[glyphmon]

MrFixr55,

Here are some updated voltages from dialing the Isotap down to 115V, which translated to 116.6V AC at the power switch

6F6G
Plate: 228V (240V)
Screen Grid: 250V (250V) (not fluctuating at time of initial test)

RF 6K7G
Plate: 246V (245V)
Screen Grid: 76V (70V)
Suppressor Grid: -3.2V (?)

6A8G
Plate: 243V (245V)
Pin 4 (grids 3 and 5): 76V (70V)
Osc Control Grid: -9.8V (?)
Osc Plate: 159V (150V)

6Q7G
Plate: 181V (150V)

IF 6K7G
Plate: fluctuated constantly in the range of 120V to 123V (250V)
Screen Grid: 75V (70V)
Suppressor Grid: 0V (?)

Now, the notable differences I saw were the lack of fluctuation on the 6F6 screen grid, and the significant drop in still fluctuating IF 6K7 plate.

Aside from that, the voltages look better at this setting, so I'm not too worried about much other than the fluctuation and the noise.

To the point of the noise, I tried removing the 6Q7 tube as you suggested and of course no signal came through.  There was also no extraneous speaker noise.  Whether smart or not, I inserted the tube with the power to the radio on and observed there was still none of the random noise, though there was a hum and very weak signal until I connected the wire to the cap.  When I connect the wire to the cap, the hum goes away, but the random noise returns.  Removing the cap connector, the noise goes away and hum returns.

For the heck of it, I checked the 6F6 screen grid and the IF 6K7 plate voltages in these two configurations (with and without the cap to the 6Q7 connected).  With the cap on, the 6F6 screen grid tended to fluctuate around 265 +/- 1V, with it disconnected, it was a steady 266V.  Cap connector state didn't seem to impact IF 6K7 plate fluctuation at all, and during this test, it was pretty constant in fluctuating between 188V and 210V.

I see that the Stewart-Warner 91-621 that I have downstairs also has a 6Q7.  I'll try to pull that tomorrow and see if swapping it out makes a difference.  Google suggests that the solder point for the cap inside the tube can crystallize and crack, creating small arcs, and that gently heating the cap with a soldering iron can help.  Of course, I'm sure overheating can do way more damage, but if swapping the tube makes a difference, maybe it's worth a shot?

Not sure if it's just coincidence that the 6F6 grid is fluctuating only when the cap is connected to the 6Q7.  Would that make sense?  I don't see a connection between the two on the schematic; of course logically, when the noise is present there's more power drawn to produce the sound.

If feel like the fluctuation of the IF 6K7 plate (and at other points between it and the field coil) is a separate problem from the noise.  Would a failing electrolytic (C60, in this case) cause this kind of fluctuation?  Like I said, these were replaced, but barely used in the last couple of decades.  Should I just replace the electrolytics again at this point?

I have not gone through and methodically heated all of the joints yet, but I still will try after I swap the 6K7s to see if that makes a difference.  Wanted to give an update and pick your brain based on the behavior of the 6Q7 cap.

Best wishes,
Roy