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Fixing Heathkit Oscilloscope

Discussion in 'Electronic Basics' started by Ivan Vegvary, Nov 20, 2012.

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  1. Ivan Vegvary

    Ivan Vegvary Guest

    Picked up the above for $ 10. Fuse was blown. Don't have a 125V 1A fuse but used 125V 1/2 amp fuse instead. Front indicator light went on and 30 seconds later the 1/2 amp fuse blew. Will pic up a 1 amp fuse. If it keeps blowing, where would be a good place - method - to start diagnosis? I did get a manual and schematic with the scope.

    I realize that I could probably buy an equivalent "working" scope for about$ 50, but I would like to get this one going. Tools available are VOM, VTVM and one hand safely in the pocket.

    Thanks, Ivan Vegvary
     
  2. Phil Allison

    Phil Allison Guest

    "Ivan Vegvary"

    Picked up the above for $ 10. Fuse was blown. Don't have a 125V 1A fuse
    but used 125V 1/2 amp fuse instead. Front indicator light went on and 30
    seconds later the 1/2 amp fuse blew. Will pic up a 1 amp fuse. If it keeps
    blowing, where would be a good place - method - to start diagnosis? I did
    get a manual and schematic with the scope.


    ** But we did not.

    What model number is it ??


    I realize that I could probably buy an equivalent "working" scope for about
    $ 50, but I would like to get this one going. Tools available are VOM, VTVM
    and one hand safely in the pocket.

    ** You need a scope to fix a scope.



    ..... Phil
     
  3. ehsjr

    ehsjr Guest

    Well, based on the available information, it's guesswork.
    That said, check the electrolytic capacitor(s) in the DC supply
    with your VOM set to measure resistance. If you don't know what
    you should see on the meter, get a new 'lytic of the same
    value and measure it to get an idea. You want to leave the
    meter connected for a while to watch the measurement climb
    up toward infinity. 'Lytics often fail with age and can
    show leakage on that test.

    Ed
     
  4. People have said in the past that the transformers on the Heathkits were
    pretty cheap, and did often burn out in the long run. I have no
    experience, but I can imagine that. The problem being of getting a
    replacement that not only has the needed windings, but fits the sapce.

    SO that's something to look into.

    Michael
     
  5. My Tektronix from 1959 (the model came out that year, don't know the date
    mine was manufactured) developed ripple in the trace. So I opened up the
    case, and put the probe on each of the filter capacitors in the power
    supply. At one point, the ripple is worse, so that's the bad capacitor.
    I was surprised (this was about 20 years ago) that the local "surplus"
    place had a capacitor of the right value, since it was higher capacitance
    than the average tube-era power supply filter capacitor. I put in the new
    capacitor and the trace was nice and flat like it had been before.

    Michael
     
  6. Fred Abse

    Fred Abse Guest

    A perennial problem with tube 'scopes is the CRT heater winding on the
    supply transformer going leaky or short to frame, or to another winding.
    The heater is connected to the cathode, to take the stress off the
    heater-cathode insulation, and is floated at negative HV, hence the
    transformer insulation must stand several thousand volts DC.

    The only cure for that is to fit a separate heater transformer with
    sufficiently good insulation, such as one of those that used to be sold to
    fix mono TV heater-cathode shorts. They used a split bobbin.

    BTDT many times ;-(
     
  7. Phil Allison

    Phil Allison Guest

    "Michael A. Troll"

    ** There is no "always" in my comment.

    You illiterate moron.
     
  8. Ivan Vegvary

    Ivan Vegvary Guest

     
  9. Ivan Vegvary

    Ivan Vegvary Guest

    Heathkit Model 10-18 Oscilloscope
    Placed 2 amp fuse (only one available) and does not blow. Here is a link tothe power supply portion of the schematic.

    https://lh5.googleusercontent.com/-...AAAAAMA/b0oSZQ5lFZQ/s720/CCF11212012_0000.jpg

    The 1V2 tube does not light. Tube has no continuity between pins 4 and 5 (with tube removed). Voltage at either pin 4 or 5 reads circa 1065V to ground. However, I can discern no voltage between pins 4 and 5. Is this OK? Do I have a bad transformer? Should I go ahead and order a new tube?

    Thanks, Ivan Vegvary
     
  10. Jon Kirwan

    Jon Kirwan Guest

    Yeah, there should be continuity between 4 and 5. The heater
    voltage is supposed to be 0.625VAC with current at 0.3A, from
    what I see at the following link:

    http://www.radiomuseum.org/tubes/tube_1v2.html

    This suggests, hot, that the resistance is about 2.1 ohms.
    But according to this Wiki:

    http://en.wikipedia.org/wiki/Vacuum_tube

    the operating temperature of oxide-coated tungsten filaments
    was about 700 C. The following chart on tungsten:

    http://hypertextbook.com/facts/2004/DeannaStewart.shtml

    gives a resistivity of about 24.93 at 1000 K and about 5.65
    at 300 K. So about a factor of 4.4. From that, I'd expect to
    see slightly less than 1/2 Ohm, or so.
    The voltage difference should be, as cited above, about
    0.625VAC. Reading the difference, though, may be difficult if
    you are measuring each lead individually referenced to
    ground. (600 ppm isn't an easy determination.) You may need
    to measure across the two socket pins.

    The tube appears bad. But I'd want to make sure, before
    plugging in a new one, that the filament voltage appears
    about right, too.

    You could avoid worrying about measuring AC voltage at high
    potential by instead testing the filament voltage by placing
    a resistor across the two pins and powering up, feeling for
    some heating effect. 2.2 Ohms there would be close to the
    nominal value and would create less than 1/4 watt of
    dissipation. With a 1/2 watt resistor, it should be enough to
    feel and not enough to be worrisome.

    Jon
     
  11. Jon Kirwan

    Jon Kirwan Guest

    I should have added: But only feel the resistor AFTER turning
    the system off and safely removing the resistor, beforehand.
    Please do NOT touch it while operating the system.

    Jon
     
  12. Ian Malcolm

    Ian Malcolm Guest

    Patch in a 1.5V maglite bulb - it should glow noticibly on 0.625 Vrms and
    you can get a rough check of the voltage by visual comparison with an
    identical bulb powered from an adjustable low voltage DC supply.

    OTOH a floating measurement with a good quality multimeter and leads isn't
    really a problem. Attach lead clips with power off of course.

    If the meter is rated less than Cat III 1500V, I would advise putting it on
    an insulator - e.g. a sheet of at least 4mm glass or clear plastic and
    making sure the multimeter lead insulation does not touch any other lead,
    component, metal or conductive object. Use nylon cable ties to dress the
    leads if you have to and do NOT touch the meter or leads with power on FOR
    ANY REASON.
     
  13. Ivan Vegvary

    Ivan Vegvary Guest

    Thanks Michael,
    Very kind of you to go through the trouble. As you will see in a new post, I did get the scope working.
    Thanks again, Ivan Vegvary
     
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