Fluke Scopemeter 97 - What's so special about those BNC jacks - Page 1
Forum section: electronics / repair

author: k1mgy
author_rank: Regular Contributor
timestamp: July 11, 2017, 06:07:23 pm
content: I realize there are adapter kits available so the scopemeter will work with a standard BNC probe, however being a cheap SOB I'm tempted to replace the BNC's they use with standard jacks. I realize this will expose the GND ring to possible finger contact. I can handle it.Supposing there's no other reason to keep these jacks as stock, has anyone replaced these?

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2017, 06:19:25 pm
content: Why would you need to replace them assuming they are still good? Do the ground fingers of the probes you want to use not reach?

author: Armadillo
author_rank: Super Contributor
timestamp: July 11, 2017, 06:22:09 pm
content: Try measuring 10mv signal and a whole new world open up for you.That proof a point, having oscilloscopes and using it is a world of difference.Edit:Anyway I think portable scope and entry level scope are not for low level measurement and I don't think the vertical resolution can be as low as 1mv/div.So If your measurement centers in the volt range, emm! maybe you could do without it.Ok you can skip the noisy world.Edit: fluke 97 does have the ability to measure 1mv/div signal, so it will be a waste to change to the standard jack, IMHO.

author: alm
author_rank: Super Contributor
timestamp: July 11, 2017, 06:23:04 pm
content: The Fluke 97 has no isolation between the DMM black common jack, the common for channel A and the common for channel B. So connecting the black DMM lead to a dangerous voltage puts that same voltage on both BNC shields. Brushing your hands against non-insulated BNC sockets could give you a nasty shock or worse.If you decide the BNC jacks, which I strongly recommend against, then you should clearly mark the meter for a maximum voltage of 30 Vrms / 42 Vpeak, and not use it for circuits beyond that.

author: k1mgy
author_rank: Regular Contributor
timestamp: July 11, 2017, 07:46:13 pm
content: Correct. The ground ring of my probes do NOT make contact with the special BNC jack in the Fluke. I can sometimes get it to work if I put some side-tilt into the plug assembly, but this will stress my probe and is totally unreliable.Another poster reminds that exposing this shield poses a hazard. I understand that. I suppose the same would exist if we used the commercially-available adapter that allows for a standard BNC connection to the 97's BNC jacks.

author: alm
author_rank: Super Contributor
timestamp: July 11, 2017, 08:16:01 pm
content: Which is why they put the attached safety notice in themanual.If you are unwilling to use the proper insulated accessories, then I suggest you sell the ScopeMeter and use a standard mains-powered scope with a solid ground connection to earth. At least then any exposed metal will be at earth potential, instead of whatever dangerous voltage the leads might be hooked up to.

author: EHT
author_rank: Frequent Contributor
timestamp: July 11, 2017, 09:48:06 pm
content: If your normal BNC probe isn't connecting properly with the Scopemeter, then one or other is damaged. Try swapping the probe from channel 1 to channel 2. If the same behaviour on Ch 2 then i think your BNC on the probe is damaged. The ground connector on the BNC is internal. On the scopemeter the outer ring is plastic for safety reasons - as explained by the other contributors. Normal BNCs for use on earthed equipment will work but are not safe unless you only use on low voltage circuits. You could also test with a BNC lead to a signal generator. There is no reason to swap the jacks on your scopemeter with non-insulated ones.I use probes from this range with a 2nd hand Scopemeter 95 which I purchased with no leads. They are insulated to 1,000V CAT II. They were also rebranded by Fluke & Pomona formuchhigher cost. Look for the brand MultiContact or Staubli.http://www.farnell.com/datasheets/65421.pdfAvailable from Farnell & Conrad. Just noticed Conrad also supply the accessories so I think it might 'pimp' mine up

author: k1mgy
author_rank: Regular Contributor
timestamp: July 12, 2017, 12:53:28 am
content: I tried several and get the same result. The internal ground ring *appears to be* shorter than a standard BNC jack.From an outfit that makes adapters for these:https://www.aeswave.com/BNC-Adapter-Repair-Kit-for-Fluke-ScopeMeter-p8621.htmlQuoteWhat they describe is exactly the problem I have.I'll go ahead and order up the adapters. Easier than changing out the jacks.Thanks, folks.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 12, 2017, 02:10:00 am
content: I'm not too surprised. You could make your own, or maybe try the adaptors in the link. The difference is slight. I have two the 97s I bought last year. They work great for their age. I have one complete set of original probes and those make very good contact. I have some very inexpensive 10X probes that I tried with them and the ground fingers will not make good contact. The OD is just slightly less than the ID.I wanted to the scope with my HV probe and ran into the same problem. I measured some different adaptors I had and the finger OD varies. I found some Pomona adaptors I had have a slightly larger OD. These make good contact and I can then plug the HV probe's cable into it. Like you, I am not too concerned with using the scope this way with what I am doing. It's no worse than my other scopes. I actually bought them to use in the garage which is where they came from.You could try and spread the fingers ever so slightly. Maybe 0.005 per finger. Be very careful not to touch the OD surface of the fingers. I tried this with a T I had before I started using the Pomona part.

author: BravoV
author_rank: Super Contributor
timestamp: July 12, 2017, 02:17:54 am
content: Quote from: k1mgy on July 11, 2017, 06:07:23 pmNot sure if this fits in ... its relatively cheap though.https://www.aliexpress.com/item/P7002S-5pcs-plastic-coating-BNC-Receptacle-50KY-connector-is-compatible-with-most-digital-oscilloscope-handheld-oscilloscope/32660417647.html

author: alm
author_rank: Super Contributor
timestamp: July 12, 2017, 02:24:00 am
content: Quote from: joeqsmith on July 12, 2017, 02:10:00 amNo worse? Do your other scopes also have exposed metal that is not tied to earth ground but is directly connected to the device under test?

author: joeqsmith
author_rank: Super Contributor
timestamp: July 12, 2017, 02:37:34 am
content: Quote from: alm on July 12, 2017, 02:24:00 amNo worse meaning I know what I am doing and can assess my own risks.

author: Armadillo
author_rank: Super Contributor
timestamp: July 12, 2017, 08:07:18 am
content: Ever heard of hot chassis? Yeap!, you brush your hand on it, you will know what is hot chassis.What I mean is, you ought to know what you are doing and this kind of work is not for the average Joe on the street, right? DUT are always exposed.Yes, warning message is necessary and responsible action from manufacturer, part of the usual disclaimer of liabilities for the average Joe using their instrument.but they already said they can handle with it, they are trained, so to speak. So what's the worry?

author: alm
author_rank: Super Contributor
timestamp: July 12, 2017, 08:42:19 am
content: Quote from: Armadillo on July 12, 2017, 08:07:18 amAppliances with a hot chassis will be at least a number of decades old. Because of safety, you will not find new equipment with hot chassis, at least in western countries. I would not advise anyone to wire an appliance with 'neutral' to chassis. Real transformers are not that expensive. I have encountered equipment with a mis-wired plug with neutral and ground swapped. Fault tag said breaker trips.Quote from: Armadillo on July 12, 2017, 08:07:18 amThat someone else reads it who is not infallible and invincible.

author: EHT
author_rank: Frequent Contributor
timestamp: July 12, 2017, 08:50:53 am
content: QuoteOK, all I can say is on my 95 the Pomona insulated probes plus a Pomona 350MHz one i have, a couple of standard BNC leads and T-piece all work in it. Recommend you get an insulated probe if you can.

author: Armadillo
author_rank: Super Contributor
timestamp: July 12, 2017, 08:51:27 am
content: Quote from: alm on July 12, 2017, 08:42:19 amThe primary side of today's switch mode power supply are also live, though they do not have a chassis, but I think you wholly miss the point.The point is they said they are trained and can handle with the risk, so what's your worry?

author: carl_lab
author_rank: Frequent Contributor
timestamp: July 12, 2017, 04:46:55 pm
content: Quote from: BravoV on July 12, 2017, 02:17:54 am"Sorry, this item is no longer available!"

author: joeqsmith
author_rank: Super Contributor
timestamp: July 12, 2017, 04:58:13 pm
content: Quote from: EHT on July 12, 2017, 08:50:53 amNotice the title specifically calls out the 97, not the 95. If you took the time to follow the link OP provided, note they do not call out the 95. If you measure the depth of the ground ring inside the 97's BNC from the plastic body, it's roughly 0.195" (5mm). I am not sure what the shield's ID measures. This would be easy enough for you to check if you feel the 95 uses the same connector.

author: k1mgy
author_rank: Regular Contributor
timestamp: July 12, 2017, 05:26:33 pm
content: It wasn't a matter of not taking the time, but rather misunderstanding there was a distinction between the 95 and 97 as to the construction of their jacks.Thanks for pointing me in the proper vector./m

author: joeqsmith
author_rank: Super Contributor
timestamp: July 12, 2017, 06:05:22 pm
content: Quote from: alm on July 11, 2017, 06:23:04 pmI guess when I think isolation, I am thinking no current paths between them. This is certainly the case with this scope. They are not isolated. I believe the DMMs black lead can be switched directly to the internal reference. The BNCs use a capacitor in parallel with a two PTCs (each in parallel) that tie back to the reference. I doubt these were added for safety so much as to protect the scope from people who may try and connect the probe clips to different potentials. I have two different revisions of the 97 and both are like this.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 12, 2017, 06:10:55 pm
content: Quote from: k1mgy on July 12, 2017, 05:26:33 pmI don't know if there is or not. This is why I gave you the dimensions so you could do a quick check. I have attached a picture of one of the BNCs on my 97. Maybe a quick visual is good enough.I contacted Fluke to see about getting some replacement parts for mine and I think that all four connectors and the plastic part they fit into along with maybe the shield were all one assembly. No matter as they told me they had no inventory for any of the 97's parts.

author: teus
author_rank: Contributor
timestamp: April 09, 2021, 11:25:12 am
content: Resurrecting this old topic...At work we've got an old Scopemeter with these insulated jacks. It's quite a puzzle to figure this all out. We got one PM8918 probe with a bent tip, and it's missing the piece to attach the grounding. Not sure when you need the grounding, or not. We've also got a Testtec probe with it, I thought it worked but I need to order a new tip you can just screw in.I mostly need it with a BNC-SMA adapter, so from Pomona I'm ordering an insulated BNC plug (73104-0) to make a cable.This thing eats batteries, the battery pack long died and we got no external PSU. I've 3D printed an adapter to fit 2x 18650 li-ion cells in series. I'm still waiting for some protected 18650 cells, so they just cut out when the voltage gets too low. Loose terminal clips from Aliexpress, printed in PETG. The flex from the accordeon ensures good contact with the terminals, since only one of the 18650 contacts has a spring.http://satsuki.nl/teus/projects/3D/scopemeter/2020-05-20%2013.43.25.jpghttp://satsuki.nl/teus/projects/3D/scopemeter/2020-05-20%2013.44.01.jpg

author: alm
author_rank: Super Contributor
timestamp: April 11, 2021, 10:16:49 am
content: About the reference lead: it's not called a ground lead because there's no guarantee that this lead is at ground potential, hence also the extra insulation compared to normal scope probes. Asking if you need a reference lead on an isolated scope on an isolated scope is like asking if you really need a negative lead on your DMM: How is current supposed to flow with only a single conductor? I'm guessing there will probably be some capacitive coupling from the scope meter to ground. But the measurement will be extremely inaccurate. And the low-pass filter inside the PM9818 probe is likely not going to work at all, since I imagine that it uses an LC/RC filter between the signal and reference leads. So yes, you do need a reference lead on your probes