[glyphmon]

Well, in case more information will be useful for others to chime in, I managed to do some tracing through the band switch today.

With the band switch in the broadcast position I tested for continuity between the following points:
Antenna terminal to ground terminal - good
B12 to B6 - good
6K7G plate to Resistor 33 - good
any pin on 6A8G to Resistor 44 - no continuity
F1 to Capacitor 27 - good

Someone can point out the correct terminology to use with these if I get it wrong, but to me it means the primary and secondary are good on the broadcast antenna transformer, the primary is good on the broadcast RF transformer, and the oscillator transformer is good.  As far as the secondary on the broadcast RF transformer, I think my test points were not ideal because lo and behold, I later discovered that pushing in on the band switch knob in AM position allows me to receive AM and receive it quite well.  And after cutting power with working AM I tried the same test points and still had no continuity.  AM came back when I turned the power back on, anyway.

So I think it IS in fact the band switch, and since everything else tested fine, I suspect the contacts on the D wafer (secondary side of the RF transformers) is where the weak point is.  BUT since I know about this now, I'm not inclined to go much deeper on this rabbit hole IF I can continue to get the radio to work on AM with this trick of pushing on the band switch knob.

So, that is one mystery now a little less mysterious.

Secondly, I still have some noise with the volume control all the way down, but now that I could be sure AM was working, I started out with voltage measurements per the schematic.  The loudness of the noise varied from very quiet to quite loud throughout much of testing.

Now, I didn't remember to try measuring the voltages on the electrolytic that is marked with the other tube voltages - I can easily enough, I hope, if that becomes necessary, but here are the measurements for the tubes with expected values in parentheses, Isotap set to 120V:

6F6G
Plate: 250V (240V)
Screen Grid: fluctuated constantly in the range of 265V to 270V (250V)

RF 6K7G
Plate: 266V (245V)
Screen Grid: 78.5V (70V)
Suppressor Grid: -3.3V (?)

6A8G
Plate: 260V (245V)
Pin 4 (grids 3 and 5): 78.3V (70V)
Osc Control Grid: -10.5V (?)
Osc Plate: 179V (150V)

6Q7G
Plate: 193V (150V) ***

IF 6K7G
Plate: fluctuated constantly in the range of 166V to 180V (250V)
Screen Grid: 77.8V (70V)
Suppressor Grid: 0V (?)

So, there are some significantly high voltages on some of these pins.  Not sure exactly what that suggests or if people think that it's problematic.  The sensitivity seems pretty good.

Of note: the 6Q7G plate, when making contact from the probe to the pin, I got a loud pop of noise (no smoke) and it blew the 1A fuse.  Because there was no way I had inadvertently made a short, I put a 3A fuse in that came with the fuse holders, and tried again.  Still got the loud pop but noticed the noise present with the volume all the way down went away.  In fact, I can just barely hear the station come in with the volume all the way down though maybe that's always just been buried in the noise.  Could tune through much of the AM band without the noise recurring.  Eventually, though, the noise came back, and while making contact with that pin didn't guarantee it went away the first time I touched it, it did by the second contact with the probe.

I'm hoping someone has some thoughts about this revelation with the noise at minimum volume?  

Also, would it be expected that the screen grid voltages on the 6F6 and plate voltage on the IF 6K7 fluctuated so constantly?

Thoughts on the low plate voltage on the IF 6K7 in general?

Best wishes,
Roy