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Thread: TV Typewriter replica project back on track

  1. #651
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    D27 is +2.7V (?!). R20 is +9.1V. Not sure how this could be happening.

  2. #652
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    Have you double-checked D27's part number? Perhaps you have it installed backwards. If D27 is an 8V zener, you should read 8V across it. Just the nature of the beast.

  3. #653
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    It's a 1N4738 according to what is printed on the body of it. I took care to align it according to the parts diagram. I also observed how I had the previous one aligned. It's possible the placement diagram is wrong..

  4. #654
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    This is the configuration for a Zener/Avalanche diode:



    Note that you should get 4V = 12 - 8 across R20. If the diode is hooked in backwards, then you'll get only the forward drop across the diode, which can be > 1.2V (according to the datasheet). The keyboard circuit differs from this only slightly--instead of being between the cathode and supply rail, R20 is between the anode and ground. Otherwise, it's the same idea.

    Note that the diode is reversed from a normal rectifier. That is, the higher positive voltage is connected to the diode cathode.
    Last edited by Chuck(G); Today at 11:13 AM.

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