help troubleshooting HP1740A oscilloscope

Discussion in 'General Electronics Chat' started by pastinsain, Sep 13, 2012.

  1. pastinsain

    Thread Starter Member

    Aug 25, 2012
    160
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    Just picked up a HP1740A oscope and need to fix it. The problem is that
    it will not calibrate . I have the repair manual but you need a phd to understand it. So maybe someone could give some advice

    IMG_2865.JPG

    this is the image I get with volts set to .5 and time set main div .2
    I tryed to adjust the prob resistor but it has very little effect.
    When connected to a wavetek 180, can get a sine,triangle and square wave but only at high hf 1meg setting.
    Also the dc volt measure will not work. For example if connected to a nine
    volt bat it will not show the image. or any dc voltage. however, it will
    show the amplitude of a sine on hf.

    thanks for any help
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    There is a trimmer capacitor in the 10x probe, not a trim resistor.
    Make sure the probe is switched to 10x and adjust the trim capacitor.
     
  3. pastinsain

    Thread Starter Member

    Aug 25, 2012
    160
    2
    I do have it set on 10X and tryed to adjust but that has very little effect.
    Its a tektronix probe. does that matter?
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    I suspect your probe is faulty.
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,009
    3,233
    Try doing the measurement directly using a wire (coax preferable) without the probe.
     
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    is your scope set to AC instead of DC input?
     
  7. pastinsain

    Thread Starter Member

    Aug 25, 2012
    160
    2
    Ive tryed 4 different probes all the same. One of the probes is just a bmc with two
    clips. all wire no caps. all the same
     
  8. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    I think your scope is on AC and should be DC
    as you said it wont measure your 9Volt battery!!!
     
  9. pastinsain

    Thread Starter Member

    Aug 25, 2012
    160
    2
    The scope is set on dc but ac or dc same result.

    But when its connected to the function generator, it works on sine,triangle and square at 1mhz settiing but the square is lost as you turn the freq down. and no DC output at any setting. however, the sine stays displayed as you slow the freq and the triangle will get distorted as well at low freq.

    the generator works perfect with the Tektronix 475a. so its not that
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  11. pastinsain

    Thread Starter Member

    Aug 25, 2012
    160
    2
    Yes same on both channels .

    yes thats the one

    yes Dave thats the one. HP1740A
     
    Last edited by a moderator: Sep 13, 2012
  12. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Well , unless you have the settings wrong , it must be the scope at fault?

    Can you input a sine wave at different frequencies,at 1vpk, 1Khz, 10Khz, 100Khz, 1Mhz into both channels separately AC then DC,
    then try it on square,
     
    Last edited: Sep 13, 2012
  13. pastinsain

    Thread Starter Member

    Aug 25, 2012
    160
    2
    Absolutley the scope got a problem. now its a boat anchor. I will have to wade though the service manual on this one.

    I did take the covers off and its got old original power caps. and one the the power board capacitors had been replaced at one time .Also, a rectifier was replaced looked somewhat of a hack butcher job. I have not taken the test voltages as of yet
     
  14. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    OH DEAR! have you got the schematics?
     
  15. pastinsain

    Thread Starter Member

    Aug 25, 2012
    160
    2
    dogdydave I have tryed every conceivable configuration but still no resolve.
    It will not take low volt dc test That distortion you see in the example screen shot
    is the dc and ac result. I just hope its not a proprietary chip thats gone south!
     
  16. MrChips

    Moderator

    Oct 2, 2009
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    Without studying your situation more carefully, I suspect your scope is measuring AC and not DC. Hence the problem is in the AC-GND-DC switch. Try switching it back and forth many times to try and clean the switch (or use a contact cleaner).
     
  17. pastinsain

    Thread Starter Member

    Aug 25, 2012
    160
    2
    Yep I got all the schematics and service manual in pdf format.

    I need to know what board it need to look at?

    Power supply?
    it could be anyone of five or so boards.
     
  18. pastinsain

    Thread Starter Member

    Aug 25, 2012
    160
    2
    The ac -gnd-dc switches on both channels are the same so I dont think its the switches. The interior and boards are very clean with no visible problem. The scope looks in excellent condition.
     
  19. pastinsain

    Thread Starter Member

    Aug 25, 2012
    160
    2
    Mrchips I will try a litlle CRC contact cleaner on the ac-dc switches and see what
    happens.

    Well I took the cover off and there is no way to get to the back ac-gnd-dc switches. without major surgury.
    It looks like it would take days to get to them. providing I did not damage somthing else in the process.
     
    Last edited: Sep 13, 2012
  20. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
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    look where the inputs go to, there will be a common circuit that these inputs use , any chance you can post the pdf?
     
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