Thanks for your help FloobyDust!
Here are the voltages of the two original Motorola MPF102's and the new National in each band. And just like you said, RF noise may be present. Also note, Knight interchanged the Source and Drain connections of the JFETs in this radio too:
For an experiment, I matched three new National JFETs with the DCA75. They have nearly the same Vgs "on" and "off" voltages and so I installed them to see how they affected the performance of the radio.
Well, the performance got worse. The local radio station still comes in but it's edgy and gritty and the station was harder to lock onto. Here are the new voltage measurements below. Are the new JFETs pushing the radio into distortion/clipping?
The voltages moved outward to the higher and lower extremes but stayed in proportion. There is a pattern here, unfortunately, there are no notes anywhere on the schematic as to what the conditions of the radio should be in when taking voltage readings.
I'm not sure if this drawing applies to how the JFETs in this radio operate, but it depicts the Drain voltage resting mid-point between the Source voltage and the supply voltage while at idle:
Once the JFETs are brought to their idle state, couldn't the surrounding resistors be tweaked to bring the JFET to its correct bias?
It reads that the lower Source voltage should be no less than 2v at idle, and knowing that the supply voltage in this radio is 8.70v, the Drain voltage should be set somewhere around 5.35v.
Or am I'm on the wrong track and maybe just swapping JFETs would be a better approach?
Who knows? and what are the odds that the printed schematic voltages were taken while a signal was applied to the antenna terminals? wouldn't that bring the upper and lower voltages of the JFETs to their near-maximum limits before distorting
*We have plenty of options and my goal is to get the radio working the best it can. The radio works. The new resistors are surely more accurate than the old ones and the old resistors wandered in value. Maybe, the two original/working JFETs should go back in and we just deal with "grading" another for TR-2's slot? The copper traces are super-tough so component swaps are not a problem. In another article, someone made a MPF102 tester using a breadboard and simulated the circuit it needed to work in.
And about Bill Meacham's findings, He described the (Drain resistor?) R4 change as "slightly noisier." Didn't that resistor adjustment also push the Source voltage even higher (I heard that resistance changes make the D&S voltages raise or lower in unison) and closer to the supply voltage causing the signal to go into clipping? Shouldn't have R6 been adjusted too to counteract that?
Up to this point, the only thing we have touched is the JFETs. I have not turned any coils or touched/replaced any other parts except for the new resistors.
...and I'm in no rush in getting this radio done
The more I look at the numbers of the matched JFETs, they don't look so bad compared to the voltages on the schematic, except for TR-1's voltages under the "Source" column. Hmm. We got Bill Meacham's issue
*One other thing, the Band switch is on "A" in the schematic so look only across that line when comparing the JFETs voltages.