I am using a vtvm for my measurements, and, in fact, I did get it on ebay (and
a good deal, too!). As far as I've been able to determine, it's measuring dc
voltages quite accurately. I think it may be about 5% off on resistances.
When I'm certain I'll recalibrate it.

My oscillator voltage isn't nearly as bad as my last posting would lead you to
believe. The measurements were in response to a question regarding a 6A8G
generating a very large negative current from its grid in a Zenith radio at SW
frequencies. Depending on frequency, I get 6J5G oscillator grid voltages
varying from -5v to -10.5v on BC and from -3v to -12.5v on SW. The schematic
calls for -28v, but that's with an oscillator plate voltage of 160v coming
through a 10k plate load resistor (R5). The resistor actually in the
radio---which appears to be original---is 50k and, consequently, the plate
voltage is 75v. I tried a 10k. It raised the voltages, but the radio
performed worse. So my theory is that the manufacturer discovered this after
the schematic was published? Or, in other words, "beats me!"

That 600 ohm IF 6U7G cathode bias resistor (R4) is the cause of my original
posting. (It also biases the suppressor grid and the 6A8G translator cathode.)
There are two peculiarities: First, the radio actually had what appeared to be
an original 220 ohm R4 resistor, rather than a 600. I replaced it with a 560
(the closest my Radio Shack carries). The second peculiarity is that the
cathode bias resistor is only used on the BC band. The band selector switch
grounds those cathodes on SW bands. The weird part is that I
discovered that using a 220 ohm cathode bias substantially improves the
radio's performance on the SW bands. This is in accord with Norm Leal's
information that a 6U7G requires a total grid bias of -3v (suggesting that I
should go higher than 220, maybe, since I'm getting total bias of about -2.6v
now). But if this is such a good idea, I'm sure it's not that the manufacturer
needed me to tell them soAfter getting your last posting, I replaced the two resistors inside the
output IF transformer can (the 50K R7 and the 250K R9 to ground). They were
both 30%--40% high. I can't find any leakage through the mica caps. The
schematic doesn't show any resistances for the IF transformer coils. I
checked the IF alignments, and they are good. The oscillator alignments are
good, too. I did find that the screen resistor for the IF amp (R6) had drifted
very high (210K, where it be 75K) and replaced it, bringing up the screen
voltage. I don't know how I missed this before. But none of these changes
affected the AVC voltage. On BC it sometimes gets up to -3v. But on SW (with
a big antenna) it's still between -.5 and -1. Maybe that's what it's supposed
to be? But then why a 6U5 that takes -22v to close?

I do still need to try two things you suggest. I need to see the effect of
removing the 250K resistor R9 in the detector circuit. (I thought it served
simply to complete the DC circuit to ground that begins at the 6Q7G diode.)
And I need to try replacing the 6U5 tuning eye. It is a bit dim, but ok
in indoor light. On Norm's advice, I think I'll try to get hold of a 6E5,
which closes with -8v. It doesn't seem likely I'm going to be getting -22v in
the AVC!

Thanks again for all of your ideas and suggestions, Mack
.

:Regarding AVC voltage, in most of my radios it does not go above about 3 or 4 volts with a strong local station. Your average 20,000 ohms per volt meter is not sensitive enough to detect the accurate voltages within the AVC circuit. Think about how this meter will load down a 1 million ohm resistor, and you got the picture. Some digital meters are sensitive enough to take accurate readings. Vacuum Tube Volt Meters are extremely sensitive, and will give you accurate readings in this circuit. They aren't extremely easy to come by. You can try eBay. They may need repair.
:
: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