[NostalgiaRadioTime]

It's been a long time since I've had a radio this stubborn and uncooperative! I spent the afternoon on it, totally redoing the joint on the terminal strip where R4 and C10 join, even replacing both the cap and resistor to eliminate the possibility of one or the other being defective. That caused no change. So I monitored voltage fluctuations at these points when the radio was playing normally, verses when the volume would change (now either really loud, or really quiet and somewhat garbled initially before clearing itself up again):

On Pin 6 of the 12SK7 RF tube at normal volume I get 38 VDC, when the fluctuation and sputtering begins, it drops to 27 VDC

Measuring on each side of the 1200 ohm resistor between the e-cap positives (R3), I measure 115 VDC normal / 113 VDC when the volume drops at the junction with R4, and 127 VDC on opposite side of R3. I re-soldered the joints on either side of R3 just to make doubly sure there's no bad or cold solder joint there.

Measuring right on the point where C10 & R4 join, I get 38 VDC at normal volume, and as it sputters and fluctuates so does the voltage, down to 28 VDC at low volume.

So then I did another test. When the volume dropped, I plugged in my iPod (the 3.5mm cord that is attached to the high side of the volume control). This cuts off the RF section. I had to turn the volume up on the radio, which is normal when using the cord, and the volume level never changed. At one point, I could hear the strong local station bleeding through, which indicated the volume again changed back to normal on the RF side. What this seems to indicate is the problem is indeed isolated to one or more of the tubes before the volume control (the two 12SK7's and the 12SA7) and not the Audio section 12SQ7 and 50L6.

The next test I did, as i monitored the voltages above, was when the volume dropped, I immediately poked on each of the pins on those 3 tubes, and none of them caused any change. I thought that would rule out a bad tube pin connection in the socket. But now I'm not so sure.

I then tried once again a completely different set of NOS tested good tubes. Every tube except the rectifier. And the problem was still there. So I reinstalled the original tubes I had in all along and the radio was all but silent. Very low volume and a bit garbled. Eventually it came back to life at normal volume. But this is with the same tubes I had been using all along, even put the 12SK7's back in the same tube sockets. So I still can't entirely rule out a bad tube socket, but which one!

I'm totally at a loss as to what to replace next. There's not much left that I haven't! Could it be:

1.) a faulty volume control that only acts up at odd times and only at lower volume levels? (I checked the connections there too and nothing I poked on triggered the problem). The radio has TONS of volume when playing normally

2.) Faulty RF coil (the plate of the 12SK7 RF tube connects directly to it)?

3.) Faulty OSC coil?

4.) An intermittent short in one of the IF cans?

5.) Replace tube sockets (these are the type of sockets with a molded bottom, so the pin sleeves are not exposed on the bottom where they could be crimped tighter for a better connection)?

All of these replacements require a lot of work, especially the tube sockets. I really don't want to go down that rabbit hole unless I could isolate it to one single socket because I know what will happen. Replace #1, no change, then do another, and another until finally I have all 6 replaced and it still acts up. I think I have one or two of the Universal RF and OSC coils here yet that AES used to sell but if there's an intermittent short in the IF can, that might be a deal breaker unless I could locate a similar can to replace it with.

Any thoughts on where to proceed next would be greatly appreciated!