[Elecraft] A funny thing happened while debugging a friends K2...

[Bruce Thompson]

I was helping a friend debug his K2 and I noticed that his audio was
nice and loud. (We weren't debugging an audio problem.)  I was checking
out the DAC on the control board to make sure it was working so I
checked his sidetone setting.  His was set to 050 and seemed nice and
strong.  Well, we went off to see why his power output control was not
working.  We found a lot of stuff that was working but never did figure
out what was going on.  (He and Gary think it is the output amp going
into oscillation.)

Well, I'm the guy with the low audio who has to run with the AF and RF
gains to max with the preamp turned on in order to hear any SSB signals.
Anyway, I finally had some time to play a bit with mine and thought I
would check my sidetone level.  Mine is set to 240 and it still sounds
weak.  Interesting.

So, I thought I would start walking down the circuit to see what is
happening.  I checked the output of pin 1 on the DAC that sets the tone
level.  It goes from 0 to 5V with the max setting at ST Level = 255.
OK.  I then checked the output of the op amp (U10B pin 7) which looked
good at 5 volts.  I checked it with a scope and the waveform looked a
little strange with a 600Hz ripple (sidetone freq) on the 5 volts.
However, for half of the cycle, there was a "blob" with a much higher
frequency component.  I then checked the input to the AF AMP (the LM380)
pin 8 marked "bypass".  It was a 6.8V signal like it says in the manual
and has about a 1V Peak to Peak 600Hz ripple.  The "blob" was nice and
filtered out by then.  The output of the AF amp also has the same AC
peak to peak signal.  I assume "bypass" means don't amplify but I
haven't looked in the part spec.

I also measured the audio out to the speaker by replacing it with a 4.0
ohm resistor and used a true RMS meter across it.  It reads 0.336V,
which sounds right for a 1 V P-P signal.  

Note that I have made the TF3MA sidetone modification to make it nice
and clean, which it does very well.  

Now for my question, what values are people seeing on K2s that work?  I
would love to know what AC values you see on the output of the op am
(U10 pin 7), the bypass input to the AF AM (U9 pin 8) and the output of
the AF AMP (U9 pin 6).  This would help me see where my system is
different to help identify the problem. 

The next step is to inject a calibrated input RF voltage into the
receiver and check to see what the AGC is doing.  Right now, I have to
set R1 to 10K to get it close to working properly.  I still can turn off
the AGC and have a noticeable boost in audio.

Bruce "Can you hear me now?"  KG0SH
Golden, CO 




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