I received a call about a LED video display board that had some black squares.

The first thing that would come to mind while troubleshooting this unit was that it had a bad LED module. Almost all LED signs like this one use shift registers in series. This can mean if a LED module isn’t processing its data correctly it might not pass the data down stream to the next module. The data on this sign is passed down the line on a horizontal axis, which I will call a row. When a module is bad at the start of a row it can cause all the other modules down stream from it to also stop displaying images correctly. On this sign there are 2 modules in the top row that where not working. Normally this would point to a bad LED module. However in this case the row directly below it is also out. Since the data stream doesn’t jump between rows it means we should start looking else where for the cause of this malfunction.

Zoomed in here you can see that at the start of the right side the LED’s are actually lighting up. They don’t appear to very bright however. This gives us a hint as to what the real cause of this problem is.

This sign runs off of 5 volts DC.