Hi - sorry, from what I can see, this is a well-trodden topic. I've read what I can to find out if I'm doing this right on both this and the arduino forums, but I'm running out of ideas. Basically the displays are completely unreliable, and drop out or show garbage regularly. Some displays are better that others, but others will only work for 30 seconds before going blank.
In my current setup, I'm trying to run a homemade Boeing 737 autopilot panel. "Behind the scenes" this is 2 arduino (Mega 2560) boards running a number of switches and LEDs (all switches just route an arduino pin to ground, and LEDs each run through 100ohm resistors and then to ground), as well as 4 x Max7219 blue 7-seg display boards. I must have bought over 20 of these max7219 boards from various suppliers on ebay and amazon, but I cannot get anything to stay reliable and I'm tearing my hair out. I have a reasonable if not great electronics background, and I'm a software developer by trade so I know my way around the arduino software side.
On the software side, this is supposed to be using "Mobiflight" which handles flight-sim interaction and code on the arduino to interact with the displays, but I've tried swapping out for my own code in the arduino editor with no real change, so I think this is hardware/electronics related.
Firstly I'm limiting this to a discussion about one of the arduinos for simpilicity as I've taken the other and it's associated bits entirely out of this and still have the problem. I've also removed the switches/LEDs so this is really entirely a 7-segment display related query.
The 7segment displays are all powered in parallel from a 6v, 2.1amp mains DC supply (I tried various 5v supplies, but up'd to 6v to see if that helped with any voltage drops). The ground from that also goes to the arduino ground (i.e. yes, I've read the forums). As these were never reliable, I've barely ever attempted to daisy chain these, so at the moment all 4 7-seg displays have their data lines (DIN, CS and CLK) fed directly from dedicated arduino pins.
Here's my wiring diagram: https://i.ibb.co/ydT3SqY/Blank-diagram.jpg
I tried writing some simple arduino code which initialises the boards and then sends '11111111', '22222222','33333333' etc to each display every second. I then swapped in and out lots of the displays (like I say, I have a pile of them...) to see if I could get a reliable set. Even with what I thought was a reasonably beefy DC supply, you can see a drop in reliability if I add displays (going from 1 up to 4). I don't think its the voltage of the arduino data lines as I set up another arduino as a primitive oscilloscope and the voltages looked good (and I even managed to decode the wire format and check it was sending the right signals - go me!).
I would write this method of running displays off as totally unreliable, but I can't help thinking that many people seem to have built things with these boards, so I'm still assuming it will work, but I'm clearly missing something. 4 x 7-segment displays can't be beyond the wit of man surely?!