Scope Probe Cut-Out Problems

Discussion in 'General Electronics Chat' started by sjgallagher2, Jan 24, 2016.

  1. sjgallagher2

    Thread Starter Member

    Feb 6, 2013
    111
    7
    Hey all,
    I'm having a devlish time with several oscilloscope probes I own. I have several probes for my oscilloscopes, including some cheap ones that came with a cheap scope, a bunch of old ones that I don't trust too much, and a pair of 200MHz probes that I bought recently to make sure I was getting clean measurements on some of my RF projects.
    One issue appeared recently in the cheap "came-with-the-scope" probes where they don't connect to the oscilloscope at all unless they're at the right angle, like a poor pair of headphones. It seemed that pushing in towards the scope would bring the probe back to life (I'd just sit there and put my finger on the end and mess with things to see when it came back) but all in all it is too finicky to use. I took one of them apart, because the other didn't have the problem. I couldn't make much out of it, because the probes are fortress-like in their assembly. (So much rubber!) I was tearing thing with cutters to get to where I thought the problem was, but couldn't find much.
    I assumed it was the cheap probes, and continued using my better probes, when suddenly the exact same problem happened to both of them, rendering them useless most of the time. But, again, sometimes I can push in towards the oscilloscope and get them to jump back to life for a second. I hate breaking things, because I'm in college, and I don't have money. I love fixing things, but after that first probe I lost some confidence.
    I hang my cables on a pair of hooks next to my work station and grab scope probes just above the probe itself to distinguish them from the rest of the cables and wires, then give it some tugs- only on the cable itself, not near connections.
    So I'm thinking this must be a common problem. It seems unlikely that multiple probes from different manufacturers have the exact same problem. I'm trying to retrace my steps to find where the problem is, because this all started once I got to college. But that's also where I used them most, so it's not sure-fire. For what it's worth, I have four oscilloscopes- a digital, two analog, and a USB. Same problem on all. In the mean time I'll continue investigating. (Pictures upon request, though they all look identical to one-another.)
    If anybody out there knows what's going on, and how to fix it, I would be forever grateful. Fixing = good. Buying new ones = bad. Bonus points for identifying where the problem is. Thanks so much in advance.
    Sam Gallagher
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
    3,242
    619
    I have Tek/HP and cheap probes and have not had reliability problems with any of them.

    Stop abusing them by tugging on the cables. I think that's your problem... Treat them like their worth hundreds of dollars because that's what good Tek and HP probes cost.
     
  3. #12

    Expert

    Nov 30, 2010
    16,261
    6,770
    I recently started having trouble with my probes at the BNC connection to the scope. Probably because I bought them in 1978!

    "Everything put together, sooner or later falls apart." (Paul Simon)
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,000
    3,229
    It may just be the BNC socket in the oscilloscope has developed a bad contact.
    You might try squeezing the BNC center metal contacts slightly together with a small needle-nose pliers to see if that helps.
    A more elaborate fix to try would be to replace the BNC socket with a new one
     
  5. dl324

    Distinguished Member

    Mar 30, 2015
    3,242
    619
    If it's happening on 4 different scopes, it must be the probe connector as you've stated.

    Can you post a close up picture of the bad probe connector? Some connectors use the center conductor as the pin, others have a gold plated pin. For ones with gold plated pins, the center conductor can be soldered, crimped, or just friction fitted.

    EDIT: Scope probes use a flexible center conductor, so scratch the "conductor as pin" and "friction fitted" options. Don't apply in this case...

    It has been my experience that the tip end is more delicate; but it's supported/protected by the probe "housing".
     
    Last edited: Jan 25, 2016
  6. mescab

    New Member

    Jul 17, 2015
    7
    0
    I also think it is BNC socket problem, replace with new if your problem not yet solved.
     
  7. sjgallagher2

    Thread Starter Member

    Feb 6, 2013
    111
    7
    I have always treated my probes with respect- no probe should be that delicate. But I have to face the facts. Upon taking apart one of my better probes, I see no connection between the coax and the solder gob. I added pictures of what I'm looking at.

    20160125_173220.jpg 20160125_173237.jpg 20160125_173251.jpg
    Now I'm working at stripping that tiny piece of insulation there, teflon or something, not easy to strip. I tacked at it with a find blade and touched with my soldering iron. I also prodded it with fine tweezers, yanking as I can. It's really tough. Not even that, but I'm finding no conductor inside. I expected something visible to the naked eye, but nothing yet. I'm running out of room here. What to do next, you think? Better question, what size conductor am I expecting to find in here?
    Thanks again everyone for all the help.
    Sam Gallagher
     
  8. sjgallagher2

    Thread Starter Member

    Feb 6, 2013
    111
    7
    Update: I've found the conductor! A tiny, stiff little thing. New pictures:
    20160125_175837.jpg 20160125_175921.jpg
    You'll need to look close but it's clearly there, coming out of the white. Now I have to manage to reconnect it, so that'll be a good problem. Thanks for good advice on probe-care all!
    Sam Gallagher
     
  9. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    I am not seeing a good trouble-shooting methodology here. You said you have oscilloscopes, (more than one.) Does the problem duplicate between scopes??

    I have a Tektronix 2445A that had a similar problem on the second channel. Long story short, it was not the probes, and not the channel BNC connector, it was a bad (intermittent) input module.
     
  10. sjgallagher2

    Thread Starter Member

    Feb 6, 2013
    111
    7
    Saying that I meant I had the same problem on other scopes. The problem duplicates. Upon measuring continuity between the tip of the probe to the small wire I just located, by the way, there was no connection. I am able to touch some anti-static tweezers to the solder gob and see noise when I do (vividly!) but that small wire is not connected to the probe tip on x1 or x10. A similar test with some of my other probes showed connection, so that's where Im at. Not sure where to go next to check for connections, the probe itself seems harder to take apart...
     
  11. sjgallagher2

    Thread Starter Member

    Feb 6, 2013
    111
    7
    Now I've dissected the scope and have a mere piece of coax with no connection from end-to-end. But for a few flickering minutes I got no resistance. Some odd is going on. Wires are tiny so I'm trying to prevent them from slipping from test leads. Continuing work...
     
  12. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    I bought the cheap Tektronix scope probe copies on EBay and they compare very well against the old school Tektronix 100 and 200 MHz probes.
     
  13. RichardO

    Well-Known Member

    May 4, 2013
    1,230
    382
    You may not be getting a good solder connection to the wire when you do repairs. The wire is not "normal" wire -- it has resistance -- it may be NiChrome. I don't know how to solder the wire. NiChrome is normally welded.
     
  14. sjgallagher2

    Thread Starter Member

    Feb 6, 2013
    111
    7
    Here's where we're at people. The problem, stated, is now this: I have a coax cable where when you pick it up in the middle, the resistance is <1k. But if you set it back down it's an open. I posted a video on google+ in an attempt to show what I mean! Don't know if this link will work, though. Here's a try. Let me know if anybody has seen this problem before. Thanks a lot guys!

    Postscript:
    Link to original probes I bought is here.
    The problem came from the middle of the cable, which now makes sense. The way I would get to the probes put a lot of stress on the middle of the cable. It's my own fault, and I've figured it out. But the problem remains: how in the world can I fix this these probes.
    Sam Gallagher
     
    Last edited: Jan 26, 2016
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