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SPA112 has tons of 60 Hz hum on both FXS ports

Since I bought it, my SPA112 has had tons of 60 Hz hum on both FXS ports. It's at least as loud as dialtone.  The supplied wall wart and every other wall wart I've used has the same problem.  BUT, a nice bench supply does not.  I opened it up, and touching the ground plane on the PCB kills the hum.  But a 220 uF cap across the incoming DC supply had no affect.  Any ideas?  Has anyone else ever had this problem and cured it by using a better power supply?  If so, which did you use?

 

Thanks.

 

...R