Rob French (KC4UPR)
Sorry for the delay on this. Life.
The black wire corresponds to the purple/magenta wire in the instruction manual, i.e. it connects across the two middle terminals of the switch. The switch is kind of buried at the moment, but if needed I can try to dig it out. I will note that with the the switch in the OFF position, my S-meter display goes away (as expected), and it reappears (albeit not very good) in either SLOW or FAST. I'm fairly confident that I do have the switch wired up correctly.
I've tried injected a 1V RMS 1500 Hz audio signal into the yellow audio line. I was hoping to see that result in AGC action being force, but no luck. That caused little-to-no change in the value of my S-meter/the corresponding ADC (I will note in general that the ADC input voltage I'm seeing never gets above about 0.3-0.5V). (As an aside, I also heard the 1500 Hz signal through my headphones... I'm not entirely sure how it's getting into the audio chain???)
Final note, if you look in the photo I posted before, right above the two RG-174 lines that go to the RF gain control, there are two solder pads with nothing between them. Looking at the manual for the AGC, it appears there is supposed to be a capacitor here. This appears to be an addition relative to the original ND6T design, but I imagine maybe it could have been an intentional change to the kit? What was this capacitor for, what's it's value, and could its absence be the cause of any of my issues?