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Very high input impedance high voltage meter???

Discussion in 'Electronic Repair' started by Norm Dresner, Sep 17, 2004.

  1. Norm Dresner

    Norm Dresner Guest

    I have an HP200CD that needs some loving care. The manual says to measure
    the difference in the voltage between two different points in the circuit --
    it shouldn't be all that much but the voltages between these points and
    ground is in the hundreds of volts range. The manual also says that I
    should be using a meter with 220M ohm or greater input impedance to make the
    voltage measurements. Any suggestions where such a beast could be
    bought/rented/stolen?

    TIA
    Norm
     
  2. Dbowey

    Dbowey Guest

    Norm posted:

    << I have an HP200CD that needs some loving care. The manual says to measure
    the difference in the voltage between two different points in the circuit --
    it shouldn't be all that much but the voltages between these points and
    ground is in the hundreds of volts range. The manual also says that I
    should be using a meter with 220M ohm or greater input impedance to make the
    voltage measurements. Any suggestions where such a beast could be
    bought/rented/stolen?
    When the HP200CD was manufactured there weren't any meters better than a VTVM,
    so you can use one if you have it.

    Don
     
  3. Wild Bill

    Wild Bill Guest

    I believe that the FET input analog meters have very high impedance, but I
    don't know the specific range for any specific model.

    Adding a high voltage probe to a meter's input will increase the impedance.
    For example, the Beckman 300 series DMMs have an input impedance of 22 M
    ohm, and the high voltage probe increases it by x 1000.
    The HV probe sheet lists the input resistance as 1000 M ohm for the 50 kVDC
    HV-211 probe.

    There are other multiplier probes for meters to increase the DC input range
    of meters to the 5 to 10 kV range. These would also produce a similar input
    impedance increase.

    A typical combination analog meter built into the HV probe might be a less
    accurate instrument.

    Cheers
    WB
    ..................
     
  4. Norm Dresner

    Norm Dresner Guest

    Come to think of it, I have a high voltage probe for my VTVM that'll
    increase it to IIRC 100M ohm -- that's getting much closer and probably will
    be sufficient.

    Norm
     
  5. Jerry G.

    Jerry G. Guest

    Any good DVM with a high voltage probe, such as what is used for TV
    servicing should do the job. Most are about or greater than 100 meg/Volt
    when
    used with the standard 10 meg/Volt DVM.


    --

    Jerry G.
    ==========================


    I have an HP200CD that needs some loving care. The manual says to measure
    the difference in the voltage between two different points in the circuit --
    it shouldn't be all that much but the voltages between these points and
    ground is in the hundreds of volts range. The manual also says that I
    should be using a meter with 220M ohm or greater input impedance to make the
    voltage measurements. Any suggestions where such a beast could be
    bought/rented/stolen?

    TIA
    Norm
     
  6. Jim Adney

    Jim Adney Guest

    There are such devices, but any modern DVM with a 10 MOhm input
    impedance will work just fine to measure the voltages in your 200CD.
    That warning is there because when the 200CD was made the common VOMs
    of the day had much lower input impedances, so HP wanted to keep you
    from using those.

    I've worked on 200CDs with 10 MOhm meters with complete satisfaction.

    -
     
  7. Norm Dresner

    Norm Dresner Guest

    Jim

    Thanks for the encouragement. I've almost decided not to try to repair it
    but to build a modern, solid state oscillator using the mechanical
    components as the basis. The dual variable capacitors are an almost
    impossible item to find and there's more than enough space in the chassis
    for anything with transistors or IC's. And, as a bonus, the bottom of the
    interior foundation should serve to isolate the incoming AC from the
    sensitive circuitry in the upper compartment.

    Norm
     
  8. James Sweet

    James Sweet Guest

    If you do decide to retrofit it with modern internals, a guy I know has a
    book, I think it's called Analog Circuit Design, anyway he showed me a
    schematic in there of a solid state oscillator using an incandescent lamp to
    stabilize it just as the HP uses, he built it and said it performs very
    well.
     
  9. Norm Dresner

    Norm Dresner Guest

    Yup. The book is "Analog Circuit Design" . It's published by
    Butterworth-Heinemann in their EDN Series for Design Engineers. The editor
    is Jim Williams and the article you're referencing is #7, "Max Wein, Mr.
    Hewlett, and a Rainy Sunday Afternoon" by Jim Williams, p 43ff. The book is
    on a shelf in my library already.

    Norm
     
  10. Jim Adney

    Jim Adney Guest

    I'm sure that with Jim William's help you could do a good job at this,
    but the 200CD was really a very nice instrument by the time HP
    discontinued it. I think it had a nearly 30 year lifetime, during
    which it got many significant upgrades. If you have an early one,
    there might be some reason to "rebuild" it, but I suspect that it
    would be hard to do better than the late versions. Plus it would take
    a lot less time to fix than to repair.

    I have some NOS 200CD replacement parts here (caps and light bulbs.)

    If you send me your serial number I can tell you how early or late
    yours is.

    -
     
  11. Norm Dresner

    Norm Dresner Guest

    See replies in-line.

    I'm not quite sure what the distinction you're making between "fixing" and
    "repairing". Please elaborate.
    I think that the first step would be to replace the tubes, wouldn't it?
    005-28015


    Norm
     
  12. Norm Dresner

    Norm Dresner Guest

    See replies in-line

    I'm quite unclear what distinction you're making between "fix" and
    "repair". Please elaborate
    I'd think that the first step would be to replace the tubes, wouldn't
    it?
    005-28015


    Thanks
    Norm
     
  13. Jim Adney

    Jim Adney Guest

    Oops, sorry, Mind fade. Make that fix vs. "rebuild."
    Not necessarily, but I'll admit that mine needed tubes. It worked fine
    on the higher frequencies, but crapped out down low. The longer period
    at low frequency operation appeared to be able to completely dissipate
    the space charge in the tubes, while they worked just fine up higher.
    1965 That would be a middle era for the 200CD. I think HP sold these
    from about '50 or '55 thru '77. Not great, but not bad either. OTOH,
    none of them were awful. ;-)

    -
     
  14. Norm Dresner

    Norm Dresner Guest

    Still, there are published schematics for Wein Bridge oscillators with
    <.001% THD and the hardest parts to find to implement a continuously
    variable one are dual tracking pots or variable capacitors. The dual
    capacitor in the 200CD looks to be "ideal" for putting into a modern design.
    If I decide that the criterion for deciding which way to go is just the
    quality of the result, there's no contest, solid state wins hands-down. If
    it's effort, changing a few tubes can't be beat. But if it's the ratio,
    quality/effort, then it's a much harder decision. But I'm coming up on
    retirement very rapidly so the equation changes drastically ...

    Norm
     
  15. Chuck Harris

    Chuck Harris Guest

    The only real reason that HP required you to use a 220M ohm input impedance
    meter is their HP410C was a 220M ohm input impedance meter. As was their
    earlier HP410B, and earlier HP412A.

    -Chuck
     
  16. Norm Dresner

    Norm Dresner Guest

    Gotcha. It's pretty much a nonstandard value these days -- and I think it
    was back then too.

    Related questions: The HP200CD service manual says, in part, "The DC
    voltage between the cathodes (pin 3) of the 6AU5 tubes should be 1 volt or
    less. If this voltage is excessive, it indicates a bad tube in the
    oscillator (V1-V4) which must be replaced to insure low distortion in the
    output waveform."

    1. What is "excessive"? 1.5V? 1.1V? anything over 1.000V? ...

    2. The cathode voltage for each tube is given as 3.5V. Clearly if one of
    them is way off, it's pretty much the one that should be replaced. What if
    they're both close but the difference is a little over 1.0V? Should I just
    replace both?

    3. Should I just replace all 4 tubes in the circuit? There's also a
    rectifier tube whose output is filtered by a couple of 10 uF capacitors
    [actually three sections of a single capacitor) and a 6H choke. How much
    60/120 Hz ripple on the DC output of the power supply would suggest that the
    tube and/or the capacitors should be replaced too?

    4. The really hard question: Am I still trying to polish a turd and should
    just replace all of the electronics with a good quality IC-based Wein Bridge
    using the dual variable capacitors as the basis for this?

    Norm
     
  17. James Sweet

    James Sweet Guest

    Anything over 1.0v is excessive.
    Can't help you there.
    Have you tested the tubes? How much are replacements? If they're cheap then
    it wouldn't hurt to replace them to see if it helps. Don't forget to change
    out any electrolytic capacitors too.

    I would absolutely attempt to restore function of the original instrument,
    vacuum tubes can still perform very well and that oscillator will provide
    excellent performance once you get it going.
     
  18. Chuck Harris

    Chuck Harris Guest

    What they are trying to show is the balance between the different tubes.
    Anything over 1V is excessive (in their opinion). If you let the balance
    get too far off, the oscillator will be prone to distortion, and hum.
    It probably doesn't matter what the voltage is exactly, as long as it is
    around 3.5V, and less than 1.0V difference.
    I would suggest that you pay more attention to how much ripple is on the
    output of the oscillator. What I generally do is test the caps with a good
    capacitor tester. If they seem a little leaky, I reform them for a while.
    If they don't settle down to very minimal leakage current, or if the ESR is too high,
    I replace them.
    Keep the 200CD! They are virtually bullet proof. They will last virtually
    forever. You can always build an IC Wein bridge oscillator if you really need
    super low distortion.

    -Chuck
     
  19. Norm Dresner

    Norm Dresner Guest

    As a semi-final coda to this saga, I've ordered a set of three [one spare]
    6SH7 tubes and I'm actively looking for a good source for the 6AV5GTs as
    well. I'm also looking to pick up on eBay a non-working HP 33x distortion
    analyzer or similar oscillator from which to scrounge the dual tuning
    capacitors so I can make a solid state low-distortion instrument as well.

    I'll also look into replacing the electrolytic capacitors.

    Thanks for all of your help and suggestions

    Norm
     
  20. James Sweet

    James Sweet Guest

    Is there any reason you couldn't gang together a pair of ordinary variable
    capacitors using a gear or belt system? You could probably get away with
    some cord as used in tuning dials on old analog tuned radios too.
     
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