[SOLVED] Fluke i400 AC current clamp, need repair help

[HighVoltage]

This Fluke i400 current clamp is dead and has no output at all.
I tested it with 1A AC 50Hz.

First I checked the coil and it has following values:
800 mH inductance
10 Ohm resistance
Although I do not know the correct values, this seams plausible.
So I suspect something wrong with the PCB.

May be someone here has a schematic for this little PCB?
Or an idea of how this circuit works?

Here are some pictures of the teardown

[HighVoltage]

I de-soldered some components and they all checked out alright.
Since I do need another working AC current probe right away, I ordered a "like new" one of the same i400 type.
and when it arrives, I will trace the signals of the good one in comparison to the bad one.
May be I get lucky.

[commongrounder]

Before stripping it down, I would have tested it with a current higher than the bare minimum 1 amp.  The circuitry in the clamp derives its power from the clamp coil, and will drop out if there isn't enough current flow. This clamp is best used in the tens or hundreds of amps range.

[HighVoltage]

Hello commongrounder
Thanks, I tested the also at 10A and 20 A AC and the output was just 0
With the new probe I ordered, I hope to trace the signal, to find the problem.

[ElektronikLabor]

The LT 2051 it an dual OP-amp.
Can you mearuse the inputs and outputs of that OP amp?

[ManateeMafia]

The wires look pretty fine. Is there any chance one has an internal break?

[rsjsouza]

Or an idea of how this circuit works?

Such small board is easy to reverse engineer, especially with the help of a very similar section of the circuit depicted in Fig1 of the XTR115 datasheet and with the help of the LT2051 datasheet. 

I was curious to find out what the "LTTG" part meant, but it matches the part markings of a LTC1966 RMS-DC converter as shown in its datasheet.

I would then probe pins 1 and 7 of the LT2051 to see if anything voltage shows up, as well as pins 5 and 6 of the LTC. I would then check the outputs of the XTR and even the beefy transistor (perhaps someone slapped it on the output of a power supply  )

Just a tip: if you want to increase the current sensed by the clamp, simply wind two or more turns around it. 20 turns @ 20A shoot it to its 400A limit.

[HighVoltage]

Ok, to be upfront, this repair took a very different turn.
I never would have expected this.

The new current clamp arrived yesterday and today I wanted to take the time to trace through the existing and working new PCB in the new clamp.
So I open up the brand new i400 that looks the same as the old one from the outside.
And I am finding a very different PCB inside.

Actually the PCB in this new clamp is just a straight connection from the coil to the cable.
the few SMD pads are not even populated.

So, I just remove the old PCB from the old and broken clamp and hook up a new cable and guess what.... it works perfectly within specs.
Huh
What is going on?
Why such a relative complicated circuit if there is NO need for any circuit at all?
What was Fluke thinking?

It seems the SMD pads on the new PCB are for the placement of calibration resistors?
If so, why not having a few potentiometers instead of the pads?
This does not make much sense to me at all.
What do you all think went on here with Fluke?

Well, At the end I compared the two probes and they are working perfectly.
13.87 A compared to 13.73 A is about 1% difference.
I also tested at 25A and 50A with a few more loops through the clamp and it is just working perfectly.

[SeanB]

Possibly it was modifies for use with a line powered amplifier, such that it could be used with an external power supply and converter for a low current application. that would explain the opamps and line driver built into the case. In your use case just using as a plain current transformer they prevent operation.

[HighVoltage]

Hello SeanB,
Possibly, but there was no indication, that this current clamp was ever opened up before. It looked all original and factory soldered.
May be one day I will come across another one and I will have a look of what is inside.

This Fluke i400 has an output of 1mA / A (current output)
But there is another clamp, called the FLUKE i400s that has an output of 1mV / A (Voltage output)
I first thought, that this was the PCB of an i400s, but then that is not possible, because the i400s has a small switch on the PCB to change the range.
(see picture below)

Still a little mysterious.

[Andy Watson]

It would be interesting to reverse engineer that PCB to find out what does (or did). My guess would be that it gives a true RMS conversion - something that most DVMs have built-in these days.

[SeanB]

Very possible this was a custom made probe for an OEM and the circuit was factory installed for them, so that a replacement probe would not then be available direct from Fluke but from the equipment vendor.

That it used a standard probe in a non standard way would not worry Fluke, as they would simply refer enquiries for the probe for the equipment to the OEM, or possibly the OEM simply bought a job lot of probes and modified them themselves then closed them up with a new seal again, or simply bought the probes as kits for assembly themselves.

Having a current loop would make it a nice bit of test kit for a instrumentation test set, removing the need to disconnect a cable to measure current, and there probably was a matching voltage to current converter that would be connected to measure the line voltage, so you could have apparent power displayed.

[HighVoltage]

It would be interesting to reverse engineer that PCB to find out what does (or did). My guess would be that it gives a true RMS conversion - something that most DVMs have built-in these days.

I think I will do just that, as soon as time permits.
It really would be interesting to find out what the circuit does.

May be a true RMS circuit or an OEM version, as SeanB points our.

Thanks to everyone for your suggestions in the repair as well.

[David_AVD]

So did this unit stop working while you had it or did you buy it as a non working unit or

[retiredcaps]

Actually the PCB in this new clamp is just a straight connection from the coil to the cable.
the few SMD pads are not even populated.

Finally, a schematic and pcb board I can understand! 

Good job on the repair.

[HighVoltage]

So did this unit stop working while you had it or did you buy it as a non working unit or

Hello David_AVD
I bought the clamp used but it was supposedly in "working condition, like new"
After I confronted the seller, he claimed that the probe was working for him and he also claimed
that he was the original owner. Just now I contacted him again and he said that the probe was
bought new a few years back and never opened or modified.

Hello Retiredcaps
That was funny!
 

[SeanB]

So likely it was originally a process control probe that was not used and went up on ebay in the first place.

[rsjsouza]

Nice job on the repair, HighVoltage!

Now the baffling question in my mind is: why in the world this probe costs so much having so little inside?!? (I know, I know, it is the molded package, the brand, etc. but still...  )

[HighVoltage]

Nice job on the repair, HighVoltage!

Now the baffling question in my mind is: why in the world this probe costs so much having so little inside?!? (I know, I know, it is the molded package, the brand, etc. but still...  )

I guess, the answer would be ... "because they can"
The materials are high quality but at the end it probably comes down to the brand name and the so familiar colors.
The good thing, if we are not in a rush, we can find them for low cost on our favorite ebay site.
After a partial refund, I paid Euro 20 for the probe plus a new cable.

[rsjsouza]

I guess, the answer would be ... "because they can"
The materials are high quality but at the end it probably comes down to the brand name and the so familiar colors.

Hehe... That is the game.

The good thing, if we are not in a rush, we can find them for low cost on our favorite ebay site.
After a partial refund, I paid Euro 20 for the probe plus a new cable.

That is a good price. Here in the US I could spot only this one at that price level. All the others are above US$100.00. That is why I ended up with a Y8100 similar to this one. Not the best but enough for what I need.