[Elecraft] K3 Hum on transmit

[Hunsdon Cary III]

K3 Experts:  My K3-100 has been flawless in its performance until yesterday.  I had a schedule with W5ZNN who told me I had a hum on my signal; I put my headphones on and sure enough there it was on transmit.  I tried plugging the Astron 35M directly into the AC outlet instead of the outlet strip - no luck, hum was still there.  I connected the K3 to my SEC 1223 switching power supply and, you guessed it, the hum is still there.  I hate to think there's something amiss with the K3 but it looks that way.  Any ideas?
73,
Cary, K4TM
Lynchburg VAh
h3cary at gmail.com