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10ms timebase cap

Discussion in 'Electronic Repair' started by Creamed_emu, Feb 3, 2004.

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  1. Creamed_emu

    Creamed_emu Guest

    Im restoring an old dual trace valve oscilloscope and after geting the set
    functioning i have found there is no 10ms horozontal deflection. I found
    that cap that is short circuit. it is an old clear plastic/foil cap. the
    problem is part of it has melted, the part with the value on it. all that is
    there is ***,000p and 669 written under it. Does anyone know the value of a
    10ms cap that belongs there? i dont have the schematic. I was thinking mabey
    if it used the same resistor for the rc oscillator (that is how it works?) i
    could compare the cap values of other scan rates. i believe the 10us uses
    190pf and the 1us uses 360 pf. is there any way to find the value of the
    missing cap? This scope has an enormous CRT in it and becuase it is designed
    with valves, its virturally indestructable - Perfect for a beginner like me.

    Anyway, thanks for any help you can offer - Ben
     
  2. Jim Yanik

    Jim Yanik Guest

    Check the other timing caps,the unknown cap should be a decade multiple of
    the rest of them,and then using resistors for the 1,2,5 submultiples. They
    were selected and sold as matched sets,and there were letter codes to
    indicate the selection groups,like +C,-A,etc.(for TEK scopes) Maybe
    10,000pf? For that slow a range,it would have to be fairly large,more in
    the uF range,IMO.

    669 may be a date code.

    It would also help if you mentioned the make and model of the scope.
    Perhaps a TEK 500 series?

    The sweeps are generated with a gated Miller integrator(ramp generator),not
    an oscillator.(at least in a TEK scope.)
     
  3. Ben
    It would be helpful if you tell us the make and model of the 'scope.
    Martin.
     
  4. mike

    mike Guest

    Good thing that every scope ever made used the same cap.
    That way you don't have to disclose the model.
    Absent any possiblilty of getting relevant information,
    maybe you should just
    stick in a cap and measure the sweep speed. Shouldn't take more
    than two tries to plot the graph and determine the correct value.
    If you care about the calibration, it's unlikely that you'll be able
    to buy a (single) stock cap close enough.
    mike

    --
    Return address is VALID.
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    Tektronix Concept Books, spot welding head...
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  5. And it's pretty easy to try one with an adequate voltage rating and
    extrapolate from there. :)

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  6. Jim Yanik

    Jim Yanik Guest

    Well,he could parallel caps to trim it to spec,given a reliable cal
    source.I've seen that done in TEK 500 series scopes.
     
  7. mike

    mike Guest

    Hence the use of the word (single)...
    My point is that knowing the number printed on the cap won't necessarily
    solve the problem.
    mike

    --
    Return address is VALID.
    Bunch of stuff For Sale and Wanted at the link below.
    Toshiba & Compaq LiIon Batteries, Test Equipment
    Honda CB-125S $800 in PDX
    TEK Sampling Sweep Plugin and RM564
    Tek 2465 $800, ham radio, 30pS pulser
    Tektronix Concept Books, spot welding head...
    http://www.geocities.com/SiliconValley/Monitor/4710/
     
  8. Asimov

    Asimov Guest

    "Creamed_emu" bravely wrote to "All" (03 Feb 04 12:05:12)
    --- on the heady topic of "10ms timebase cap"

    It doesn't make sense that a longer timebase uses a smaller value cap.
    Make/model missing, etc...

    Cr> From: "Creamed_emu" <>

    Cr> Im restoring an old dual trace valve oscilloscope and after geting the
    Cr> set functioning i have found there is no 10ms horozontal deflection. I
    Cr> found that cap that is short circuit. it is an old clear plastic/foil
    Cr> cap. the problem is part of it has melted, the part with the value on
    Cr> it. all that is there is ***,000p and 669 written under it. Does anyone
    Cr> know the value of a 10ms cap that belongs there? i dont have the
    Cr> schematic. I was thinking mabey if it used the same resistor for the rc
    Cr> oscillator (that is how it works?) i could compare the cap values of
    Cr> other scan rates. i believe the 10us uses 190pf and the 1us uses 360
    Cr> pf. is there any way to find the value of the missing cap?
     
  9. Jim Yanik

    Jim Yanik Guest

    TEK's timebase caps were *standard values* that were matched to eliminate
    tolerance deviations.Knowing the original value would be a great help.
     
  10. mike

    mike Guest

    I'm in a mood to rattle your cage so I'll ask, "Isn't "matched"
    the very definition of "non-standard"???? ;-)
    Since the OP refused to give any clues about the scope in question,
    that's not possible.

    I'm really RANTING about people who ask questions
    with insufficient specificity to allow ANY useful information to be
    provided.
    Unless you have the Psychic Hotline on speed dial, you ain't gonna
    be able to answer the question.

    If the OP had merely mentioned the make/model, (and it was a TEK), you
    or any number of us could have given the value, part number, cautions on
    cap type and Deane Kidd's phone number to get one.

    As for "great"...well...to make this work, you're still gonna have to
    stick in a cap, measure the sweep speed and either parallel another cap
    or find one closer to the short end of the tolerance that can be
    paralleled up to the needed value. Knowing the start value would save
    a step in the experiment...but that would save less time than the OP
    spent (not) asking the question. Don't get me started on cap
    construction, temperature sensitivity, dielectric absorption...and the
    zillion other things one might want to think about to match the other
    caps and compensation in the system.

    mike



    --
    Return address is VALID.
    Bunch of stuff For Sale and Wanted at the link below.
    Toshiba & Compaq LiIon Batteries, Test Equipment
    Honda CB-125S $800 in PDX
    TEK Sampling Sweep Plugin and RM564
    Tek 2465 $800, ham radio, 30pS pulser
    Tektronix Concept Books, spot welding head...
    http://www.geocities.com/SiliconValley/Monitor/4710/
     
  11. Jim Yanik

    Jim Yanik Guest

    No."matched" is *selected* standard caps,like picking the caps (out of a
    big bin of them)that are -5% of *marked value*,then picking the ones that
    are +3%,and using them with other caps of the next decade standard value of
    the SAME deviation,-5% with -5%,+3% with +3%,until you get a -matched-
    SET.But the values are the standard ones the manufacturer has.
    Like the ones listed in Digi-Key,where the chart says "standard values".
    (like .033uf,330pf,.1uf,10uf,etc)
    Sure,you want to use polypropylene caps or silver mica depending on the
    value needed.If you can't get those,then polystyrene.But considering most
    scope timebases are only 3% accurate,that's good enough.

    BTW,I worked for TEK for 21.5 years as a scope/instrument repair and cal
    tech;Indianapolis and Orlando Field offices.
     
  12. mike

    mike Guest

    My statement was that it's unlikely you can go buy a cap that will plug
    right in and be (specified in such a way that the sweep will be)
    calibrated. Even if you got the exact valaue, it's not guaranteed that the
    other caps in the matched set would be within 3% of nominal.
    Of course, there's always random luck.
    I understand we're arguing semantics, but that's why arguments were
    invented.
    Congratulations. You got me beat. I only spent 20 years at
    TEK Beaverton as a design engineer.
    mike



    --
    Return address is VALID.
    Bunch of stuff For Sale and Wanted at the link below.
    Toshiba & Compaq LiIon Batteries, Test Equipment
    Honda CB-125S $800 in PDX
    TEK Sampling Sweep Plugin and RM564
    Tek 2465 $800, ham radio, 30pS pulser
    Tektronix Concept Books, spot welding head...
    http://www.geocities.com/SiliconValley/Monitor/4710/
     
  13. Jim Yanik

    Jim Yanik Guest

    I agree that it's unlikely to plug in a single cap and have it within
    tolerance for the scope. (Although I used to do it at TEK,on 200 series
    miniscopes.)

    What products did you work on?
     
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