[Elecraft] Lost side tone

[Pete Meier]

Before I order a new K2 MCU, which I suspect has partially failed, I 
wanted to ask you folks for advice.

First a little history, I added the KIO2 some months back. From the 
start I had trouble with receive data from the K2 to the computer. 
Transmit from the computer to K2 worked.  I found I had low or missing 
voltages in the receive path and had also lost sidetone. A faulty Maxim 
chip was suspected. It turned out I also needed to replace the K2MCU to 
fix the problem. After this all worked fine.

Work has kept me from playing radio much so recently I discovered the 
KIO2 had failed again. I decided not to pursue fixing it at that time 
as I had the KPA100 upgrade almost complete with it's own KIO2 on 
board. All went well with the initial tests but when I added the 20 amp 
power and powered up the K2, I still had the NO PA PS message and yet I 
had 13.4 volts to collectors on Q1 and Q2 and elsewhere on the PA 
board. I wondered if somehow information was not being routed back to 
MCU via the I/O line. So I decided to check the KIO2. Same problem 
appeared as before; the computer could change modes of the K2 but never 
received info back from the K2.  Next, I disconnected the KPA100 and 
reconnected the old cover and it's options to check basic operation and 
that is when I discover the loss of sidetone. Everything else works 
fine both transmit and receive.

I have checked Q5 (even replaced it) and ensured the menu settings for 
tone origination (U6 vs U8) were not the problem.

My suspicions are that somehow the original KIO2 might have damaged the 
K2MCU via the P4 AUX I/O lines.

Am I way off base? Your thoughts?

Pete WK8S
pmeier@mac.com