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Thread: TV Typewriter replica project back on track

  1. #691
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    Quote Originally Posted by Chuck(G) View Post
    Take a look at what the base voltages are when you press a key on Q2,3 and 4. If they all change state no matter what key is pressed, then look at the base voltages of Q5, 6, 7. Also, check the polarity of the diodes D3-D11. A shorted one or one reversed could really throw a wrench into the works.
    I regret I've never been good at testing diodes. I put my DMM on continuity.. put black lead on anode and red lead on anode and get:

    D3 - 8.5M
    D4 - 8.5M
    D6 - No continuity
    D8 - No continuity
    D11 - No continuity
    D12 - 35.5M
    D14 - No continuity

    Reversing the leads I get:

    D3 - 3.8M
    D4 - 1.0M
    D6 - 1.0M
    D8 - 1.0M
    D11 - 1.0M.
    D12 - 1.0M
    D14 - 1.0M

  2. #692
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    Doesn't your DMM have a diode range? My inexpensive one does. If not, I can help you set up an ersatz tester.

    The general idea is this:



    You measure the forward voltage drop of a diode directly using a current-limited low-voltage source. In the reverse direction, the voltage read will be nearly that of the source; in the forward direction, the voltage read will be that of the forward drop of the diode (about 0.7V typically).
    Last edited by Chuck(G); June 21st, 2018 at 10:02 PM.

  3. #693

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    Keep in mind that in-circuit testing of components can sometimes produce other than expected results due to other components attached to the diode. You might need to lift one leg of the diode to get a valid test.

  4. #694
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    Lift one side of the diode and test for a 10 to 1 front to back ratio using the resistance (ohms) scale on your multimeter. Works sufficiently in most cases.

  5. #695
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    Depends on the DMM and the voltage used.

    Fluke has a good tutorial on checking diodes with a non-diode-enabled DMM. here

    I'll add that small-signal diode failures in normal conditions are extremely rare.

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