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CRT testing analysis etc. - hp8505a

Discussion in 'Electronic Repair' started by dansawyeror, Apr 14, 2007.

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  1. dansawyeror

    dansawyeror Guest


    I just picked up a used hp 8505a. The CRT display is blank. I am looking
    for advice on the order of testing. It has been years since I have done
    any HV work. There is a possibility the tube itself is bad.

    The CRT is a vector display, not a raster display. I have the schematics
    and some expected voltages. The main connections are straight forward:
    filament, cathode, grid, focus and vector controls, and HV.

    I have measured the filament, it is a few Ohms.

    What is a reasonable range for the control grid?

    What measurements can be made of the tube?

    What reasonable measurements can be made of the PS?

    How can the HV be tested?

    Thanks - Dan
  2. Using safe procedures where needed, measure the voltage across the heater.
    Does it glow?
    Using safe procedures where needed, measure the voltages on the elements.

    Campare with design values.
  3. Guest

    dan what exactly is it you dont know?

  4. dansawyeror

    dansawyeror Guest

    Thanks. The initial readings are inaccurate, all I have is an old analog
    Simpson meter with a 5000V range

    I cannot see any filament glow. The CRT resistance is 3.2 Ohms, the
    transformer reads close to zero. It is difficult to measure the filament
    voltage as it floats at 2k DC.

    - Dan
  5. Guest

    attach leads to unpowered filement connections, sit meter & leads on
    cardboard, switch on, kee paws off.

  6. You can measure it if you don't touch the meter or leads while doing it.

    Or, hook up a flashlight lamp to the filament supply leads with the tube
    disconnected and see if it glows.

    Apply 1.5 VDC to the filament and see if current flows.

    I'd bet on the other voltages being the problem. You probably have a divider
    chain for those voltages and resistors may have drifted.
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