[Elecraft] [K2] Odd AGC problem with K60XV installed

[Bill Coleman]

A few weeks ago, I managed to cobble enough of the parts together to build a K60XV. It came together great, and works both on 60m as well as a base for a transverter.

I thought that would have been the happy end of the story, except I’ve had some odd trouble with it installed. I saw this briefly the day after the SS CW contest, and on the second day of the SS Phone contest. In each case, removing the K60XV solved the problem and it did not recur.

Here’s what I’ve observed. After leaving the K2/100 turned on for several hours, the rig is in receive mode, but the AGC is fully activated, with all ten bars showing on the S-meter. The receiver audio is entirely blanked. 

This condition is intermittent and random. It tends to last for several seconds to many minutes. It happens on all bands at the same time EXCEPT it does NOT happen on any TRN band that is configured (I have one configured for 50 MHz/28 MHz). This lead me to believe the K60XV was involved. During the SS Phone, I removed the K60XV and replaced it with a Re-work eliminator and K2/100 and the rig worked just fine for the next 10 hours. 

During the last three days, I opened the K2 up and laid the KPA100 on the workbench and left the K2 powered up with the K60XV installed. The problem manifested itself within hours. I was able to run a few diagnostics.

The AGC activation happens in all all AGC modes, SLO, FAST and OFF. I was able to get a scope probe on the emitter of Q12 on the Control board. When the blanking occurs, I see a train of pulses around 2 mS apart - around 500 Hz. 

I tried poking a finger around various places on the K60XV board to try to exacerbate the problem. I wasn’t able to determine anything conclusive, although a couple of times I thought something in the vicinity of the K2 relay might be involved. 

I’m completely dumbfounded by this problem. There’s nothing in the K60XV that interconnects with the AGC system in any way. What could possibly be going on here?

Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr at arrl.net
Web: http://boringhamradiopart.blogspot.com
Quote: "Not within a thousand years will man ever fly!"
            -- Wilbur Wright, 1901