Norm, I'd learned a lot from your earlier postings about "tuning eye" tubes,
so I knew to expect less response from a 6U5 than a 6E5, but I was surprised
that it takes -22v to close. I never get AVC voltages greater than about
-2.5v, and only that high in the first minute or so of operation. Afterwards
AVC voltages are around -1v, maybe -1.5v. Now I only trust my second-hand
vtvm to a point, since I haven't figured how to find reliable voltages and
resistances to calibrate it against. But it's not off by an order of
magnitude. I'm certain. Also, even adding a 220 ohm cathode bias, the total
grid bias on the 6U7G IF amp is still less than 3v on the SW bands. This makes
me think the real problem is somewhere else in the radio.

Thomas, this could be where your suggestion comes in. I did replace the
capacitors and the 1meg (rather than 2.2meg) resistor linking the detector
circuit to the AVC, but I didn't replace two that are inside the output IF
transformer, a 50K and a 250K that I think are about 30% high. That is
definitely what I'll try next.
Richard, I pulled the 6Q7G detector and took some readings. In this radio, the
AVC voltage fell to -.6v (to the accuracy of the aforementioned vtvm) on both
BC and SW bands, so apparently the 6A8G translator isn't generating high
voltages on its grid like it is in your Zenith. On the chance that the
difference is in the voltages applied to the 6A8G's in the two radios here are
the actual voltages on the 6A8G on the SW bands with the cathode grounded: K
(pin 8) 0v, of course; G2 and S (pins 6 and 4) 54v; G1 (pin 5) -3v (from the
grid of the oscillator 6J5G); and P (pin 3) 250v.

It's irrelevant, but I might add that the -3v from the oscillator grid is way
low according to the schematic, which shows -28v. That's in large part
because the 6J5G oscillator plate voltage is supplied through a 50K ohm
resistor---original to the radio, I'm pretty sure---rather than 10K per the
schematic. I tried a 10K. The voltages look better, but the radio works
worse. I also tried a NOS 6J5 (vs OEM 6J5G), and got better voltages, but no
difference in the radio's performance. The patient presented with a 6J5GT that
tested ok, but didn't work.

Despite all these problems, the radio works surprisingly well. For example, I
usually have no trouble getting (on the West Coast) the BBC, 125kw from
Singapore to Australasia or Radio New Zealand, 100kw to the Pacific islands.

Thanks again to the three of you for your expert help, and to NostalgiaAir.
I'm new to this and have found both the schematics and Forum postings
absolutely invaluable.