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Tektronix 2215A

Discussion in 'Troubleshooting and Repair' started by AndrewBrolowski, Mar 7, 2014.

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  1. AndrewBrolowski

    AndrewBrolowski

    1
    0
    Mar 7, 2014
    I just bought a Tektronix 2215A o-scope and it all functions work well EXCEPT that the both traces on the CRT gradually fade to becoming nearly invisible when the "A and B SEC/DIV" is decreased to below 0.2 microseconds (...and 0.1 and 0.05 being the last lowest time settings). I suspect it is degassing of the CRT but would really appreciate additional opinions before I tear into it.

    Andrew
     
  2. davenn

    davenn Moderator

    13,553
    1,852
    Sep 5, 2009
    Hi Andrew
    welcome to Electronics Point :)

    tho you may get some answers here
    if not there a really good tektronix group on yahoo groups
    that I have belonged to for many years and lots of very good help will be found :)

    http://groups.yahoo.com/group/TekScopes/

    cheers
    Dave
     
  3. elebish

    elebish

    177
    12
    Aug 16, 2013
    Most scopes use a brightness enhancement circuit to brighten the trace when the time base switches to higher speeds. Sounds as though your problem is the opposite! I would check the crt cathode voltage at different sweep speeds. The potential should be approx. a negative 2400 vdc. There are also "unblanking" circuits that are used for eliminating the retrace lines that are controlled by sawtooth waveforms that are produced in the horiz sweep circuits. The trigger circuits provide the trigger to start the sweep. Make voltage measurements in the Z axis and horiz sweep circuits first but be careful of the higher voltages that can bite.
    Another problem can be the persistence of the phosphor is degraded enough to not maintain a bright enough trace when sweep speeds are slow. That would indicate a bad crt. NOTE: The high voltage to the anode of the crt should be about 12000 to 15000 vdc, enough to cause severe dilatation of the eyeballs! Ed.
     
  4. OLIVE2222

    OLIVE2222

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    25
    Oct 2, 2011
  5. shrtrnd

    shrtrnd

    3,712
    468
    Jan 15, 2010
    All good advice above.
    The first thing I'd do however, is increase the CRT 'intensity' pot, to see if those signals become visible.
     
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