Service manual for Fluke i50s / Lem PR 50 current clamp

[0xdeadbeef]

I recently noticed that my Fluke i50s is quite a bit off regarding gain. There are two trimpots on the PCB so I would guess these could be used to fix that (gain/offset maybe), but I would feel better if I had a service manual. Anyone got one by chance? One for the Lem PR 50 should work as well, the Fluke seems to a rebranded Lem.

[0xdeadbeef]

I bump this up since I didn't really make progress here ... and kinda forgot about it anyway due to other issues.
When I last thought about opening the amplifier module, I faintly recalled that I originally saw some kind of communication going on at the connector pins 1/2 and found an EEPROM close to these pins.
So I wonder if the calibration is done with the trimpots at all or through some kind of serial communication.
A service/calibration manual or any kind of hint would be much appreciated.

[0xdeadbeef]

OK, so I decided to have another look myself. For pictures see the original thread.

I identified the following components in the amplifier module:

Voltage regulators
L7808   +8V (screwed to case)
L7908   -8V (screwed to case)
9L05A   -5V (above the keyboard connector)
78L05A +5V (lower left, close to EEPROM)

Amplifiers
3x LM7171AIM - 200MHz difference amplifier
2x OP27G   - OpAmp
1x INA132  - Difference amplifier for current measurement (upper right)

Other
MC68HC705P6A - taken from the original thread, there's a sticker ("SONDE 24") on mine that I don't want to remove
DG442DY - SPST CMOS analog switch, normal open (lower right)
24c02 - I2C EEPROM (2K, lower left)
MAX504 - 10bit SPI DAC  (below µC)


First of all I tried to investigate the serial communication that I observed on pins 1/2 during my original investigation in 2015 or so. To my surprise, I couldn't find any communication there (anymore).
I traced back connector pin 1 to CPU pin 5 and connector pin 2 to CPU pin 3. There are resistors in series, so a continuity tested won't find the connection from pin to pin.
Honestly, I'm not sure why the communication is gone. Maybe it was only there while I had the supply issues and was some kind of debug message?

Anyway, my original assumption was that the EEPROM could be connected to pin 1/2 for calibration. This is however not the case. Indeed, the SCL pin is connected to pin 1 of the µC and SCA to Pin23 (6th from the left in the upper row). Since there is an EEPROM and a DAC, I still assumed the EEPROM could store calibration data, so I tried to measure the communication on the SCA/SCL pins. However, from what I can tell, it reads the bytes 0x35 0x30 0x30 0x30 0x30 0x30 0x31 0x31 0x31 0x31 from address 0 once after reset. This doesn't look like calibration data, indeed every byte seems to be an ASCII number, so I guess the EEPROM start stores the string "5000001111". So, well, a "50" maybe for the i50s/PR50 followed by "0000" and "1111". While it's beyond me y why somebody would add an EEPROM just to store something as useless as this, I guess we can rule out that the EEPROM stores calibration data.

So I had a look at the DAC (MAX504) - which in my original theory would be used to apply the calibration data. It's an SPi device, so I traced the communication as well and measured its output.
Note: Bipoff is connected to Refout/Refin and RFB to Vout, so it bipolar configuration (-2.048V .. +2.048V) - which, well, already hints that it's not used for gain control.
Directly after reset, a 0x800 (0V) is sent and after the degaussing there's a sequence of 7 communications with a period of ~51ms (and 30.5kHz clock).

0x800 : 0V
0x150 : -2.048V*(512-(0x150>>2))/512= -1.712V
0x60  : -2.048V*(512-( 0x60>>2))/512 = -1.952V
0x800 : 0V
0x4B0 : -2.048V*(512-(0x4B0>>2))/512 = -0.848V
0x470 : -2.048V*(512-(0x470>>2))/512 = -0.912V
0x470 : -2.048V*(512-(0x470>>2))/512 = -0.912V

My scope measurement on Vout showed the same behavior.
So IMHO this looks like the auto-zero functionality in work. Maybe in two cycles for the two ranges.
I just wonder if should concern me that the offset are a bit on the high side given that the DAC is limited to -2.048V.

So yeah, so much about that electronic calibration idea.

Unfortunately, I's a bit difficult to access the other side where the amplifiers and trimpots are located (either desolder two TO220 or remove the sticker on the back of the case).
From the pictures I took in 2015 (and the better one in the thread), I assume the three LM7171  form an instrumentation amplifier. I would think that the trimpot in the middle of the three LM7171 might be used to trim the gain of the instrumentation amplifier. Looks like the feedback resistors of the two inputs OpAmps are 390Ohm and the trimmer is maybe connected in parallel with two 1k2 resistors between the feedback loops of the two LM7171.

I don't really have a clue yet what the other trimmer is for. It doesn't really seem to be connected to the instrumentation amplifier. Maybe it's related to the INA132 on the other side of the PCB. Unfortunately, I wasn't really able to follow the traces. Seems to be at least a four layer PCB. Actually I'm also not sure what the INA132 is used for. Maybe for current measurement during degaussing or something.
Also the two OP27 can't be really part of the output circuit as their bandwidth seems to be too low. I assume they are used for overcurrent protection.
Last but not least, I didn't really understand yet how the range switch works. I wonder if the analog switch (DG442DY) could be used for that.

Then again, my main interest is to calibrate the i50s to reduce the gain error. Currently the readings are only ~92% of what they should be in the 1A/1V range. My best guess at this point is to use the trimmer in the middle of the three LM7171  for that. But it's a drag to get there and I'm still a bit puzzled if this is really the official approach.

[_Wim_]

Wow, you already spent quite a bit of time on this! Next weekend I will try to log the communication on my probe. This should give a good comparison of the data transmitted.

[0xdeadbeef]

This would be great. Also any input on my idea to use that trimmer for gain correction would be appreciated.
And I'm still not 100% sure how to remove the label on the backside without damaging it. Heat maybe.

[_Wim_]

I think heat should do the trick, but it is never as nice afterwards as before. My label has been off as far as I remember, because I replaced the regulators.

[0xdeadbeef]

As discussed in the original thread, I once desoldered the regulators to get access to the backside. But firstly I don't want to repeat that again if not needed and secondly, I need the regulators in to do the calibration.
So I'd need two desolder/solder them twice for one calibration. As the label removal is also somewhat destructive, it seems weird that there is obviously no other way to calibrate this thing without tearing it apart. Especially given that it has a microcontroller which is accessible in some way from the RJ45 connector.
So either it's not meant to be calibrated (in the sense to minimize the error instead of just documenting it) or I'm overlooking something here. I mean there are also some trimpots in the probe itself, but I wouldn't dare messing around there without completely understanding the circuit.

[_Wim_]

I had some free time in the lab today, here is the log of my fluke i50. Indeed probably no cal data in there...

[0xdeadbeef]

Yeah, weird.
Your string seems to differ in one character though. Mine was "5000001111"" and yours is  "5000001101".
My best guess is that it's something like a (hardware) revision number. Like 1.1.1.1 and 1.1.0.1.

[_Wim_]

Just checked the accuracy of mine in the 1V/A range (compared to my Brymen BM869S), I am also 7% low on low frequencies (until 2kHz), and from 7khz they match to within  0.1% (probe connected to Picoscope 5000 running in 16bit mode). So it seems to be low frequencies only which are off.

[_Wim_]

Just thought I would map the entire frequency response.

Setup:
-   Picoscope 5000 with FRA app.
-   Frequency generator output to LT1210 buffer 
-   output buffer to 50 ohm load.
-   Measure both voltage on 50 ohm load (as input in FRA) and current to 50 ohm load (as output in FRA).
-   Fluke probe set to 1V/A.
So theoretically we should get a value of -33.97dB.

So we see a max deviation of about 1 dB (12%), with 7% undershoot at low frequencies, and 5% overshoot at high frequencies, which is quite a bit out of spec.

Remark: I am unsure if the values above 1Mhz are correct, as these fluctuations are probable causing by phase delays caused by the fluke probe.

[0xdeadbeef]

Hm, I must admit I didn't really consider a frequency dependency of the gain since (according to the manual) the frequency response is as flat as a pancake until at least 2MHz or so.
It is possible of course that this flatness is the result of specific trimming which derated over time. I guess fiddling around with the gain of the instrumentation amplifier won't really solve the problem then.
Did you try connect the ground terminal btw? The manual hints that for higher frequencies, using the ground terminal is recommended.

[_Wim_]

Did you try connect the ground terminal btw? The manual hints that for higher frequencies, using the ground terminal is recommended.

No ground terminal connected in this test. I have done in the past (but not in a test like this), and did not see any difference.

My best guess was that the hall effect sensor has lowered a bit, but the current transformer is still normal. But I am not sure this current probe uses such a hybrid approach (hall+current trans), and also not sure the crossover between them is then typically around a few kHz.
 
But I must admit that absolute accuracy to less than a dB is not so important for me, as I typically use this probe to see the FFT spectrum of the current, and typically am more interested in the frequency than in the amplitude. And now I have the table with correction values, it is even less of a priority. Still, a service manual with schematic would be very nice, because I always like to understand how some piece of gear exactly works. This is one of the main reasons I like buy broken test equipment, because it “forces” me to study its operation principles.

[TC14MKL]

Now i also own a LEM PR50 and need to test it. The information in this thread is very helpfull to do so  . Is there also some kind of a users manual available for this current probe ? With some search in the internet i could only find a two page datasheet but nothing else. Was there really no manual from LEM / Fluke ?
Working with a current probe is not too complicated and not much buttons there to press but even then i would have expected a user manual.

[0xdeadbeef]

I could offer a very short user manual. Actually it's mainly for the power supply, but a few pages deal with the probe as well.

[TC14MKL]

Thank you very much for the manual. In the meantime i could test my PR50. It starts up as it should once i connect the +/– 12V so no delay of the 12 V necessary.  Unfortunately the readings with 1A and 3A DC current are a bit out of specification – out of spec in both measurement ranges.
One reason is a small airgap between the slider and the fixed part of the clamp.
Pressing both parts together is giving significant better readings but still higher than 1.5%.
Did you have any success in getting your readings improved ? Any idea what the trimmer in the probe itself is used for ?

[_Wim_]

Unfortunately the readings with 1A and 3A DC current are a bit out of specification – out of spec in both measurement ranges.
One reason is a small airgap between the slider and the fixed part of the clamp.
Pressing both parts together is giving significant better readings but still higher than 1.5%.

Today I made this little 3D printed ring that I glued on the probe head (printed with 100% infill). It firmly closes the gap, and in my case the deviation on DC is now less than 0.7%. STL file is included in the zip-file. It takes only 2 minutes to print 

[Hydron]

In the tek version the slider does two jobs - it both closes the jaws, and pushes the top half of the core (which is kinda floating in there) down firmly on the bottom half in the last bit of travel, after the jaw looks closed (and also closes a contact so that the amplifier knows the jaw is locked).
There is a little tab and slot arrangement at the end of the probe that is engaged before the slider pushes the core down, ensuring that the outer plastic is held firmly and the core pusher thing has something to work against. Has the Fluke/LEM probe got a similar thing (looks like it might from your pics), and if so, have you got a broken tab or something that should engage but isn't doing so? Or is there something in the slider part that is out of alignment?
As long as it's not ultrasonically welded or something then I wouldn't worry too much about opening it up, just be careful and don't leave finger oil/debris on the core faces (I once did some pretty serious surgery to a broken tek one and it works pretty well afterwards).

[_Wim_]

There is a little tab and slot arrangement at the end of the probe that is engaged before the slider pushes the core down, ensuring that the outer plastic is held firmly and the core pusher thing has something to work against. Has the Fluke/LEM probe got a similar thing (looks like it might from your pics), and if so, have you got a broken tab or something that should engage but isn't doing so? Or is there something in the slider part that is out of alignment?

Yes, originally it had indeed a tab and slot arrangement, but has was already broken off when I bought the probe. I did open up the probe and checked everything inside also when I bought it. Had to repair/glue also some other broken plastics (probably the previous owner dropped the probe). I am quite sure all inside are aligned correctly now, I just still had the (small) issue of insufficient closure. It did not bother me that much (I typically do not need that kind of accuracy), but as I recently bought a 3D printer and am learning fusion 360, it was a fun little project...

[_Wim_]

Yes, originally it had indeed a tab and slot arrangement, but has was already broken off when I bought the probe.

Close up of the missing part

[Hydron]

If it had been dropped enough to break a bunch of plastic I'd say you're lucky that it works at all! (the core with hall sensor is know for fragility). Fix looks like a pretty reasonable option given the results, you're only losing a bit of ease of clipping onto wires with the DIY ring thing.

[_Wim_]

If it had been dropped enough to break a bunch of plastic I'd say you're lucky that it works at all! (the core with hall sensor is know for fragility). Fix looks like a pretty reasonable option given the results, you're only losing a bit of ease of clipping onto wires with the DIY ring thing.

Yes, I agree. The plastic does become brittle over time however, so it probably does not take a very hard hit to break with older probes.

The seller who I bought the probe from had multiple probes with similar issues. User 0xdeadbeef also bought one of these, hence I figured I would post the little adaptor ring here. Luckily the cores were not broken, as we bought these "as is" on Ebay, and finding a replacement core is unlikely...

[Kirkhaan]

Hi guys, I am selling my PR-50. See this post on the Buy/Sell/Wanted forum:

https://www.eevblog.com/forum/buysellwanted/fs-(eu)-lem-pr-50-universal-50mhz-30a-current-probe/new/#new