Silent Knight SK-5208 Weird Problems

I have 3 SK-5208 panels. 1 of them (shown in the attached video) seems to at least boot up fine, though I don’t have any alarms or pull stations to test it properly. When I tried the other two, however, all I got was a continuous buzzer and a completely blank display. Interestingly, I noticed that the large capacitor on the functional panel has a black plastic top, while the two that were nonfunctional did not have that plastic top, instead, it was just the bare metal top of that rather fat and large capacitor.
SK-5208 Video

I have an SK-5208 myself: mine has a capacitor that’s completely enclosed in plastic like the one you say you have that works. Not sure what could be wrong with the other two though aside from something on the main board being screwed up (possibly permanently).

About the AC LED flashing: it’ll flash when there’s a battery trouble.