High Latency

I felt I needed to blog this just because it was so very odd, and there seems minimal documentation of it already on the web: an organisation I help to support has a good few Cisco 3550 switches- WS-C3550-24-EMI. They’re egtting on now, and are due for replacement (having done well, at about 13 years old).

Anyway, we’ve now had three of them fail- not while in use, but after a power failure.

Or so it seemed…

The unusual part of this is that they failed- completely dead, no lights, no fan, no anything, but left alone with power connected for some time (where “some time” could vary between 20 minutes and 3 hours), they would eventually start up and work as normal.

There’s little mention of this online- I could only find this old post, which suggests capacitor failure as a likely cause- a diagnosis I’d agree with. Taking one switch apart didn’t reveal any obvious failures, but that doesn’t mean there weren’t any, of course. As it is, the switches are on a maintanance contract and due for replacement- and the one I tool apart was a spare, so I doubt we’ll ever bother with them. PSUs sell for around a hundred euros or US dollars, and a whole switch can be had for less than that, so unless you have spare time, a bag of capacitors, and soldering skills, it probably isn’t worth it.


A different search found this article and this Cisco tech note.

One Response to “High Latency”

  1. species5618 Says:

    switched mode PSU in thing like this use a bleed resisitor to pre charge the primary side capacitor to the point the control circuit will start !

    once started the transformer will keep the primary side capcitor charged

    so typicallly either the resisitor or the capcacitor go soft, so the capcitor will not hold, or does not get enought energy so survive the start process, in this failure mode you may here a ticking noise,

    if it is completely dead, the capictor may be leaky resulting in suffcient voltage being achieved

    in the consumer world it is often easy to get repair kits for set top boxes, and other devices.

    not only have i repaired a few, i design a switched many years ago too