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Tektronix 211 Scope Probe

Discussion in 'Electronic Repair' started by Wild Bill, Sep 23, 2004.

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  1. Wild Bill

    Wild Bill Guest

    Hi.. a friend asked me to have a look at the probe for this model 211 mini
    scope, which had been working properly until he disassembled it, which was
    quite a while ago.
    I'm hoping someone can comment on the center conductor path components of
    this probe.

    Tek p/n marked on body 010-0378-00
    10x ~ 20pF 4.4 M ohm

    Resistor markings 4 M 1% T0 PME 62S
    PFR 8216 (date code I suppose)

    The various parts of the probe assembly are threaded together in a modular
    fashion. I just have the parts of the probe here (not the cable, which
    appears to plug into the back end of the probe assembly).
    Within the body (the hollow cavity which the spring-hook screws onto), there
    is the precision resistor 4 M ohm.
    The resistor has a peculiar mechanical connection at the back end.. it's a 3
    fingered friction fit contact connection to the metal tube that's part of
    the other section of the assembly.

    So far in the voltage path, there's the exposed thread for the hook tip,
    then the resistor, then it's friction contact to the metal tube.

    The center conductor from the rest of the assembly is a solid tinned copper
    wire that's bent into a small loop (folded back over itself to form this
    loop).
    This wire loop extends down into the metal tube when the probe parts are
    assembled, but the loop doesn't appear to be the intended electrical
    connection.

    It would seem that there was/would be some other method of contact between
    the wire loop and the metal tube, or to the peculiar end terminal of the
    resistor.

    The metal tube also seems unusual, in that it's part of the center conductor
    circuit, and subject to a lot of inductive pickup since there's no shielding
    for it.

    Thanks
    Cheers
    WB
     
  2. mike

    mike Guest

    This appears to be a 10X attenuator that the attached 211 probe plugs into.
    The assembly drawing calls for a 825K resistor. That doesn't make any
    sense to me, but that's what the drawing says.

    Not at all clear from the drawing, but it looks like this is a shunt
    resistor. There's no mention of another resistor. It's possible that
    it's built into the 204-0554-01 "attenuator body".

    mike


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  3. TekMan

    TekMan Guest


    The metal tube is part of capacitive divider compensation.

    And the unsual resistor value has to do with the special contruction
    od the tek 2xx probes. These differ from other known 10:1 10 Megohm
    probes.

    The tek 2xx series is special in many details, so the manual is a big
    help.


    Andreas
     
  4. Wild Bill

    Wild Bill Guest

    Thanks Mike and TekMan, I'll have him bring the scope over to see if the
    probe and lead check out OK.

    My main interest was whether the 10x attenuator section is missing a part.
    The 4 M ohm resistor in the body section is connected to the metal tube by
    the peculiar 3-fingered connector/terminal, but the wire from the back
    section just protrudes into the tube, without a solid connection to the tube
    or the resistor.

    Thanks
    Cheers
    WB
    ................
     
  5. Jim Adney

    Jim Adney Guest

    That's a variable capacitor bypassing around the resistor that sits
    inside it. By screwing the probe part more or less together, you get
    more or less capacitance around the resistor. The ratio of capacitance
    to resistance needs to be the same at every stage of the voltage
    divider (which is what the probe really is) in order to get equal
    voltage division at all frequencies.

    You would attach the probe to a known good square wave and adjust the
    capacitance to display a good square wave on the scope, then you knew
    that your probe was properly "compensated."

    Keep in mind that some Tek probes used coaxial cables that had
    reasonably large resistances built into their center conductors. This
    often leads to the resistors you can find not adding up to the 10 MOhm
    that you expect.

    -
     
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