[colug-432] mostly dead switch — diagnostics for fun

[Rick Hornsby]

Woke up this morning to a network dead switch at home. Well, not totally
dead but enough that the only remaining light was power, and it was
flickering like a fluorescent tube in horror movie.

It’s an old D-Link 1Gb/s piece of junk, DGS-2208. I’m surprised it’s lasted
this long. I had a spare Netgear something or other, so I popped that into
the closet and all was well again.

It’s probably trashed, but I wanted to at least try to figure out what’s
wrong with it, for fun. Maybe there’s an EE or two who can suggest some
ideas? This image [1] is pretty close, if not identical, to my switch’s
PCB. I’m not an EE, so please feel free to correct anything I get wrong
here.

First thing, I think the 5V power supply is shot. I put it in my scope
(which I barely know how to use - this might be the first productive thing
I’ve done with it.) and it looks like there might be some kind of failure
in a regulator(?) because it’s not putting out a steady 5V.[2] I don’t
think this is normal, but I don’t know because I didn’t look at it before
the failure.

Secondly, there’s something screwy on the PCB, apart from the PSU.
Everything looks fine visually, but I discovered trying to power the PCB
from the bench supply that the board responds the same way to that as it
does to the wall wart - the power light flickers weirdly, and it sends the
bench supply into a fit. Without any power applied, I started probing with
my multimeter and found that the power pins on the PCB barrel short circuit
(continuity beep) for about 1/8 second, and then the meter shows ~500mΩ
when it should probably show open, or least something more than 500? I
double checked the polarity with what’s marked on the case, and even tried
in reverse and got the same result.

If you remove the multimeter probes and immediately put them back, you get
the 500mΩ, but not the dead short. Give it 10-15 seconds, and then you’ll
get the same quick dead short and then back to 500mΩ. In my ignorance, that
suggests maybe there’s a bad cap(?) The reason I say that is that it seems
like there’s something holding a charge for a short time (cap?), and when
the charge dissipates, there’s a dead short for that brief moment.

I dunno, I could be completely wrong. This thing is junk and probably not
worth the effort, but I maybe I’ll learn a little more through the process.

Thanks!


[1]
https://www.smallnetbuilder.com/images/stories/lansrouters/8port_gigabit_roundup/dlink_dgs_2208_board.jpg
[2] https://www.dropbox.com/s/fcrc9h8lo7xm1a5/SDS00003.png?dl=0
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