Do Oscilloscope probes just "die"? - Page 1
Forum section: electronics / repair

author: danielbriggs
author_rank: Regular Contributor
timestamp: August 20, 2020, 02:31:06 pm
content: Hello Team-EEVBlog,I am just curious if it is normal for oscilloscope probes just die and give up working instantenously?I have had a Keysight MSOX2022A for a number of years and have been using the stock N2890A 500MHZ Passive 10:1 probes for a while.Today one just stopped working. It is definitely not the scope channels as swapping probes over moves the fault, so it's definitely the probe. I've tried copious amount of cable wiggling and checking the tip, taking the shroud off, and no joy. It just behaves on screen as if it's not plugged in; absolutely zero signal of any variety shown.I wasn't probing anything dangerous when it failed... just a pretty simple 5V TTL serial signal.I don't want to repair it, I'm just curious on the failure mechanism and if it's common should anyone knows? Coax fatigue?There can't be much inside these passive probes to go wrong surely.All the best,Dan

author: trevatxtal
author_rank: Contributor
timestamp: August 20, 2020, 03:07:36 pm
content: In my 55 years in Electronics I have never had or heard of a probe that failed.So the answer is very unlikely!

author: Tom45
author_rank: Frequent Contributor
timestamp: August 20, 2020, 04:50:08 pm
content: One of my first eBay purchases, 20+ years ago, was a Tektronix 7704 mainframe. When it arrived I was surprised that it came with a box of a few dozen scope probes that weren't even mentioned in the eBay listing. I'm guessing that the seller was cleaning out a lab and just gathered every probe in sight and put them in the box.At least half of those probes didn't work. At least the price was right. Considering the value of the probes that did work, the 7704 was free.Just last week while using one of those probes that had been OK for the 20+ years I've had it, it was working one minute and then just nothing.So yes scope probes can and do fail. The half that were DOA had failed for someone else before I got them. Perhaps 2 or 3 of the ones that did work have failed since then while I was using them.

author: AndyC_772
author_rank: Super Contributor
timestamp: August 20, 2020, 05:09:48 pm
content: Scope probes certainly can fail. I have an Agilent N2890A which became annoyingly intermittent, which is a total no-no for any kind of diagnostic tool. It's now tied so it dangles over the arm of a chair and is doing a fine job in its new role of cat toy.I've also encountered several dead Tektronix P6139A probes, which could be temporarily restored to life by connecting a few volts directly to the BNC connector. Legend has it these were from the early days of lead free solder, and the failure mode was tin whiskers which grew between the conductor and GND on the scope side of the 9M input resistor. These would short out the high impedance signal, but were easily (though temporarily) cleared by having a small voltage applied directly across them.This is, to this day, theonlyconfirmed case of a product actually failing because of tin whisker growth that I've come across in 25 years.

author: DimitriP
author_rank: Super Contributor
timestamp: August 20, 2020, 05:11:28 pm
content: QuoteNo much , only five things.1,The receptacle for the probing pin brakes/loosens.2,3 The coax brakes at the strain relief point of either the probe side or the oscilloscope side4 the BNC connection to the compensation assembly goes bye bye5 Or the coax gets pinched/ twisted at a random location somewhere along it's lengthYou could determine exactly how it failed by taking it all apart.

author: Tom45
author_rank: Frequent Contributor
timestamp: August 20, 2020, 06:19:26 pm
content: Quote from: DimitriP on August 20, 2020, 05:11:28 pmOr use it as an excuse to play with a TDR in an attempt to learn where the break is.

author: Gyro
author_rank: Super Contributor
timestamp: August 20, 2020, 07:16:40 pm
content: Quote from: Tom45 on August 20, 2020, 04:50:08 pmThere was a period when certain Tek probes used to suddenly develop shorts in their cables - I think it was early early models of the P6136.I don't know about Keysight probes but a resistance measurement across the BNC plug might possibly throw up something.

author: kg4arn
author_rank: Unknown
timestamp: August 20, 2020, 07:28:37 pm
content: I have caused probe failure by over voltage at high frequency looking at HF power amp voltages.

author: AndyC_772
author_rank: Super Contributor
timestamp: August 20, 2020, 08:01:11 pm
content: Quote from: Gyro on August 20, 2020, 07:16:40 pmProbably the same tin whisker issue as on my P6139A.

author: danielbriggs
author_rank: Regular Contributor
timestamp: August 21, 2020, 05:09:02 pm
content: Thanks for everyone's comments + replies; all really interesting.So with a bit of extreme dissasembly I found the cause! A break in the coax (where black sharpie mark is on the image below).Happy I found the reason, but still sad it failed as I take really good care of all my kit + has never been abused with sharp kinks etc.Also, the actual core is stupidly thin; I measured ~0.084mm diameter (~AWG 40??)Inside the BNC connector (scope end) is a board with one cap and two trimmers. The board is quite badly soldered // not cleaned.Probe end there is a very thin PCB and the varable tuning cap.All the best,Dan

author: Gyro
author_rank: Super Contributor
timestamp: August 21, 2020, 06:00:35 pm
content: Yes, the inner core is very thin - to get the correct capacitance / impedance. It is also resistance wire rather than copper for damping.An accidental tug is more than a risk than kinking. I can't see whether that one is, but Tek probe inner cores are formed into a zig-zag which gives them some stretch.P.S. I don't know what's under the heatshrink, but as it's near the end you might try shortening it and still have a chance of it remaining within the compensation range.

author: trevatxtal
author_rank: Contributor
timestamp: August 22, 2020, 05:07:22 am
content: Well Well so they can fail and in some numbers, it is the first time I have heard of it!So is it poor quality manufacture, poor handling, or just age, no some of my probes are 50 plus years old .Thank you every one for your input I have learned a lot.

author: tggzzz
author_rank: Super Contributor
timestamp: August 22, 2020, 08:13:10 am
content: Quote from: danielbriggs on August 21, 2020, 05:09:02 pmThe probe cable is a transmission line, as is a piece of coax. Yes, that means you can get reflections at impedance discontinuities, e.g. where the cable is connected to the scope.The problem is minimised by making the cable alossytransmission line, by having a distributed resistance along it length. High resistance is achieved by having thin wire.FFI about the fundamental theory of probes, plus many practical considerations, seehttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

author: cgz2001
author_rank: Contributor
timestamp: August 22, 2020, 08:54:45 am
content: About 14 years ago I was working as an electrical engineer for electronic controls in commercial aircraft. There were returns delivered which, after failure analysis, became defective due to tin whisker growth between what were then fine pitch parts. The board was well engineered and conformally coated. I don't recall the details such as pitch or conformal coat model number, but I do recall the unit was in service for about 4.5 years and the units have a warranty of 5 years. However keep in mind these units see temperature extremes others do not, such as hot sunny days in desert countries around the world followed by colder temperatures up in the sky, then repeat.

author: David Hess
author_rank: Super Contributor
timestamp: August 22, 2020, 09:42:39 am
content: Oscilloscope probes have many age and use related failure modes. The fragile center wire can break with repeated flexing. The cable insulation can crack with age. Stress when pushing with the probe tip can fracture the 9M series resistance.

author: tggzzz
author_rank: Super Contributor
timestamp: August 22, 2020, 11:30:49 am
content: Quote from: cgz2001 on August 22, 2020, 08:54:45 amA NASA report on whiskers :http://nepp.nasa.gov/whisker/reference/tech_papers/2012-Panashchenko-IPC-Art-of-Metal-Whisker-Appreciation.pdfSome of the pictures are "interesting".

author: Refrigerator
author_rank: Super Contributor
timestamp: August 22, 2020, 11:46:24 am
content: In my university lab it was common for the BNC plugs on the test equipment to be worn out and horribly intermittent but i don't ever recall a probe failing.

author: Circlotron
author_rank: Super Contributor
timestamp: August 22, 2020, 12:09:48 pm
content: Quote from: kg4arn on August 20, 2020, 07:28:37 pmYep. Just because a probe is rated at 400V doesn't mean it will tolerate 0-400V in 1uS... No sir.

author: tggzzz
author_rank: Super Contributor
timestamp: August 22, 2020, 12:52:37 pm
content: Quote from: Circlotron on August 22, 2020, 12:09:48 pmThey ought to have a frequency derating curve, and users ought to look for them. The max voltage at rated frequency ismuchlower than people would guess.

author: AndyC_772
author_rank: Super Contributor
timestamp: August 22, 2020, 02:19:59 pm
content: They do. Page 6:https://literature.cdn.keysight.com/litweb/pdf/N2889-97001.pdf?id=3014705

author: Circlotron
author_rank: Super Contributor
timestamp: August 22, 2020, 02:57:59 pm
content: ^^ Going by that graph, many signal and function generators would have enough output to pop a probe!

author: TheMG
author_rank: Frequent Contributor
timestamp: August 22, 2020, 03:15:51 pm
content: This has me thinking... I've got a P6137 probe that is hopelessly intermittent. Actually, it seems to work for a while with small signals but the moment you put any larger voltage on it it dies completely, then starts working again some time later. Very weird fault.

author: Gyro
author_rank: Super Contributor
timestamp: August 22, 2020, 04:18:14 pm
content: Reading backwards, that's the P6136, P6137 and P6139 - doesn't bode well for the P6138 does it!It might be worth constructing a probe 'reformer'. The resistance of the centre conductor can't be more than around 100R, so a small capacitor discharging into the BNC end at, say, 50-100V ought to blow out the Tin whiskers for a reasonable period of time.

author: Berni
author_rank: Super Contributor
timestamp: August 22, 2020, 04:21:11 pm
content: I seen Agilent probes fail at work.They often get borrowed by people that barely even know how to use a scope. So im guessing they don't get a very gentle treatment there. Eventualy i come across a probe that was acting weird and after a bit of swapping around determined its indeed the probes fault. Have not really gone into why it failed but i couldn't spot any obvious damage on it, i just tagged it as non working and grabbed another one.So yeah they do fail.

author: trobbins
author_rank: Frequent Contributor
timestamp: August 23, 2020, 07:39:11 am
content: I 'fixed' two P6139A probes a few weeks ago that had the 'whisker' fault. Luckily the 9meg tip element screws out of the probe tip end, and then squirting some contact cleaner in, and some healthy knocking to dislodge whatever was in there gave me back correct function. But yes I guess passing a current from probe lead to ground would 'open' the whisker. Whether one fix is better than the other may only tell with time.

author: AndyC_772
author_rank: Super Contributor
timestamp: August 23, 2020, 03:24:36 pm
content: I'd be inclined to consider a probe as "dead" as soon as it becomes unreliable.I have a rule of thumb which says any piece of diagnostic equipment needs to be at least an order of magnitude more reliable than the thing it's being used to investigate. If I probe a test point on a board and see 0V, it needs to beoverwhelminglylikely that the board really is outputting 0V, and thatitis therefore the correct focus of my time and attention.If I end up wasting time and effort trying to find out why a signal isn't arriving somewhere it should, and it turns out the probe was broken all along, then that probe is scrapeven if it can be temporarily fixed.

author: danielbriggs
author_rank: Regular Contributor
timestamp: August 23, 2020, 03:32:24 pm
content: Quote from: AndyC_772 on August 23, 2020, 03:24:36 pmAbsolutely this!That's why mine got cut up (out of curiosity) and then went in the bin + a new one ordered.All the best,Dan

author: RoGeorge
author_rank: Super Contributor
timestamp: August 23, 2020, 03:45:55 pm
content: Oscilloscope probes do not just "die", they go to probe Heaven.