question about oscilloscope probe

Discussion in 'General Electronics Chat' started by lokeycmos, Mar 23, 2013.

  1. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
    i picked up a scope probe today at a flea market. it is a 100mhz switchable between x1 and x10 with a compensation trimmer built in. I have attached some pictures. before I got this probe I was simply using a piece of coax stripped back for my probe with a ground clip soldered on. when I connect my function generator to the scope using my homemade probe and set to 10v/division I get a proper square wave signal. but when I connected the new probe set to x1, the only way I get a reading on the scope with everything else the same is to turn it all the way down to .01v/division. expectedly its is even less when I switch to x10. so my question is, if it is at x1, why is it so attenuated compared to just a straight piece of coax? is it possible I got a bad probe? ty

    one more thing, why are the edges so round with x1?
  2. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
    more pics of probe
  3. JohnInTX


    Jun 26, 2012
    First, select DC coupling on the vertical and see how that works.

    If it doesn't, and I were going to guess I'd say that the probe is open at the switch in 1X mode. The spikes you see look like a (stray)capacitive-coupled input. Pomona made generic probes back in the day but I never used them so don't know how good they are but would expect them to at least outperform wire with tape on it. You may have an old one with issues. Have you checked continuity at 1X?

    One thing to consider is that your scope input is likely not matched to the probe. 1M 45pf is pretty old school and newer 1M 10pf probes may not have enough compensation range to get good edges. That said, I would expect a rolling off of the input, not a differentiation.

    As shown, your waveforms suggested an overcompensated probe but that's usually not a problem in 1X mode, so.. I'd check the switch, whether the hood is making good contact with the center pin etc.

    Awesome old scope though. My first personal scope was a Tek 545(?) with a 1A1 plugin. Loved it until traded up for a 465 which I used into the 90's. Now its a DSO but, gotta love the old stuff!

    Thanx for the trip! A scope with an astigmatism control! Love it.
    Last edited: Mar 23, 2013
  4. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
    thanks for the help guys! the probe does work. for some reason the component lead clip was not making connection to the probe. when I switched tips it works great!
  5. crutschow


    Mar 14, 2008
    Those probe tips sometimes have to be firmly pushed unto the probe or they don't make contact.