Clarifying the question made, relevant to the grid designations made on olde tyme 1st det. tubes (mixers) :
GS . . . for Grid Supressor (Which is also tied in to G3) redesignated as G3,G5 grids by the time of the advent of the 6SA7 and 6BE6's.
GA . . . .for Grid Anode (This grid acts as the Anode or plate of the "triode" local osc function within the tube)
GO . . . for Grid Oscillator (Designated as the grid of the osc triode functioning of the tube.)
After confirming your plate and screen levels,the next main thing I was interested in was that cathode reading of +4, VDC . . . so that confirms that set is not having a floating ground.
PLUS if you were to stab your Vtvm’s / DVM positive lead into the 6A7 cathode connection and then monitor the 1st grid with the negative lead, I think that it should reflect a -4 VDC reading and be indicative of a healthy local oscillator output from your 6A7.
Lets come back to that aspect later, with the utilization of an exploratory test.
ODDITY:
With that series of schematics, inclusive of your set, having been around for 70 years passage . . . . I now wonder if you are the ONLY one with that brand of set with a problem .
Nor have others not had problems in the past, in the sets mixer section which required the use of that schematic.
I am placing a thumbnail of the mixer circuitry as the schematic SHOWS and then another, as I THINK that you will find, is the way that the set actually is being wired.
Its only concerning the oscillator portion, so please check your set and confirm. I am placing in the most common and guesstimated 1st grid bias resistor and the cathode resistor value . . .correct me as per your set.
It seems that the sets schematic . . . along with several other adjunct "Bleep" chassis' schematics . . . all seem to equally be showing, a "Wheres Waldo" in respect to "floating grounds" . . . . or specifically . . . just how is the cathode of the 6D6 and the 6A7 getting their DC ground paths ? ? ?
Review the provided schematic and note its inset blow up that I have placed at the bottom, for being in confirmation to your sets ACTUAL wired circuitry, as being compared to that original schematic.
Now try this procedure for performing a dynamic evaluation of your RF circuitry on that AC64 set :
Testee-Testee:
Sooooooo . . . take one each . . . pocket transistor radio . . . or a small table model radio.
Best connection access . . . will be if the AC64 chasis is being placed on its side, so that a sole connection can be extended out at a right angle to the chassis..
Use a test lead with its end alligator clips . . or 2 . . or 3 daisy chained, as per the 2 sets proximity requirement. . . . the closer the two are,the better.
One clip end goes to a connection to the the plate of the 6A7 and then the end of the required clip lead length(s) is wound 1 turn around the monitoring receiver, with that alligator end merely clipping onto the clips central insulated wire.
We're ONLY wanting some degree of capacitance coupling here.
If having to use a conventional small table radio for monitoring, treat the coupling to it in the same manner as you would be doing with a transistor radio, except you will be using its loop antenna.
Warm up the AC64 and see if it is working or not.
If its not playing for you, bring down any of its volume to the extent that you can then concentrate only on the monitoring receivers audio and then tune that monitor set to the second "harmonica" . . .of 456 kc which would be roughly 912 kc . . . and hope that you are then going to be hearing incoming stations on the monitor receiver WHEN the tuning of the AC64 is being run from one end to the other of the BCB.
If you then find yourself hearing received stations on the monitoring receiver, your AC64's RF front end, the mixer and its local oscillator is functioning for you, but with no reception functionality being had from the rest of the AC 64 for you.
To further confirm why that is, now lift the clip connection from the 6A7 plate and move it on to the plate of the 6D6 I.F. amplifier and see if you are then receiving the stations same as before, but even stronger now.
Last step is to pull the clip from the 6D6 plate and shift it to the 1st grid of the 76 detector tube and do the same monitoring as was done previously, but with a tuned in stations strength coming in the loudest yet.
Should you have lost the reception during any of the previous steps, that would be indicative of the point where you were loosing your signal, and the inoperationality of the AC64, being able to work alone by itself.
If the set plays on the monitor all the way up to that last point of the 76's first grid, then, one needs to look towards the aspect of the sets fault being in the Det-6F5 1stAF -AVC , 42-AF output stage or power supply feed or possibly some bandswitching action or miswire fault of the AC64.
Standing by for your feedback . . . . .
73's de Edd
