Maybe a cold solder joint on the header of the board with the LEDs? Easy enough to reflow all the solder on those and see if that works (or replace the header with a new one.)

Continuity may be there even with a cold (broken solder joint). As the board heats or shaken, the connection can break, causing the LEDs to flicker. I'd still start there -- easy to check, easy to fix (just heat up and reflow the solder on the header at the joints). Make sure to rule it out completely before moving onto the next thing.

Sorry that didn't help. Can you take a photo of the board and post a video of the flickering?

Thanks for the videos. So it really isn't flickering, but what you are seeing is the strobing of the signal doing down the row or column of the light matrix. If you notice, the first LED (Duffman) is out. Test its LED and its resistor -- and replace the bad one (or just replace both of them). I'll bet that fixes your problem.

I can't think of a time where I have seen an LED blow and create a short (they fail open). However, there is probably something wrong in that circuit. Replace the LED and the resistor. I checked the manual, and there isn't a breakdown of parts for that board, so I am not able to tell you what you should put in. You should be able to figure out the resistance of the resistor based on the color bands. Also, measure the base of the LED in millimeters. With those data, it shouldn't be too hard to guess at the LED (I can't imagine they used anything exotic). Feel free to post a picture of the board and I'll see what I can come up with. For ordering components, check out https://www.greatplainselectronics.com. Ed is an active poster on pinside, he sells very reliable products and his prices can't be beat (and he has helped me numerous times on pinside).

You'll want to replace the top "beige" resistor. Color code is red-red-brown, or 220 ohm. I'm not sure if it is a 1/2 or 1/4W, but here are the links to buy them from GPE:

1/2W: https://www.greatplainselectronics.com/proddetail.asp?prod=RCF1/2-220
1/4W: https://www.greatplainselectronics.com/proddetail.asp?prod=RCF1/4-220

They are $0.05 so get some of each.

The LED is probably probably either a 3mm or 5mm:

3mm: https://www.greatplainselectronics.com/proddetail.asp?prod=HLMP1790
5mm: https://www.greatplainselectronics.com/proddetail.asp?prod=HLMP3507

Each is $0.20, again, get a couple of each.

I think there is a $5 minimum order from GPE, so it is a good time to stock up on connectors, headers, transistors, etc.

No problem. Hope this solves your problem, and post back either way as it may help others down the road.

Congrats! Doing a quick search for the correct triangle shaped LED, I came up with this: ebay.com link: 20 x VQA29 Miniature LED TRIANGULAR GREEN 20 pieces

Hope that helps.

Digging some more at digikey, this looks like this is a good replacement: https://www.digikey.com/catalog/en/partgroup/4-0mm-x-4-5mm-triangle-with-flat-top-led-lamp/35917. It has looks like it can handle 20mA, with an input voltage of 2.2V. Using this resistor calculator: https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-led-series-resistor and assuming an input voltage of 6.3V, the 220 ohm resistor is perfect. (But keep in mind that I am just guessing at the size of the LED.)

Screen Shot 2017-08-02 at 3.03.11 PM (resized).png

The one your ordered looks good. How long do you think it will take to get it and report back? (I hope it works!)

Excellent!

Nice work! If you can, post the part number of the LED.

This looks like the part: https://www.digikey.com/product-detail/en/panasonic-electronic-components/LN212RP/P318-ND/10510

But I can't find it for sale.

Maybe this will work: https://thesaberarmory.com/product/4-8mm-triangular-led-red/
Specs are correct, but not sure about the size