Author Topic: Agilent N2772A (Fluke DP120) differential probe repair attempt + teardown  (Read 5716 times)

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Offline 6151kokodef

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Hi, I'm new to the forum and this is my first post.

I recently bought this Agilent N2772A differential probe for measuring mains. However the probe inputs measures open circuit and there's no output at the BNC.

I then took the probe case apart, removed the red&black cables and applied signal directly to the gold connectors on the PCB. There is output at the BNC, but it is the input multiplied by ~6.72 rather than divided by 20 or 200. So I suspect the red and black cables have internal resistance and therefore stripped them. They turned out to be shielded coaxial with a super thin core wire. However I just couldn't measure any continuity through either cables.

The two PCB inputs measures only 85kohm in between and 45kohm to ground. I tried to put 5.1MEG resistors in series with the input to fake the stated 5MEG input impedance. But this produced a really poor square wave response with ~12us rise time (c.f. spec states 17.5ns).

What could be done to make the probe functional again?

Here are the instruction and service manuals for the probe:
http://media.fluke.com/documents/dp120___iseng0600.pdf
http://media.fluke.com/documents/DP120___sieng0100.pdf

I also noticed nobody has posted internal photos of the N2772A or Fluke DP120 before - so I've attached some here.

Thanks!
 

Offline TheRuler8510

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6151kokodef,

Thanks for the pics.

Did you ever get it to work?

Thanks,
"There are no facts, only interpretations."
--Friederich Nietzsche
 

Online Mechatrommer

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how much price did you get this? last time i got 1141 diff probe at cheaper stated as broken. all i have to do is resolder the unsoldered coax output cable and all are working 100% again. your unit probably just a broken cable, i believe the board is still 100% functional. be carefull not to change those trimpots and variable capacitor they should be already calibrated from factory (but later you are going to recalibrate anyway so who cares? just dont further damage the undamaged. see below). what type 5Mohm resistor are you using. carbon? wire wound? use carbon is better, dont use wire wound thats a sin.

freq respond can be compensated by adding parallel (or to ground) C to 5Mohm, or several "parallel RC" in series to simulate distributed/characteristic impedance of the original coax cable, time to fire up spice simulator now?. also looking at how a normal dso probe is compensated by RC network is a good learning experience, if you need to, buy a cheap china dso probe and destruct it to see the internal to get the feeling.

after you sorted it out you need to recalibrate the device again anyway for both input matching, lets hope the designer have put that in the board among those trimpots/varcaps and in manual on how to recalibrate. this is only way if you cant find spare part of the original cables. hope that help. glad to see the internal of it thanks, just cant figure out the ICs used and dificult to trace since its using more than 2 layer board.
if something can select, how cant it be intelligent? if something is intelligent, how cant it exist?
 

Offline TheRuler8510

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how much price did you get this? last time i got 1141 diff probe at cheaper stated as broken. all i have to do is resolder the unsoldered coax output cable and all are working 100% again. your unit probably just a broken cable, i believe the board is still 100% functional. be carefull not to change those trimpots and variable capacitor they should be already calibrated from factory (but later you are going to recalibrate anyway so who cares? just dont further damage the undamaged. see below). what type 5Mohm resistor are you using. carbon? wire wound? use carbon is better, dont use wire wound thats a sin.

freq respond can be compensated by adding parallel (or to ground) C to 5Mohm, or several "parallel RC" in series to simulate distributed/characteristic impedance of the original coax cable, time to fire up spice simulator now?. also looking at how a normal dso probe is compensated by RC network is a good learning experience, if you need to, buy a cheap china dso probe and destruct it to see the internal to get the feeling.

after you sorted it out you need to recalibrate the device again anyway for both input matching, lets hope the designer have put that in the board among those trimpots/varcaps and in manual on how to recalibrate. this is only way if you cant find spare part of the original cables. hope that help. glad to see the internal of it thanks, just cant figure out the ICs used and dificult to trace since its using more than 2 layer board.

How much $$ did you pay for yours? Is $300 a good price for a new one?

"There are no facts, only interpretations."
--Friederich Nietzsche
 

Online Mechatrommer

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How much $$ did you pay for yours? Is $300 a good price for a new one?
$50 (incl the 1142 power module). but i have to pay extra $60 for shipping. i dont think its available new in ebay. the best is "like new" (fully functional) in box that cost $500-$1000 excl shipping. i believe this agilent 2772 is also around that range if not cheaper (used) no i just checked its actually more expensive in ebay than 1141 unit. the one-hung-low brand new is around $300 but goes to ~20MHz BW only.
if something can select, how cant it be intelligent? if something is intelligent, how cant it exist?
 

Offline EEVblog

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Thanks for sharing.
Haven't seen that one before.
Not the usual high voltage resistor ladder input you'd expect to see on the input?
 

Online Mechatrommer

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it has peculiar arrangement "circled" in the picture. why they want to short those passives (C or L or both) is a mystery to me. i suspect this is why this kind device is overly expensive (read black magic)
if something can select, how cant it be intelligent? if something is intelligent, how cant it exist?
 

Offline David Hess

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They turned out to be shielded coaxial with a super thin core wire. However I just couldn't measure any continuity through either cables.

The center conductor is resistance wire used as part of the probe compensation and usually very difficult to solder to.

Quote
The two PCB inputs measures only 85kohm in between and 45kohm to ground. I tried to put 5.1MEG resistors in series with the input to fake the stated 5MEG input impedance. But this produced a really poor square wave response with ~12us rise time (c.f. spec states 17.5ns).

What could be done to make the probe functional again?

I would try replacing the probes with a pair of cheap x10 oscilloscope probes that include their compensation at the probe end instead of the BNC end.  The attenuation factor will be screwed up but maybe you can fix that either in the differential amplifier or at its output.

The AC and DC common mode rejection ratio would need to be recalibrated which may not be possible.

The bandwidth is low enough that it a pair of attenuating probes made from RG-174 with 5 MOhm resistors on the end could work if you can get the compensation right.
 

Online MarkL

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it has peculiar arrangement "circled" in the picture. why they want to short those passives (C or L or both) is a mystery to me. i suspect this is why this kind device is overly expensive (read black magic)
Looks like two passives in parallel to me (left and right), and there are two sets of them (top and bottom) in each of your circles.  Maybe not so magical?
 

Online Mechatrommer

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Looks like two passives in parallel series? to me (left and right), and there are two sets of them (top and bottom) in each of your circles.  Maybe not so magical?
look carefully the trace in the middle (longest trace in circled area). 2 sets of C in series, and then they are paralled. hence we have 4 passive in series and parallel combo, thats not magic, whats magic is the solid copper trace in the middle shorting those parallel making them useless to my eye. edit: ok i got it now you are right, parallel C and then seried, i need sometime on drawing board to figure it out. i still find its funny though.
« Last Edit: July 23, 2014, 07:14:56 am by Mechatrommer »
if something can select, how cant it be intelligent? if something is intelligent, how cant it exist?
 

Offline TheRuler8510

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To anyone interested:

Techni-Tool has these units in their clearance category for $298.  I just ordered one...seems like a bargain.

Link:
http://www.techni-tool.com/431TE396

Cheers!
"There are no facts, only interpretations."
--Friederich Nietzsche
 

Offline eurofox

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I have one like that in "new" condition and pay 120 Euros on ebay France  :-DD
eurofox
 

Offline giovannirat

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Re: Agilent N2772A (Fluke DP120) differential probe repair attempt + teardown
« Reply #12 on: December 08, 2016, 05:31:37 am »
I recently bought a Fluke DP120 differential probe, ecaxtly the same as desribed by as6151kokodef in the first post.

Unfortunately I  have the same problem as6151kokodef because I couldn't measure any continuity through the red cable. These cables turned out to be shielded coaxial with a super thin core wire with a 5MOhm resistance.

I asked Fluke for a spare part, but it is no longer available. :--

Is there any chance to get the probe repaired?  I'm not so skilled to find a decent solution...... :-//

Has anyone solved a similiar problem or may be a spare part available? Can the failure solved with another setup (other cables with a resistance in series.....) but still maintaining the original specs (more or less) ?

Any help appreciated ..

Thank you very much

 

Offline Assafl

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Re: Agilent N2772A (Fluke DP120) differential probe repair attempt + teardown
« Reply #13 on: December 08, 2016, 08:24:26 pm »
Well, the probes are very similar to the (egregiously expensive) probes made for the isolated scopes (like the VSP210 or VPS212 probes). I don't know if the wire is the same but very well may be... They sealed the probe's ground pin with a plug, and changed the sleeve to fit a safety banana plug (very useful). The VPS series are like 200$ a probe so perhaps try to find a broken one ebay...

Another option (if lucky) is that the disconnect happened on the probe body side. In that case just shortening the wires a bit may work. The service sheet shows the configuration of the internal wire connector and splice (it isn't soldered - it is bent on the inner insulator and has a ferrule that is spring held in place).

Yet another option is to try to pry the probe apart. Not sure it isn't molded though....
 

Offline mk_

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Re: Agilent N2772A (Fluke DP120) differential probe repair attempt + teardown
« Reply #14 on: December 12, 2016, 07:45:04 am »

Any help appreciated ..


send me a PM with info about your location... maybe I can help you, I`m livin in Oberösterreich, 49xx

Regards

Michael
 


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