We have a SG300-10P as well as a SF302-08P. They are providing PoE and data for a mixture of 7961s, 7960s, SPA504s and SPA514s. The problem is that there is a very audible "hum" on the 79XX phones that are connected to the SG300.
Moving the phone(s) with the hum to the SF switch makes the hum go away (same patch cable).
Swapping the power supplies of the SG and SF switch made no difference.
Using a temporary AC power source supply for the SG switch made no difference.
It seems that the SG switch has something wrong that is introducing PoE noise.
Any suggestions or how do I contact Cisco for replacement/troubleshooting?
Hi West, the 7900 phones are special in the sense that it uses inline PoE. The SX300/500 series use a power conversion to make those phones work. Might I ask if the noise goes away if you use a Cat3 cable?
I do admit it is unusual for a "PoE hum". I do not recall hearing any kind of hum when I used to lab these telephones.
PoE can be as much software related as it is hardware related. The best first steps if I were to be troubleshooting this myself is evaluating the software version my switch uses. Per example, I remember testing these phones on 184.108.40.206 where there was no hum noise. Are you using a 1.3.5 release? It may not be a bad idea to go through a couple software revisions and see if the issue follows the software or stays with the hardware. I would also recommend making a backup file and just using a factory default switch as it is definitely best when upgrading and downgrading software to factory reset the unit to avoid any code overlaps.
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