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How does probe attenuation affect input impedance on a scope

  1. May 7, 2016 #1
    upload_2016-5-7_21-2-50.png

    I assume the 10X setting to be the most useful with audio work. Comments??
    My oscilloscope has 1M ohm input impedance when set at 10X as stated above.
    What is the likely impedance at 1X ?
    The manual indicates the above chart is based on the probe being set at 10X. As I am dealing with DC voltages more that 400 VDC (<400 VDC to <= 700 VDC) I assume I need a 100X probe to measure vacuum tube plate conditions that exceed 400 VDC. Yes?

    Cheers,

    Billy
     
  2. jcsd
  3. May 7, 2016 #2

    jim hardy

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    can you scan and post that ?

    I'm from old days
    some scopes were 50 Ω in
    most were 1 meg and a few pf as your chart says
    since the 1X-10X switch was in the probe, the scope stayed 1 meg but there was 9 more meg in the probe
    and that's why the probe had a trimmer cap to adjust out the pfs from 'scope and probe's coaxial cable.
    By the time i got old, the 'scope acquired another ring on its BNC connector through which the probe apprised the 'scope of whether it was selected for 1X or 10X, and the 'scope's internal microcomputer adjusted the numbers on the screen to match. .

    Now scopes have a computer menu so complex that i need to have it interpreted by young people.
    So I think
    your scope is 1 meg on X1 and the probe makes it 10 meg on X10
    and the max voltage is 400 at the 'scope input
    and
    the manual that comes with your probe will tell how for much voltage the probe remains safe... when selected to 10X it divides that voltage by 10 before handing it to 'scope, but i doubt it's good for 4kv.

    Here's a "bargain " probe i bought
    http://www.mpja.com/20MHz-Oscilloscope-Probe/productinfo/15085+TE/
    Quality 200MHz oscilloscope probes with a BNC connector to match to most equipment. Max Input: 600V AC P/P + DC.
    in other words, 600V at any instant, even peak of a sinewave atop DC.
     
  4. May 7, 2016 #3

    jim hardy

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    Just checked my Tektronix P6109
    500V peak+dc, drops with frequency to about 80 volts at 10 mhz.
     
  5. May 7, 2016 #4
  6. May 7, 2016 #5
    These questions are really about two things. One can use a 10x or 100X probe to measure higher voltages. Using a 10X or 100X or 1000X will also change the impedance which could be even more useful in some cases.

    As a side bar, many of my friends who mess with electronics do not work on tube amps because of the high voltage issues and also because they were never given any information about vacuum tubes in school.

    This is the best written text on how a vacuum tube works I have read.
    http://jacquesricher.com/NEETS/14178.pdf [Broken]
     
    Last edited by a moderator: May 7, 2017
  7. May 7, 2016 #6

    jim hardy

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    page 82 says
    450 volts at any instant with the supplied 10X probe set for 10X
    i didnt see anyplace a specified max for probe set to X1
    .... tech writers should have included that
    one of those things we find the hard way.

    Myself I'd buy one of those X100 probes for tube work..
     
  8. May 7, 2016 #7
    I have it on order as we speak Jim. Better safe than sorry. I have taken it slow on where and how to use the oscilloscope. There are many ways to screw up.

    I understand the issues with the scope being grounded and a short that could happen with AC.

    I am not so sure I understand why it would be any safer to use a portable battery powered scope than using mine which can be powered on battery. Well..there is the issue with the portable scopes that there is no common connection to the separate probe ground clips. I don't see why I could not use only one channel on my scope while on battery not connected to earth ground to measure the voltage drop across a resistor in a mains leg hot or netural as a example.

    Am I missing something??
     
  9. May 7, 2016 #8

    jim hardy

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    Just be aware the 'scope common floats at potential of wherever the probe's ground clip is hooked.
    I dont know whether it has any exposed metal that's wired to common
    i'd probe everywhere with an ohm-meter to find out
    and be doggone sure to use only one ground clip at a time.
     
  10. May 7, 2016 #9
    Yes,I agree with everything you said. For one thing, I have not had any reason to try such a thing. And no, everything on the outside of the scope is plastic, so no exposed metal.
    The only example is probing the secondary of a transformer before the rectifier and there is no real need to use the ground clip in the first place.

    I am trying to force myself to put the DMM down and get use to using the scope more. There are some issues with my scope because the voltage read out is very small and hard to see....getting old and blind...lol

    There is a lot of skill required to use a scope and I am not at the point I know much yet.
     
  11. May 7, 2016 #10

    jim hardy

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    It's all in selecting the trigger
    think about it
    when do you want the dot to start across the screen ? At some level on the wave you're measuring? Select trigger source to that channel, select slope + or -, AC or DC coupling, lastly adjust trigger level so trace starts at desired point on the wave. Do it in that order.

    Auto Trig reverts to Line Triggering ; if no legitimate trigger comes along by next line cycle the dot starts across screen anyway so you'll know scope is just waiting....,
    it'll go across screen 60 times a second in absence of a signal big enough to trigger....
    I prefer to have it dark when no trigger but that's just me....


    In lots of power supply work you're looking for line frequency noise so select trigger source to LINE and you'll be synced with the noise you are looking for. Adjust level to make dot start where in the sinewave you want.

    Ignore delayed sweep until you are fluent with regular triggers. Basically it lets you wait forsome time after a trigger to start the dot.
    When you need it you will learn quickly.

    Scope is like the Queen in chess, your most powerful tool. Yours has frequency analyzer built in....... FFT.

    Old jim
     
  12. May 8, 2016 #11
    Thanks Jim. I have been studying the manual and other books on the use of scopes in general. I have finally been through every menu in my scope and pretty well remember where to get to things. Little by little I am finding my way around. I have got to the point where I can inject a signal into a amp and trace the signal to where it stops to find faults. The learn curve is getting less of a problem.

    I have in mind to do some exploration into a known good working amp to see what things look like when there are no faults. I hope to be able to use the scope to identify where things are on a PCB. For example many times there are several diodes that are side by side that connect to different secondary connections on a transformer. With traces on both sides of the PCB and no PCB layout diagram, it is often hard to know what is connected to what. At other times I am working on some amp that I have no schematic at all so I need all the help I can get.

    At least for tube guitar amps in general the designs are more alike than different. Having said that, I repaired one a few day ago that the input did not go directly to the grid of the first pre amp tube. It went through a a bunch of resistors and caps and a couple of JFET's before arriving at V1. I had no signal at the grid of V1 and it stopped at the first JFET. As it turned out the JFET was ok but no current was getting to it do to a bad cap and a open resistor. I am sure there would have been a better way to use my scope to discover the fault. I never started out wanting to work on solid state stuff but it is everywhere in guitar amps now days so I have little option but to bite the bullet and learn it. Such is life!!

    Cheers,

    Billy
     
  13. May 8, 2016 #12

    jim hardy

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    You'll get more fluent, it's no different than learning a new language growth is exponential.
    In general if you see signal on one side of a cap and not the other something is fishy and caps do sometimes go open...
    You have the right approach, follow signal stage by stage. After verifying power supply is good.
     
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