[Elecraft] K2 Sidetone Problem

[Jim Scott, AD5YU]

Hi all,

K2 Serial Number 5563 was completed over a year ago, and has been operating very well ever since.

A couple of months ago I had a problem with the headphone socket, and was sent a replacement which is working fine now.

A couple of days ago, the side tone stopped working. In fact there are no tones at all, from the paddle, button presses, or even setting the side tone pitch or level. 

I checked for dry joints and bridges around the side tone components (U8, Q5, U10b etc) and even replaced Q5 as a precaution. The audio amp is working fine. I don't have a scope, so I was only able to make a few measurements with a Fluke multi-meter. I could only detect a few mV of ac on pin 4 of U8 when there should have been a tone, so I suspect either a problem there, or possibly with the micro itself. 

Before I replace any other components, can any one suggest a failure mode and a suitable method of confirming it with only a multi-meter to work with? If I need a scope, I can probably get access to one. Has anyone ever had a similar problem? Would a reset help?

Bear in mind - this K2 has been working very well for well over a year now, on CW, phone and digital modes. 

Thanks,

Jim Scott, AD5YU/VK1GF