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Peggy weirdness - Parts of Columns lighting up, others not

edited March 2014 in LED Matrix Kits
Hi - just finished constructing my second peggy board, and now it seems like parts of certain columns aren't lighting up. Prior to completion, it was working fine.
Here is a picture...



  • Hmm.  That's not good.  The LEDs in any given column are all wired together (in parallel) at their anodes (high sides), and then the LEDs in any given row are wired together (in parallel) at their cathodes. 

    Thus, if a full column were out, I'd suspect the transistor (or demultiplexer chip that drives that transistor). If a partial column is out, I'd normally suspect that there could be (for example) a cut in the circuit traces.  But there's actually no place (or set of places) in the circuit where you could cut the traces and end up with the pattern that you're seeing.   

    Instead, there's some kind of other error going on here, and I've never seen anything like it. Possible things that could cause this pattern would include:
    - A large number of LEDs not fully soldered
    - A large number of LEDs installed backwards. (Test, by pressing the leads of a spare LED across the leads of an LED on the back of the board.)
    - Programmed to display that pattern.
    - Short circuits (extra connections), due to stray wiring or over-bent wires. 
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