Your oscillator voltage is incredibly low. It should be around 4 to 7 volts, depending on what part of the dial you tune to. How well is all of your radio aligned? How well is the RF amplifier aligned? Have you checked the mica condensers for leakage? They cannot have any leakage at all.

Also, you can try removing the 250K resistor (R9) and see what this does. Most radios do not have this resistor. Incidently, I think you said that the diagram lists the IF cathode bias resistor as 250 ohms. As I saw it, the diagram shows 600 ohms. Maybe I misread your post.

Make sure that everything is precisely aligned. Check IF coil resistances against any listed in the schematic. If resistances are off, then you may have a shorted coil. Unless you disassemble the transformers, you won't be able to accuruately test the resistances because of the resistors and components in the second can. Also, have you tried replacing the eye tube? Is it old and dim? Some eye tubes lose responsiveness with age (gassiness, etc.). Most remain the same, but I have come across a few that respond less as they get older.

The reason why your radio cuts out the RF amplifier for short wave is because the RF amplifier is tuned specifically for broad coverage of the broadcast band. Radios which use the RF amplifier for both bands usually have provision for changing the frequency coverage for each band. Some have pre-tuned trimmers for broad coverage, and some have a third gang on the tuning condenser for more accurate tuning of this stage.

Thomas

:Hi, Richard, Norm, and Thomas: Thanks very much for your help. It's taken a
:few days to find Quality Time with my radio to follow up on your ideas.
:
: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.
:
:
:

:Hi, Richard, Norm, and Thomas: Thanks very much for your help. It's taken a
:few days to find Quality Time with my radio to follow up on your ideas.
:
: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.
:
:
: