[Elecraft] Digital noise in K2 audio

[TBald53892@aol.com]

Since I had the scratchy AF gain pot problem I recently implemented the 
Alternative Wiring for the K2's AF Gain Control modification by N6KR and 
N0SS. This well documented modification has fixed the noisy pot problem but 
has revealed yet another issue, at least on my K2 (sn 2638).

I find that at low AF gain control settings I can hear an approx. 900 Hz 
(very rough guess) continuous tone that seems to be the result of the mpu 
scanning the front panel keys. Push a key such as band up or down and the 
tone briefly drops in frequency to approx. 470 Hz (checked this by comparing 
to sidetone freq) as the mpu services the band change request and then 
returns to the 900 Hz key scan.

I hear this noise on two different types of headphones of varying efficiency 
and over the K2's internal speaker. Over the speaker it sounds like there is 
a small fan running inside the K2! Of course turning up the AF gain 
eventually covers the digital noise with normal band noise etc.

For diagnostic purposes this noise is best heard with the K2 AF gain set to 
full CCW. It is an issue because with the improved AF gain control action the 
noise can also be heard in the first 30 to 40 degrees of pot rotation where I 
spend most of my time listening. The noise is even more evident if you reduce 
the RF gain and run correspondingly more AF gain as I tend to do. Once you 
realize it's there it's really bothersome because the rest of the K2 receiver 
chain is so clean.

I'm not an engineer but as many of you know that darn K2 encourages you to 
look under the hood so I poked around a bit to see if I could learn anything 
and get rid of this noise. I have the KAF2 option installed on my K2 so I 
removed it to see if the noise was reduced. Nope. When the KAF2 is removed it 
looks like the audio chain is entirely disconnected from the product detector 
and the rest of the receiver. Good. Ok, so if I turn the AF gain fully CCW 
the noise should disappear...nope, no change as I rotate the pot. Ok, perhaps 
that tells me its in the LM380-8, U9 circuitry. If I ground both inputs to U9 
the noise should go away...nope. Ok, perhaps if I lift the +12V side of R16 
and feed U9 with an entirely separate 12 volt supply...nope the noise is 
still there. Well, lets put this puppy into tune mode and see if the mute 
function kills the noise. Dead silence. Out of tune and the noise returns. I 
didn't expect this result. Perhaps in tune mode the keyboard is scanned 
differently?

I believe this noise was present before the modification but with the AF gain 
scratchiness and poor pot resolution it was not as noticeable. Perhaps the 
CMRR of the new pot circuit is significantly reduced as well. I'm not sure 
where to proceed from here except to present this problem to the group mind 
and see if anyone has a suggestion or two. Thanks!

73, Tom KI5FM  


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