Rca Wo-505a

Discussion in 'General Electronics Chat' started by Danielsix-five, Apr 1, 2008.

  1. Danielsix-five

    Danielsix-five Thread Starter Member

    Joined:
    Dec 6, 2005
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    Location:
    Colorado
    Bought a surplus RCA WO-505A Oscilloscope. Got everything working okay, except the Horizontal Amplitude (the test circle thing). I can get the internal source to display a test circle correctly with 24VAC on the vertical input, but in external mode, I connect the AC Source (24V Transformer) to both the vertical and horizontal inputs (common ground) and I get back a loopy-cornered skewed rectange. Any idea what's going on or how to fix it??

    -Daniel
  2. SgtWookie

    SgtWookie Expert

    Joined:
    Jul 17, 2007
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    Location:
    In the vast midwest of the USA; CST
    What are you using for test leads?

    Try swapping the leads.
    If the loopy-corner skewed rectangle is then drawn in a mirror image, the problem is in one or both of your test lead(s).

    Your transformer may be going into saturation. This is pretty common with the cheap transformers you get nowadays. But it sounds more like you have a good bit of capacitance in there somewhere.

    With the secondary windings of a transformer connected to the X and Y inputs, you should wind up with a straight diagonal line.

    If you have a signal generator around, set it to output a 1kHz square wave @ 1v, and observe it using your test probe(s). It should look nice and square. If not, the internal caps need to be adjusted.

    If you have a CMOS 4000 series IC around that has Schmitt inputs, like a 4093 or 40106, you could build a simple square wave generator using a small cap (around 8.2nF) and a resistor (around 100k). That should give you in the vicinity of 1kHz output. See the datasheet for either IC for the oscillator diagram. Once you get the oscillator running, you should see close to a perfect square wave on your scope if you look at the output of the oscillator with a 10x probe.

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