I don't want to confuse ANYONE with facts, so I'll proceed slowly.
here is a picture of the 7510's digital input section. I believe (correct me if I'm wrong) that its essentially the same in the 7520. here is the 7510:
now, does anyone see anything wrong, here?
let me point out a few things. again, bear with me, I hope that introducing actual data and design practices won't get anyone mad at me.
first, I don't see any pulse transformers on the coax inputs. I also see the grounds are all common (if the rear panel is metal, but maybe its plastic?). we can't tell what chips TEC aka B uses since someone has scratched off the labels. would it be the user? or the photog? unlikely. it must be someone else, then.
on the 2 opto blocks, I don't see any inductors (which are often typical of even medium-end systems) on the b+ lines to the TORX blocks. I bet they're cap decoupled, only. ho hum.
looking back at those orange rca's, lets just say I've seen better quality at my local radio shack. connectors don't matter as much in digital but those look pretty below-average to me.
I'm not sure I can comment too much on the trace layout but it also looks suboptimal. look at those right squiggles near the tc-7510 print (above it). pretty ugly and not even useful (there's no timing on those lines, I would guess, so there's no reason for 'extra delay'). maybe they used a cheap pc board layout program. either way, it shows a lack of attention to detail or maybe it was ALL auto-routed (brrr!).
the 2nd board, below, isn't even 2 layer (it has topwires on it). and the op-amp seems like it might be essentially a cmoy. certainly unbuffered.
that's just a corner photo. I don't want to spend more time analysing this POS but its not even commercial grade, imho.
