16x32 LED matrix weird manufacturing

[KNfLrPn]

I was looking at http://www.hobbypcb.com/component/k2/it ... al-details as a reference for the 16x32 displays when I noticed that his display has a hex inverter whereas the ones I got from Adafruit have an extra 74HC138 in its place.

I started tracing out the circuit (the Adafruit ones) and as far as I can tell, this chip is not doing anything at all and might in fact be dangerous (to the chip, probably not to the user). The Y1 output is tied to ground, which means that in all but one case it's going to be shorting out (outputs are normally high), and that one case can't possibly happen because the A and C inputs are tied to gnd and vcc. The B input comes from the latch/strobe (display input) for some reason. The chip datasheet says the max output is 25mA so it probably won't catch fire or anything, but it can't be good for it.

An additional result of this is that I don't think the enable signals for the normal 74HC138 are connected properly. The G2B signal is floating because it is only connected to the G2A signal from the odd HC138. I think the display still works because NC floats to enabled.

The really weird thing is that the hex inverter is a 14-pin package, but this is a 16-pin chip/footprint, so it's not like they just put the wrong chip in the pick-n-place. It wasn't designed to have the inverters.

Anyone have any idea what's going on?

[KNfLrPn]

I just looked at the image on the product page (this one), and it doesn't look like it has any chip in the place in question.

I guess I need to get my displays working so I can do more tests.

[KNfLrPn]

I confirmed my suspicions.

A) One of the enable signals on the 74HC138 (the one that's supposed to be there) is floating. Touching that node (I attached a wire to it) causes the display to turn on or off erratically. Touching the node and touching ground turns the display on, touching the node and touching vcc turns the display off (resistance of my body is enough to pull it either way).

B) I removed the extraneous chip entirely with no change to display operation.

So, if you have one of these displays from this batch (the one photographed for the product description doesn't have this problem, so I assume there are different batches), you should at least tie pin 5 of U4 (or pin 4 of U3) low (with a resistor). You should also probably remove U3.

Adafruit, you might want to look through your stock and make some modifications. Having a floating pin that can turn the whole display off is not a great idea.