Had my N5200 since about 2006 and the power supply was on its last legs - the main symptom was that it would sometimes fail to power up, especially when it had been turned off for a long period of time.
Unfortunately it looks like the original power supply (Seventeam ST-200UAB) is no longer available, and has been replaced by a new model with larger dimensions (the original was 180x100x40mm) and there's not much room in there to spare.
Decided to pop open the power supply and have a look - there were two capacitors that looked suspicious (tops were bulging somewhat), a 1000mf 16V, and a 2200mf 10V.
I Found some near matches at the local electronics store and replaced them (though getting the old ones out wasn't that easy). Seems to have fixed the problem, and the unit powers-on immediately now. Will probably need to do the fan next as it's quite noisy in mine (but it's in a closet, so not a big deal).
As an alternative, I've found another PSU which may be a compatible match, the Zippy H1U-6250P, which is a 150x100x40mm 1U 250W supply.
I'm a little disappointed that Thecus isn't still developing firmware for the unit, but it's served it's purpose nicely and has protected my data through a number of drive failures over the years.
I'll probably be looking at a DroboPro FS as a replacement in a year or so. 8 bays and relatively painless hot-swapping of drives without rebuild downtimes to increase capacity are tremendously appealing to me.