Tektronix 422 High Voltage Supply Problems

Discussion in 'Technical Repair' started by m2circuits, Oct 1, 2016.

  1. m2circuits

    Thread Starter Member

    Jul 17, 2016
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    Picked this scope up not working at the local electronics store in hopes of bringing it back to life. All of the the indicators, graticule illumination, and various lights work when it is powered on. However, there is nothing on the display. I'm assuming the problem lies in the HV supply since all of the low voltage rails check out. I did notice that the neon lamp on the voltage multiplier board looks a bit odd. I'm curious to know if neon lamps can fail, and if this could potentially be the culprit for the lack of high voltage.

    Service Manual here: http://w140.com/tekwiki/wiki/422

    The lamp looks like something splattered inside, the regulator board has a neon lamp too, but it is clean and clear inside.
     
  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Are you familiar with all the scope front panel controls?
     
  3. m2circuits

    Thread Starter Member

    Jul 17, 2016
    52
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    Yes, trigger set to auto or free run has no effect, intensity has no effect and neither does either of the channel position controls(both horizontal and vertical). The 'un-cal' lights come on when the cal knob is turned and goes off when the cal knob clicks back to off. The scale illumination works fine, and all the rails supplied by the power supply check out. Just an empty screen, no trace.
     
  4. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Do you have another scope to trace signals?

    Set the trigger to Line. Then look at the output of the horizontal....it should be a ramp at 60 hz.
     
  5. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    The standard procedure for no display goes like this: (stop when/if you get something on the screen)
    Turn brightness to max, select external X input if available, otherwise use 1mS timebase.
    Check low voltage power supplies
    Check high voltage power supply - generally around 1000V - skip this if you don't have the equipment to do it safely.
    connect a voltmeter between the Y-plates on the CRT and adjust the Y-shift to get zero volts. If you can't do this the fault is in the Y circuit
    connect the voltmeter between the X-plates on the tube and adjust controls to get zero volts. If you can't do this the fault is in the X circuit
    CAREFULLY short circuit the grid and cathode of the CRT. If this gets something on screeen the fault is in the Z circuit.
    Still nothing on the screen may be the very high voltage or the CRT.
     
  6. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Did you measure the voltages on the CRT, except the hv and those over the capabilities of your meter? Do you have a hv probe?

    Have you powered down, and tested the diodes in the hv circuit or to check for shorts when de- energized?
     
  7. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    If the HV is defunct the most likely cause by far is the transistor oscillator feeding the transformer which generally has its own fuse.
     
  8. m2circuits

    Thread Starter Member

    Jul 17, 2016
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    I just saw one sweep go by the crt while rotating the time/div know rapidly from side to side, haven't checked the list AlbertHall posted but I guess this means that the crt has power?
     
  9. m2circuits

    Thread Starter Member

    Jul 17, 2016
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    Sadly I don't have a HV probe, so I will check the X and Y axis as mentioned above.
     
  10. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    If you get that single sweep it is probably the trigger or timebase circuit. Set the time/div control to EXT HORIZ. Does this get you a dot (adjust X and Y shift). If so then that is confirmed.
     
  11. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    There are some service manuals on ebay as a CD.
     
  12. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Take the scope out of auto and trigger with line.

    You should have a horizontal line.

    Although the dot works just as well.

    Make sure the x10 is pushed in.
     
    Last edited: Oct 1, 2016
  13. m2circuits

    Thread Starter Member

    Jul 17, 2016
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    Yes it does, but as soon as I touched the intensity knob the dots disappeared.
     
  14. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    You may have multiple problems. The thing with the intensity control may be the control itself. Try rapidly twiddling it from end to end to try to clean the track.
     
  15. m2circuits

    Thread Starter Member

    Jul 17, 2016
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    It sweeps, very dimly, and touching the Intensity control makes it bright and then disappear.
     
  16. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Try the twiddling the intensity control 'trick'.
     
  17. m2circuits

    Thread Starter Member

    Jul 17, 2016
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    The dots are bright, but only when the intensity control is all the way, but then as I increase the sweep it get almost to dim to see.
     
  18. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    I see your in the chopped position on the vertical. I would recommend ch 1 until you have a trace. Ch1 and ground.
     
  19. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    That sounds like a hold-off problem - the blank time in between sweeps. On some 'scopes this is adjustable but I don't think the '422 has a front panel hold-off adjuster.
     
  20. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Yes, it could be that if it is triggering from the chop waveform.
     
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