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Got it working now; replaced the 4.7M resistor and it made little difference; figuring I had nothing to lose at this point I tweaked the adjustment on the discriminator transformer a little (taking note of the starting position), and it made a huge difference - was able to get the volume up to normal on the FM side; however, it would go up and down with a pop, usually coming back if I pushed down on the discriminator tube.
I had cleaned all of the tube pins and sockets, so at first I was worried about the dreaded "Silver Migration Disease" I've heard about, but it wasn't crackling and didn't sound like "thunder" the way the symptom is usually described, so I figured there may be one or more cold solder joints. I reflowed the solder on all the connections under the discriminator tube and transformer, and now the FM volume is up to normal and it's no longer popping or cutting out.
I may ask a friend of mine for help doing an alignment sometime - he used to be a TV repair tech (including tube TVs from the old days) so he may know something about it, but for now it sounds pretty good.
Thanks again for the advice!
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