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Heathkit oscilloscope?

Discussion in 'Electronic Basics' started by Dan Beck, Dec 20, 2007.

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  1. Dan Beck

    Dan Beck Guest

    Hello all,

    I have been fooling (emphasis on fool) with an old Heathkit 10 MHz two
    channel oscilloscope, which I believe was built in 1976 (it was given to me
    several years ago). I have been able to calibrate the voltage on one
    channel, but not the other. Sometimes, after I have spent some time probing
    old pinball machine circuit boards using the one good channel, the voltage
    flakes out such that there is minimal amplitude in the waveform, and I can
    no longer observe good signals from the board in test.

    Does anyone here have experience with such a dinosaur? When the device
    works it has helped me diagnose these old boards; I am just wondering if
    there are any tips from the wisemen here in order for me to set this thing
    up better, recalibrate, etc.

    Thank you in advance for reading.

    Regards,
    Dan
     

  2. Does this scope have a model number?


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  3. Rick

    Rick Guest

    I'm thinking the IO-4510 or 20..
     
  4. robb

    robb Guest

    I assembled one of those in college as part of a physics
    project. 10 years later i purchased at an alumni day /reunion lab
    clearout sale. It came with the original assembly instructions
    manual, schematics and user manual. I also see these on THe-bay
    occasionally.

    if you have trouble solving problem, can not find info already
    posted somewhere , and i have the same model them let me what you
    need i can try to post something somewhere.

    robb
     
  5. Dan Beck

    Dan Beck Guest

    Thank you all for the responses!

    I apologize for omitting the model number. It is a Heathkit IO-4550 two
    channel oscilloscope. The individual who gave it to me also built it, so I
    have all the assembly and operation manuals.

    Another flaky behavior is when I connect to the 1 V peak-to-peak calibrator
    on the front, I do get the square wave, but sometimes it only lasts for a
    few seconds, and then the voltage craps out and I lose the waveform
    amplitude, as I discussed previously.

    Could any of these symptoms be explained by dried out electrolytic
    capacitors? I also noticed when I was attempting to calibrate the second
    channel's voltage (the one that doesn't work at all) the potentiometer I was
    supposed to turn was sticky, and rough feeling.

    Again, thank you in advance for any and all responses!

    Regards,
    Dan
     
  6. Have you made sure the probe (including the cable and connector) are
    actually good? If any of that was flakey, the amplitude would suddenly
    go away. And sometimes it's worth looking at the simplest solutions.

    Just use a piece of wire to connect the input to the calibrator output,
    and see what happens.

    Michael
     
  7. Dan Beck

    Dan Beck Guest

    Hi Michael,

    thank you for your comments. Yes, indeed it is profitable to keep things
    simple...Here are some observations:

    Y1 (Good channel) Using a plain wire per your suggestion reveals a stable
    square wave of appropriate amplitude, with the following exception. If I
    gently tap the AC-GND-DC switch for that channel, without even changing the
    position of the switch, the waveform decays to a trace with noise. This new
    trace looks like it's "trying" to return to a square wave, but doesn't. If
    I rapidly move the switch up and down it usually corrects; otherwise I have
    to power down the 'scope and return to it minutes later.

    Y2 (Bad channel) Using the plain wire test again all I get is a trace with
    noise on it, very similar to what I described above. Messing with the
    corresponding AC-GND-DC switch does nothing.Sometimes I am able to use this
    channel to read signals from one of my circuit boards, but the amplitudes
    are much lower than they should be, given the voltages I expect them to be
    at.

    I was able to perform the compensation procedure for the probe on its 10X
    setting, using the supplied calibrator square wave on the 'scope, so I think
    it is safe to conclude the probe is ok.

    Both AC-GND-DC switches feel sloppy in their respective actions; I wonder if
    they need attention...

    I do appreciate your efforts with this.

    Regards,
    Dan
     
  8. My first thought would be dirty switch contacts, but the power off/on
    remedy confuses things. Could also be a cold solder joint on the
    switch connections.
    Possibly poor connections to/around the gain switch/pot.

    You could try connecting corresponding points in the two channels
    while feeding one channel - that may help to locate the fault in the
    bad channel.

    Of course, once you have one channel working fairly well, you can use
    it to trace the signal through the bad channel.

    If the original builder wasn't too experienced soldering, you should
    check carefully for cold solder joints.
     
  9. Rich Grise

    Rich Grise Guest

    When you say, "the voltage craps out", which voltage do you mean? Knowing
    that will go a very long way toward helping us help you troubleshoot
    this thing.

    Good Luck!
    Rich
     
  10. robb

    robb Guest

    you are quite fortuneate to have those...

    have you already run the initial diagnostic setup tests ? how
    accurate were the voltages and resistances at the test points ?
    were any out of the specified range ?

    I have a note in the margin of ny manual that says "No trace"
    even though only a few of the values were slightly out of
    specified range
    I am no expert buy any stretch but the little i have leaned,
    flaky electronic behavior seems accompanied by some physical
    connection problem (i like the poor solder joint idea from one
    of te experts) or possibly a close but not exact component
    substitution

    since it was a kit i would worry about incorrect connections,
    swapped or in correct components placement. Looking inside, i
    remember how easy it was to make incorrect connections on the
    switches, using the wrong valued parts (resistors / transistors )
    and putting the parts in the wrong place etc...

    i am sure i overheated several parts soldering / de-soldering /
    re-soldering.
    scope seems like a restoration project in-itself.
    sounds like fun
    robb
     
  11. Dan Beck

    Dan Beck Guest

    Hello all,

    many thanks for all the responses to my questions. They are very much
    appreciated, and more importantly, learned, where I had to learn!

    An update: I reflowed the solders on the BNC connectors for both probes,
    and the solders on the AC-GND-DC switches. I also tried to clean out these
    switches using solvent on a cotton swap. Both switches are improved, but
    one still exhibits flakiness. I have new switches on order. As for the BNC
    connectors, the connector for the dead trace I had turned out to be the
    problem. Both traces work properly now, with appropriate amplitudes.

    Regards,
    Dan
     
  12. Rich Grise

    Rich Grise Guest

    Cool!

    But bottom-posting is the convention.

    Cheers!
    Rich
     
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