Audio clicks on the NorCal 40A from the KC1 keyer

A while back I noticed that I had a slight and slightly annoying audio click or pop on my NorCal 40A (wilderness radio edition) whenever the KC1 keyer started up.  That is, on the first element of a character or at the beginning of a played message, there was a click, which I assumed had something to do with the keyer microcontroller waking up.  It was not present at all in straight key mode, which bypasses the KC1 keyer. 

I cannot remember now if this was always a problem, or something that had cropped up at some point.  I don’t use the rig frequently, but when I do, it is usually in the field on a hiking trip.  The rig has not been handled particularly gently, so there is the possibility of damage from rough handling.  
 
NorCal 40A transceiver with the top cover removed
My beloved Norcal 40A

The click was not terrible, but the straight key mode sounded so much better, I decided to try to see if I could fix it.  Adding an extra capacitor bypassing at the KC1 microcontroller didn’t fix it.  Eventually I lifted the line that connects the KC1 to the LM386 audio amp, and I still had the problem! So, I started probing around with an oscilloscope.  My first thought was to see if there was a transient on the MUTE line, but using the single trigger mode on the scope, I didn’t find anything noteworthy.  Here, I am referring to the line labeled as “MUTE” on the KC1 keyer.  It goes to the junction of D1 and D2 on the NorCal 40A. I thought that might be a place to look for some kind of transient.   It is actually more of a “not-quite-mute” as some transmitted signal by design sneaks around Q2 via R4 in lieu of an audio generated sidetone.  The KC1 needs some kind of sidetone audio by itself, without keying the transmitter, so that it can annunciate the frequency, etc. so it has an audio connection to the LM386 as well. 

Probing around some more, I found that the issue decreased significantly when the scope probe touched the pin 2 input, so I slapped in a 68pF cap from pin 2 to ground on the LM386 and the problem is gone!

Location of added capacitor near the LM386
Placement of the 68pF cap near the LM386 is visible on the left.

Although my problem is solved, it is still a little mysterious to me.  If anyone else has noticed a similar audio click with the KC1, please leave me a comment! 

73,

Mike K6STR


 

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