Philharmonic IF wiring - schematic errors

[Kent King]

I'm working on a 1939 Beam of Light Philharmonic Deluxe and noticed a wiring error (ok, maybe)...in looking at the schematics available, it gets worse. Refer to the 3 images attached as needed. Here we go:

In the 1937 schematic, and in most sets I've worked on, pin 8 (cathode) and pin 5 (grid #3, usually tied to the cathode) go directly to ground. Pin 5 (at ground) is used as a tie point for the "pass through" caps in the IF cans. In the 1939 schematic, it looks like Scott added a cathode bypass (400ohm/.05mfd) - BUT - the schematic still shows everything grounded (look closely at the diagram, the R/C network does nothing as drawn).

Now the puzzler - the set I'm working on is wired like the 1939 schematic, but "correctly", the cathode and pin 5 are sitting on the R/C network, NOT at ground. HOWEVER - the two very original caps passing through the IF are connected to pin 5 - therefore NOT at ground potential.

Now I'm sure the primary IF bypass (goes to B+) should be coming back to ground. And pretty sure the secondary one should too. I have two options: remove the R/C network and just ground things like the 1937 schematic, or move the pass through caps over to a ground tie point. I"m leaning toward the latter solution. Would love thoughts on this from the techs in the group!!

Kent

[Kent King]

Thanks to everyone (Scott - just saw your note...I concur, I checked one of my other chassis as well).

After 14 hours of work, I have finished replacing caps and checking all the wiring. I still have to do some cleaning and work on the remote control units (the remote control volume control is still out in this picture). 

[Scott Seickel]

Kent, this is how the cathode is wired on several sets I have.  This is correct. 

[Norman S Braithwaite]

I suspect the cathode return introduced a small amount of negative feedback to the IF amplifier to improve IF amplifier stability.  In every AW23 on which I have worked, one tuned circuit is shunted by a 500k resistor not shown in the schematic for the purpose of reducing Q and gain to prevent oscillation even though excessive shielding was employed.

Norman

[Kent King]

I also spoke with Norman...I'm going to restore it as it was built and see what happens. On the 2nd IF, the B= cap had shorted, and that took out the 400ohm cathode resistor and not the 1K feeder (which is what usually goes). It will be a while before I can tell you all how it works - this is a remote control set and I've got quite a few hours of work ahead before powering it up.

Dave - that Potter cap is a replacement, but I'm recapping the set throughout.

[Bill Liers]

Kent,

Cathode bypass not at DC ground potential but still close enough to AC ground through bypass cap. No major functional difference certainly at If frequencies.

[David C. Poland]

Note - the combo Philly diagram Riders 14-5 shows this IF cathode network added only for the 3rd IF tube, and no longer in the 1st and 2nd IF tube. 

[David C. Poland]

Try it. Run the two IF caps to some other ground. Keep the new "cathode bypass network" in a functional circuit. Seem beneficial?

Seems to me Scott would introduce the modification for some good reason (adding cost of 2 additional components in each stage of the IF amp) .

That Potter cap to left be an old repair/replacement?