Okay, it looks like the low B+ was mostly a "test equipment malfunction". (Sorry!) Y'see, I figured rather than use my usual DMM, it might make more sense to monitor B+ with my big old analog VOM, since it's easy to see the meter pointer from a distance and easier to spot up/down drifting than on my DMM, plus I could just leave it connected and not have to worry about the meter shutting itself off after a few minutes of inactivity like my DMM. Plus, it has a 1000VDC range, and it's 50K ohms/volt so there shouldn't be any significant loading going on. Well, after pulling the chassis, propping it up on a table (running it on a variac, with HV and vertical output tubes pulled, but a speaker connected) and tracing B+ voltages back towards the 5U4's, I increasingly began to think something funny was going on... Even after pulling the main B+ fuse (which comes just after the 5U4's and before any of the filter caps) I still wasn't able to get more than about 200V on the meter, and it was just as erratic as before. On a whim, I disconnected the power again, switched to the 250V scale, ramped the power back up, and watched the meter pointer rise up-- to about mid-scale, and erratically. At this time I started to hear a scratching/hissing sound from the speaker. Gee, that's odd, the B+ fuse is pulled, how could there be any audio? Then I smelled something burning. Wait! That's not the *speaker* making that sound-- that's the
multimeter!!
One fried VOM, coming right up. Measuring B+ with my "everyday" DMM, everything was hunky-dory and stable.
However, running the chassis by itself turned out to have a side benefit. Rather than remove the tuner from the cabinet, I decided I'd make use of the old "Tuner-Subber" I had found at a hamfest a while back. With the substitute tuner hooked up to the IF input cable from the chassis, I was surprised to find I was now receiving TV program audio at the speaker-- all
I had been able to get before (with the chassis hooked up in the cabinet) was a little random static. As it turns out, there is indeed a problem with the tuner. I put the chassis back in the cabinet, and with the set's own tuner, it's just as before with a dull blank raster. However, with the substitute tuner, I actually get something that looks like a picture! The horizontal frequency is way off, though, and the focus is still very poor, and there's no apparent chroma signal, but at least it's *something*! At least that means the IF, video amp, and audio sections are okay. Even with a working accurate meter, B+ is actually a tad low, but not bad (I see about 280-290V on the 300V bus), so I doubt that's the main culprit of the focus/brightness issues, but it's something to keep an eye on. The 1V2 is still dark
(the heater is quite visible when running it in a tube tester; yes, I know, a lot of 1-volt miniature tubes, such as those used in old portable radios, have almost invisible filaments, but the 1V2 was quite easy to see on the tester, so I figure it should be visible here...), and I measure near-zero VAC across the heater pins (though it's also possible that my DMM can't measure AC accurately at such high frequencies). The flyback winding that powers the 1V2 heater has continuity, and it's really easy to spot on the flyback (it's just a single loop of wire) for that matter. Clipping that dodgy-looking jumper wire had no evident effect. Next I'm going to see if I can find an extra damper tube and horiz output tube just to see if replacing them affects anything (I did see some blue flickering in the HO tube, but not really bad), adjust the horiz osc coil, and check other components in the HV/horiz section...
Thanks for all the comments/advice so far! Sorry 'bout the B+ red herring, though...