HP 140A oscilloscope

Discussion in 'General Electronics Chat' started by haamer, Sep 27, 2010.

  1. haamer

    Thread Starter New Member

    Sep 20, 2010
    7
    0
    It was made in 1963, so I have replaced all electrolytic capacitors and everything works, except one clitch: when I adjust beam intensity brighter, then also signal amlitude grows at CRT.
    At max intensity sinewave or square seems to be 2 times bigger, then at minimum intensity.
    Very annoying.

    Any suggestions, what can be problem?
    Thanks
    T.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    It looks like there are problems with the high voltage.
    As the intensity is higher the high voltage gets lower and the picture is larger.

    Bertus
     
  3. haamer

    Thread Starter New Member

    Sep 20, 2010
    7
    0
    If high voltage gets lower, then image should go dimmer, but why larger?

    T.
     
  4. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    I had the 1964 US Navy version of that same scope. When I first got it, it had the problem that when you pushed on the beam finder, the picture went away completely. The problem turned out to be two high voltage capacitors; the problem was easily determined because the caps arced!

    As for your problem, it seems there is a relationship between how much current the supply is delivering and the amplitude of the vertical amplifier.
     
  5. timrobbins

    Active Member

    Aug 29, 2009
    318
    16
    Best to check the regulation of all the power supply rails as you change the intensity setting.
     
  6. gootee

    Senior Member

    Apr 24, 2007
    447
    50
    You could try asking in the hp_agilent_equipment discussion group, which can be found at yahoogroups.com .
     
  7. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    A lower HV at the anode will cause a picture to "bloom" as they used to call it on TVs. It may have a HV regulator tube in it like some older TVs of that era used 6BK4s.
     
  8. theamber

    Active Member

    Jun 13, 2008
    318
    0
    You mean the horizontal deflection opens up or the vertical amplifier shows larger amplitude? If it opens up the singnal should grow vertically and horizontall, that happends if the horizontal output is getting higer V from the flyback or the HO transistor.
    Seems to me you have a problem in the vertical amp. I do not know that scope does it has two separate channels?. It is probably a dual beam,check the gain of the transistor in the vertical amp. But it is strange the intensity control is related to that.
     
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