good probe Resistance seen by scope at BNC is stable , on bad one resistance at BNC is lower and varying
Careful application of ISO alc into comp box trim does affect but does not fix bad probe.
I have no gighometer at the moment, so I cannot measure the internal high meg series resistor, most likely 49 Megs.

1/ Probes were working originally as a pair in differertial use.  not exposed to excess voltages or mechanical abuse. Used in a normal lab well within ratings

2/ All connections for return at BNC, probe tip return, ground clip are near zero Ohms. Only BNC is the Hameg German scopes input CH1/CHII which work fine and good quality. BNC segments on probe side are like new, no wear, corrosion or abuse.

3/ The safety mark "2800VAC CATII"  is on  body of probes. (Also  X1000)   2800 VRMS max = 4000 V peak.

4/ PMK lists 4 types of X1000 PHVS that differ only in legnth (these are 2m) and presence of ID etc.

5/ Contacted PMK, and recieved note from factory to send more info and scope pix. Have done that.

Suspect the attn trimpot is intermittent OR that the HiMeg R 49M has failed.

Impossible to open comp box or probe further.

Awaiting PMK responses.

Good day,

Jon

[alm]

I have no gighometer at the moment, so I cannot measure the internal high meg series resistor, most likely 49 Megs.

49 MOhm is not exactly Gigaohmmeter-only territory. Do you not have a DMM that can measure up to 100 MOhm (not uncommon for bench meters), or a DMM that can measure conductance (Fluke 87/189 for example)? Or measure with 10 MOhm resistor in parallel and calculate back based on known value of 10 MOhm resistors. This is what many DMMs do for the higher resistance ranges.

[jonpaul]

Update 18/10

to Alm: re megohmeters:


My main lab indeed has Keysight 34465A, GenRad 1863, 1864 megohmeters, Danbridge 0-30kV insulation tester etc.

But now I am overseas with a very limited lab, VOM, scopes, Funct Gen, DCPS.
I figured how roughly to test the probe resistance: Used a VOM   200 uA scale
The  SIGSALY Quantizer has   +260VDC B+, I  appled  260V  to both probe tips.

Measured the  current flowing from  the probe tip to BNC center and shield. About  4.7 uA
both  good and bad  probes. The HV series  resistance is very close to the rated 50M  Ohms.
I conclude both probes  series resistors are fine 

Problem is failure of the tiny and delicate scale CAL adjust trimpot.
Hope for  for PMK service.

Amicalment

Jon

[jonpaul]

Rebonjour,

We  attempted to use contact cleaner (a tiny drop) on the suspect   gain trimpot of  the defective  PMK X1000 probe.

The cleaning changed the waveshape but  after LF comp adjustment the  scale error is still 9 times, about the same:

Photo 1 Scope Hameg HM203-4  photo of good (upper) /bad (lower) probe on same 2V 1 kHz CAL signal,  scope CH1/CHII is 5 mV/DIV

Photo 2 Scope Hameg HM204  photo of good (upper) /bad (lower) probe on same 2V 1 kHz CAL signal,  scope CH1/CHII is 5 mV/DIV

Same result if the CH1/CHII is interchanged.


Conclusion: As the probe series R is OK and the ground returns are near zero Ohms, we conclude that the Defect is inside the probe comp box, a bad gain trim adjust pot.

Anyone ever seen this on PMK probes? Anyone ever opened or repaired a PMK probe comp box?

I passed this on to the PMK Germany service, and await PMK's offer of repair or replacement.

Amicalment

Jon

[HighVoltage]

I had very bad experience with PMK 1000x Probes and killed a few of them without obvious reasons.

Then I switched to TesTec X100 probes and never had a problem.
TT-HV 250
(I only needed this for up to 2kV)

[jonpaul]

Bonjour, HighVoltage:

Many thanks for this note: "I had very bad experience with PMK 1000x Probes and killed a few of them without obvious reasons."

Can you detail your "very bad experience" that killed them?


As  designer, and manufacturer of SMPS, HVPS, since 1970s,  we   used the classic Tektronix P6015, P6015A for decades, never any issues.
 
We   discovered the   PMK X1000 probes recently thought to try a pair, as the PMK probe  bodies and comp boxes are much smaller  than the TEK.

We find that the PMK X1000  probe tips, bodies and comp box trimmers are much  more delicate  than the TEK.

We hope that PMK Germany service can support the bad probe and fix it.


Amicalment,


Jon

[HighVoltage]

Hello Jon

For high voltage measurements of up to 40kV I also use the Tektronix P6015A and never had any problems with any of them.

For anything above 40 kV and up to 80kV I use DIY capacitive dividers.

For coil measurements of up to 2kV the P6015A probe was too bulky and I got the PMK Probes X1000 and one failed right away. I got a new one under warranty and that one failed after a short time. I tried a 3rd one and that one also failed and got returned for a refund. Since they all had warranty, I never attempted to open them for analysis.

I switched to the TESTEC many years ago and and although they are only good for 2.5 kV, I never had a failure.   

[jonpaul]

UPDATE 23 Oct 2023:

Problem is definitely in the probe compensation box, most likely the scale/gain trimmer.

I have received an RMA from PMK service, I shall return the defective probe to Germany soon.

I have  changed the post  title as PMK support is fine!

Shall update further after probe is returned to me.

Jon